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Seed Processing and Storage: Principles and practices of seed harvesting, processing, and storage: an organic seed production manual for seed growers in the Mid-Atlantic and Southern U.S.

Authors:
  • University of Virginia (former faculty member, currently independent researcher)
SEED PROCESSING
AND STORAGE
Principles and practices of seed harvesting, processing, and
storage: an organic seed production manual for seed growers
in the Mid-Atlantic and Southern U.S.
Copyright © 2004 – 2010 by Jeffrey H. McCormack, Ph.D.
Some rights reserved. See page 28 for distribution and licensing information.
For updates and additional resources, visit www.savingourseeds.org
For comments, suggestions, or distribution information contact:
Jeffrey H. McCormack, Ph.D.
Saving our Seeds
www.savingourseeds.org
Charlottesville, VA 22911
Funding for this project was provided by USDA-CREES (Cooperative State Research, Education, and Extension Service)
through Southern SARE (Sustainable Agriculture Research and Education).
Seed Processing and Storage: Principles and Practices
Copyright © 2004 – 2010 by Jeffrey H. McCormack, Ph.D.
Version 1.6 July 21, 2010
2
TABLE OF CONTENTS
Principles of seed harvesting and processing..................................................................................... 3
Seed harvesting and extraction.............................................................................................................. 3
Harvesting..................................................................................................................................... 3
Dry seed processing.................................................................................................................... 3
Wet seed processing................................................................................................................... 4
Seed processing and handling ............................................................................................................... 6
Threshing equipment................................................................................................................. 6
Seed cleaning equipment (seed screens)................................................................................ 6
Winnowing equipment ............................................................................................................... 7
Seed drying.... ............................................................................................................................................ 8
Principles of drying..................................................................................................................... 8
Drying seeds for long-term storage......................................................................................... 8
Instructions for drying seed with silica gel........................................................................... 9
Seed storage.. ............................................................................................................................................ 9
Effect of temperature on seed longevity ............................................................................... 9
Effect of seed moisture and humidity on seed longevity................................................... 9
Relationship between relative humidity and seed moisture content .............................. 10
Determining seed moisture content ....................................................................................... 10
Relationship between temperature and moisture on seed longevity............................... 11
Illumination.................................................................................................................................. 11
Respiration and heating............................................................................................................. 11
Effects of fungi, bacteria, and pests on seed storage.......................................................... 12
Variety and harvest conditions ................................................................................................ 12
Seed moisture levels required for long-term seed storage................................................. 12
Packaging materials and containers for seed storage......................................................... 14
Seed longevity............................................................................................................................................ 15
Relationship between viability and vigor............................................................................... 15
Expected life span of stored seed............................................................................................ 17
Seed dormancy.......................................................................................................................................... 17
Federal and state seed laws.................................................................................................................... 18
Seed treatments ........................................................................................................................................ 19
Germination testing ................................................................................................................................. 19
Basic procedures for doing your own tests ........................................................................... 19
Testing by laboratories .............................................................................................................. 20
Germination enhancement techniques ................................................................................................ 20
Mixing seed lots of the same variety ...................................................................................... 20
Additional seed processing....................................................................................................... 20
Brief microwave treatment........................................................................................................ 21
Other techniques......................................................................................................................... 21
Seed statistics............................................................................................................................................ 21
Labeling and record keeping .................................................................................................................. 22
Shipping seeds........................................................................................................................................... 22
Prior to shipment ........................................................................................................................ 22
Containers for shipping............................................................................................................. 22
Appendix A: Hot water treatment times (in minutes) for vegetable seed .................................... 23
Appendix B: Vegetable seed data, germination information, and federal standards................. 24
Appendix C: Basic seed cleaning equipment ...................................................................................... 25
Sources of supplies and equipment...................................................................................................... 26
Selected bibliography and literature cited .......................................................................................... 26
Licensing and distribution information............................................................................................... 28
Seed Processing and Storage: Principles and Practices
Copyright © 2004 – 2010 by Jeffrey H. McCormack, Ph.D.
Version 1.6 July 21, 2010
3
PRINCIPLES OF SEED HARVESTING AND PROCESSING
The primary purpose of storing seeds is to save seed from one season to the next, but farmers and seed
companies often find it useful or necessary to store seeds for at least two to three years, and sometimes
longer.
There are several reasons for this: (1) seed yields and seed quality (germination and vigor) may be
unpredictable due to growing conditions, and (2), market demand for certain crops may vary significantly
from one year to the next. Market demand itself can be strongly influenced by media coverage of certain
varieties that may quickly fall in or out of favor depending on media exposure. Likewise, market demand
may also be influenced by the psychological effects of poor growing conditions from the previous season.
For example, gardeners coming off of two years of drought may become discouraged from making purchases
for the next season. National events (such as war, disasters, or media “feeding frenzies”) during peak
ordering season can have a strong effect on seed sales. Cold, cloudy, winter weather, lingering late into
spring can have an effect on impulse purchases by casual gardeners. Though sales to market growers tend
to be more predictable from year to year, many gardeners may purchase more on impulse. Because of all
these factors, seed demand is not precisely predictable. As a consequence much of the seed sold in
commerce is not sold the year after it was produced. Seed is routinely carried over from year to year, and
germination tested on a regular basis.
Because seed is routinely stored for more than one year, it is important to understand how seed
harvesting, processing and seed storage affect the longevity and vigor of the seed. Seeds are fragile, living
organisms, and the shelf life of the seed is affected at the beginning of the plant life cycle by such factors as
soil nutrition. For example, if the soil is zinc deficient, the quality of the seed will be adversely affected.
(Zinc is a co-factor in many enzyme reactions in the life of the seed and the maternal plant.) Though
providing the best conditions for crop growth and health is the foundation of seed quality, the factors that
can have the most important effect on seed viability and vigor are harvesting, extraction, cleaning,
transportation, and storage. It is easy for seed to become damaged at any of these stages. The purpose of
this publication is to provide guidelines for minimizing seed damage and maximizing seed viability and vigor
from pre-harvest through post-harvest processing.
SEED HARVESTING AND EXTRACTION
Seed harvesting and cleaning methods can be divided into two methods: dry processing and wet
processing. Dry processing involves harvesting seed that has already matured and dried within the seed-
bearing portion of the plant. Examples of dry processed seed plants include beans, broccoli, corn, lettuce,
okra, onions, sunflower, and turnips. Wet processing is used when the mature seed is enclosed within a
fleshy fruit or berry. Examples of wet processed seed plants include cucumbers, melons, and tomatoes.
Some vegetables can be either dry processed or wet processed, for example, peppers, and squash.
Harvesting:
The basic rule of harvesting is to allow the seed to mature as long as possible on the plant without the
seed or fruit becoming diseased, or overly ripe. Each type of plant has an optimum time for collecting the
seed, but factors such as climate, weather, disease, insects, birds, or predatory mammals may require that
the seed be collected at less than the optimum time. In the Mid-Atlantic and South, frequent and daily
thunderstorms and high humidity may play a large role in determining how and when seed is harvested. For
example, in dry climates, beans can normally be left to mature and dry in the field, but during wet humid
weather, it is best to harvest early and allow the beans to continue maturing and drying under cover.
Dry seed processing (pods, capsules, seed heads, etc.):
When seeds are ready to be processed, the entire seedpod, capsule, or seed head will become brown and
dry. During the maturation process, the ripening pods and capsules change color from green, to yellow-
green, to yellow, to light brown, to a darker brown, or dark gray. Ripening and maturation may be uneven
within the pod or capsule, uneven on the plant, and uneven within the stand of plants. For that reason, the
pods of many plants are harvested individually. Seeds of legumes and brassicas often develop a split along
one side of the pod. This is the best time to collect the seed, before the pods start to open and scatter their
seed. Most flower seed heads are not ready to harvest until the flower head has dried completely to the base,
Seed Processing and Storage: Principles and Practices
Copyright © 2004 – 2010 by Jeffrey H. McCormack, Ph.D.
Version 1.6 July 21, 2010
4
including a short section (approximately ¼”) of the supporting stem. Some plant families, such as the
Asteraceae (Aster family) have a smaller percentage of viable seed in the head, and the seeds continue to
mature after collection. For this reason it is best not to be too hasty in harvesting the seed. Examples
include lettuce and sunflower. Some seed may mature in the capsule or pod, even before the pod has turned
completely brown. Most seeds turn a darker color as they mature. Seeds may initially be white, turning
green or tan, and then brown or black. Once the seed pods, capsules, and seed heads start to mature, it is
important to check the crop on a daily basis. Rain or seed predators can ruin a good seed crop in a short
period of time. Plants that produce umbels (members of the carrot family, or Umbelliferae) can usually be
left in the field to harvest until the umbels are dry. Some members of this family mature their seed unevenly
causing seed to scatter, while other seeds in the umbel continue to mature. One method of dealing with
crops that mature their seed unevenly is to pull the plants and hang them upside down to dry under cover.
This allows the seed to continue to mature on the plant while the plant dries. This procedure is often used
for lettuce. Confidence in knowing when to harvest comes both with experience and familiarity with
different species and crops.
After harvest, seeds are threshed to remove the seed from the surrounding plant material. A period of
air-drying is important before seeds are threshed. Plant material should be spread out in thin layers until all
plant material is dry; otherwise, mold, decay, and heat from decay will cause damage to the seeds. As the
plant material dries, seed pods may split open or shed seed. Harvested material should be stored in a well-
ventilated room with low humidity. During this time you should be aware of insects, especially weevils that
feed on the seeds. Plant material that is ready to be threshed should be brittle. Threshing is best done
outside on a dry day. The threshing process involves application of mechanical force using a controlled
pressure and a shearing motion., and is accomplished by hand or by machine.
There are many different methods for threshing seed. Plants that have pods, such as beans and okra,
can be threshed by placing the pods in a large feed sack, which is tied shut securely, and then placed on the
ground where it is flailed, stepped on, jogged on, or danced on with a twisting motion. The sack is turned
often to redistribute the plant material for further threshing. When using this method, it is best to use
running shoes or other soft-soled shoes because seeds can develop hairline cracks and splits from too much
pressure. To separate seed from flower heads, plant material is spread on a concrete slab and then gently
walked on with a twisting motion to break open the flower heads. Care must be taken to not apply so much
pressure that the seed is abraded or broken (especially a concern with angular-shaped seed). To thresh seeds
of brassicas, place plant material in a large wheelbarrow or on a large tarp, and while wearing gloves twist
and wring the plant material through your hands until the seed breaks free from the pods. Another method
for extracting seed is to place seed heads on a piece of plywood. The seed is extracted by placing a wooden
cement float above the material and then pressing and twisting until the seed breaks free. If the seed heads
are small you can run a rolling pin over them. For threshing small lots of seed, a threshing box (Figure 1)
may be used. This consists of a wooden box, with sides slanting outward, open at the top and on one end,
and the long sides and bottom covered with corrugated-rubber floor matting. Seed is placed in the box and a
rubbing board (also covered with corrugated rubber), is moved back and forth across the seed heads to
separate the seed from the heads.
Figure 1. Threshing box for processing small amounts of seed.
Wet seed processing (crops with fleshy fruits, fermentation):
Wet seed processing is used with seed crops that have seeds in fleshy fruits or berries. There are three
steps to the process: (1) extraction of the seed from the fruit, (2) washing the seeds, and (3), drying.
Seed Processing and Storage: Principles and Practices
Copyright © 2004 – 2010 by Jeffrey H. McCormack, Ph.D.
Version 1.6 July 21, 2010
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Extracting seed:
The type of extraction process depends on the species. Soft fruits such as tomatoes are cut up,
mashed, and then fermented. Cucumbers and melons are cut in half, the seed scraped out along with
the fruit pulp surrounding the seed, and then fermented. In watermelons, the entire fleshy fruit is
fermented along with the extracted seed. These types of fruits have a gel surrounding the seed that
contains germination inhibitors. The presence of the gel also makes handling and drying of the seed
difficult. Fermentation is a natural process that occurs to a small extent as fruits decompose. When
fermentation is done in a controlled manner, the microorganisms, principally yeast, break down the gel
thus releasing the seed while killing bacteria and fungi that cause most seed-borne diseases. The
temperature and length of fermentation are important. If the mash is not fermented long enough, seed-
borne diseases will not be eliminated, but if fermented too long, the seeds may sprout prematurely. The
length of the fermentation is dependent on temperature and typically last three days at a temperature of
70 to 75
o
F (21
o
C to 24
o
C). Length of fermentation may also depend on the variety itself. For example,
varieties with high sugar content may take longer to ferment, up to four days. With few exceptions,
fermentation periods longer than three days risk damaging the seed. There are different fermentation
techniques for different crops, for example, pepper seeds are extracted from the fruits by mashing, but
the fermentation process may last only 24 to 48 hours. Though eggplant isn’t a watery fruit, it can be
mashed and fermented for about 48 hours.
A Corona® grain mill is a very handy tool to use for extracting seed of eggplants and hot peppers.
The grinding plates on the mill are set wide enough apart to break up the fruit without grinding the
seed. Small peppers can be put directly into the mill, whereas larger fruits such as eggplant are cut into
small cubes before processing. Before processing another variety, the mill has to be disassembled and
cleaned thoroughly to remove bits of small seed caught in the plates and other parts of the mill. Though
some people use a food processor and blender set on low speed to extract the seed of hot peppers and
eggplant, it is easy to damage the seed. I would not recommend using a blender. Instead, a food
processor that has a thick (approximately 1/8”) blade works well provided there is ample water in the
processor, the speed is as slow as possible, and the processing time as short as possible.
There are some differences of opinion about adding water to the fermenting mixture, the concern
being that water slows the fermentation process causing premature sprouting. In my experience this
hasn’t been a problem. As long as the ferment is not diluted more than perhaps 10 to 20 percent by
volume, this isn’t a concern. The issue of whether to add water depends on how thick the mash is and it
depends on the variety. Some varieties make a very thick mash that is hard to stir, and others make a
watery mash that stirs easily. The mash should be stirred three times a day, once in the morning, once
in mid-day, and once in the evening. If the mash is too thick to stir easily, the nutrients are not going to
circulate easily. Stirring is also important for better control of seed-borne diseases. In addition, when
the mash is not stirred, a foul-smelling, white mold forms on top of the mash. This mold can discolor
(darken) and damage seeds at the top of the mash. These seeds will later have to be hand picked out of
the dried seed. A properly fermenting mash should not have a foul smell, and there should be little or
no white mold on the top. A small amount of white mold is not harmful and can be stirred back into the
mash, but a heavy overgrowth should be removed.
Washing seed:
After fermentation is complete, the seeds are washed to remove pulp, pieces of fruit and debris, and
low quality seed. Before washing the seed, it is useful (especially for washing tomato seed) to first scoop
out pieces of pulp floating on top of the mash. This is done by straining the mash with your fingers,
pulling out the larger chunks. Whether or not there is floating pulp depends on the variety or how
thoroughly the fruit was processed. Add a volume of water at least equal twice the volume of mash. It is
important to dilute the mash sufficiently because the more dissolved solids there are in the mash, the
higher the specific gravity. If the specific gravity is high (lots of soluble solids) it will be more difficult to
wash the seeds properly. As a general rule, good seeds are heavy and sink to the bottom, whereas poor
quality seeds are light and tend to float off with the wash. The washing process is repeated until the
wash water becomes clear. Although most good seeds sink to the bottom, some vegetables have very
light seed and require extra care during the washing process. For example, it is common for a significant
amount of good pepper seed to float rather than sink during the washing. This can be avoided by
adding the wash water slowly, so as not to create tiny air bubbles that adhere to the seeds, making them
buoyant. Even with this precaution, there are a few varieties of watermelon for example, where the good
seeds tend to float rather than sink, thereby requiring special care in washing. In this case, the best way
Seed Processing and Storage: Principles and Practices
Copyright © 2004 – 2010 by Jeffrey H. McCormack, Ph.D.
Version 1.6 July 21, 2010
6
to wash the seed is to pour the wash through a ¼” hardware cloth screen, and then use a hose to force
pulp through the screen.
Drying seed:
Seeds should be dried fairly quickly after washing. Slow drying may result in mold growth or
premature sprouting of the seed. In the Mid-Atlantic and South, seeds should not be dried in the sun,
nor should they be dried anywhere where the temperature exceeds 95
o
F (35
o
C). Dark colored seeds are
especially vulnerable to damage when sun dried. Instead, seeds should be dried in a climate-controlled
environment using fan ventilation. A combination of ceiling fans and air conditioning dries seed safely
and very quickly. Seed should be spread out in thin layers (no thicker than ¼” for small seeds) and then
stirred several times a day until dry. Once the seeds feel dry, they should cure for another two to three
weeks. Curing is the final stage in the drying process. As the seed moisture content declines it comes
into equilibrium with the relative humidity. After the seeds are cured they can be placed in a container.
When drying seeds, choose plywood, window screen, or any hard, non-stick surface. Avoid using paper
towels, newspaper, cardboard, or cloth because seeds will tend to stick to the surface making them
difficult to remove. Beginning growers often make the mistake of drying squash seed on newspaper,
which adheres permanently to the seed coat. No one wants to read the daily news on the surface of
squash seed.
SEED PROCESSING AND HANDLING
Threshing: equipment:
For small-scale seed production, it is often not cost effective to purchase threshing equipment because
of the small volume of seed produced. A number of small alternative seed companies, specializing in
heirlooms and specialty seed, thresh most, if not all of their seed by hand. Some seed crops, especially herb
and flowers seeds are produced in such small quantity (and have such small seeds) that it is impractical to
use a machine for threshing. Also flowers and other seed crops, if left to dry on canvas or a tarp, naturally
dehisce (shed) much of their seed. The remaining amount of seed can be released by flailing or rolling, or
other non-mechanical means.
It is difficult to locate low-cost, low-tech shredders for small-scale seed threshing. The options are to
locate old, used equipment or to construct your own. Vegetable seeds, such as beans can be threshed with a
modified leaf shredder/chipper. Plans are available on the Web site of the University of California at Davis,
Long Term Research on Agricultural Systems project:
http://agronomy.ucdavis.edu/LTRAS/itech/thresh.html#shred.
Typical threshing rates for this machine are 22 to 30 pounds/hour for brassicas, and approximately 100
pounds/hour for beans. The research and development group at the Organic Gardening Experimental Farm
of Rodale Press developed a small-scale thresher designed to thresh a variety of crops ranging from
amaranth to soybeans. Though the machine was not fully refined it did an acceptable job. Plans for this
machine were published in Small Scale Grain Raising (see Logsdon, 1977). The book is out of print and is
available from book dealers specializing in used books. Plans for building this thresher are also available
(without illustrations) at the following Web address:
http://www.geocities.com/RainForest/Vines/4095/resources/thresher.html
Several growers have reported that small-seeded crops, such as amaranth can be threshed using an Allis-
Chambers model 66 combine which is noted for its ability to thresh and clean small-seeded crops (Weber, et
al., 1986).
Seed cleaning equipment (seed screens):
It is not necessary to have expensive seed cleaning equipment to clean seed for small-scale production.
With the exception of lettuce seed and a few others , the majority of crop seeds can be cleaned with
homemade seed screens. Winnowing will still be necessary to remove smaller chaff.
Seed Processing and Storage: Principles and Practices
Copyright © 2004 – 2010 by Jeffrey H. McCormack, Ph.D.
Version 1.6 July 21, 2010
7
Many seeds can be screened with several different mesh sizes of hardware cloth. Hardware cloth is
readily available in the following mesh sizes: 1/2", 3/8", 1/4", and 1/8". The 1/2" and 1/4" mesh sizes are
available at most hardware stores. The 3/8" and 1/8" mesh sizes are the most useful sizes, but are the most
difficult to find.
Aside from hardware cloth, a lot of other materials are useful for making seed screens. Aluminum
window screen can be used for small seeds. You'll also find various meshes of screen available in the
housewares section of department stores. Cabinets from electronic devices often have round or oblong holes
which are useful for cleaning some types of seed. Special meshes can be ordered from mail-order hardware
specialty catalogs. It is useful to collect a large variety of mesh sizes and shapes to handle a wide variety of
seed types.
Once you have located suitable screen material it should be mounted on a frame. Professional seed
screens are mounted on 12" square wooden frames. Homemade screens should also be mounted on 12"
square frames so they can be used together with professional screens. The frames can be made of wooden
lath which measures 3/4" x 1-1/2". Ideally, the screens should be constructed so that they nest together.
The nesting feature is desirable for using three screens simultaneously: rough chaff is retained on the top
screen, seeds in the middle screen, and small chaff and small seeds on the lower screen. Once the frame has
been made, the screen is nailed with small nails or brads at approximately 1" intervals all along the edge of
the frame.
Winnowing equipment:
The classic method of winnowing involves placing seeds in a wide basket and tossing the seeds and chaff
into the air. The chaff is carried away by the wind. Illustrations of Native Americans using this historic
method evoke a bucolic mood, but in reality, winnowing by this method is extremely difficult, and the results
are not very satisfactory. The most vexing part of the process is that wind speed is always changing in
velocity and direction. It can work for certain kinds of seed, but it actually works better to use two large
bowls, pouring seed from one bowl into another bowl below, while blowing on the plant material as it falls.
This method works satisfactorily if the seed is heavy and the chaff is very fine, and susceptible to being
carried away by a gentle current of air. In any case, hand winnowing should be done, not on windy days, but
when the air is calm.
A good range of equipment for winnowing includes the following: (1) an assortment of stainless steel
bowls ranging in size from 6 to 16” in diameter, preferably bowls with varying shallow and deep sides; (2) an
electric hair dryer with two speeds (heating element removed, or unheated air setting); (3) portable vacuum
cleaner with option of connecting hose to the air discharge opening: (4) household box fan with three speeds;
(5) squirrel cage blower; (6) rheostat for controlling fan motor speed; (7) a tarp for catching seed or chaff;
and (8) a dust mask for keeping chaff out of your lungs.
Most light seed can be winnowed quickly and efficiently with mixing bowls and a hair dryer. It takes a
little practice determining the correct amount of seed to put in the bowl, setting the proper fan speed, and
the distance of the dryer from the bowl. It helps to jiggle the bowl up and down or to swirl the seed in the
bowl. There must be little or no wind when you work with light seed. Until you have a little practice it is
best to put a tarp on the ground, in case you need to sweep up your mistake and start over. Heavy seed is
best winnowed by pouring the seed from one container to another in front of box fan, or alternatively
directing the air discharge hose from a vacuum cleaner into a large (16” diameter) mixing bowl.
When dealing with large volumes or certain types of seed it is helpful to use mechanical equipment for
winnowing. The University of California at Davis has plans for making your own winnowing equipment. This
information has been available from their Long Term Research on Agricultural Systems project dealing with
Appropriate Technology for Small and Subsistence Farms. Information previously on their website has
moved and is no longer available, but a copy of the information may be downloaded from:
www.savingourseeds.org/resources.html
Seed Processing and Storage: Principles and Practices
Copyright © 2004 – 2010 by Jeffrey H. McCormack, Ph.D.
Version 1.6 July 21, 2010
8
SEED DRYING
Principles of drying:
Drying is a normal part of the seed maturation process. Some seeds must dry down to minimum
moisture content before they can germinate. Low seed moisture content is a pre-requisite for long-term
storage, and is the most important factor affecting longevity. Seeds lose viability and vigor during
processing and storage mainly because of high seed moisture content (seed moisture greater than 18%).
High seed moisture causes a number of problems:
Moisture increases the respiration rate of seeds, which in turn raises seed temperature. For
example, in large-scale commercial seed storage, respiring seeds may generate enough heat to kill
the seeds quickly, or to even start a fire if not dried sufficiently. Small-scale growers are not likely to
have such an extreme condition, but seed longevity will, nevertheless be affected.
Mold growth will be encouraged by moisture, damaging the seeds either slowly or quickly, depending
on the moisture content of the seeds. Some molds that don’t grow well at room temperature may
grow well at low temperatures causing damage to refrigerated seeds. In such a case there may be no
visual sign of damage.
Unless seed moisture is at least eight percent or below, insects such as weevils can breed causing
rapid destruction of seeds in a short period of time.
Drying seeds for long-term storage:
Silica gel is the most effective desiccant (moisture absorbing material) for drying seeds. Powdered milk
has been recommended as a desiccant in older seed-saving literature, but is less than ten percent as effective
as silica gel. Silica gel is a highly porous form of silica that absorbs moisture. It is available as a powder or
as beads in different sizes. The best size bead for drying seeds is approximately 1/16” to 1/8” in diameter
“Color-indicating” silica gel is a form of silica gel that has been treated with a small amount of cobalt
chloride which acts as a moisture indicator. When the indicator gel is completely dry it is a deep blue color.
As the gel absorbs moisture from the air, it gradually changes in color from deep blue to light pink.
Though silica gel is clearly allowable under the Organic Rules, color-indicating silica gel is not clearly
covered. Check with your certifier before using color-indicating silica gel.
Silica gel can be repeatedly reactivated (re-dried) after it has absorbed moisture. The procedure involves
heating the gel and driving off the moisture, and as it dries the color gradually changes to deep blue. Drying
must be done in a controlled fashion, otherwise the beads will turn black, and the moisture-absorbing
capacity of the beads will be destroyed. There are two methods for reactivating silica gel:
Oven-drying method:
This method gives the best results, but it takes longer and uses more energy. Set the oven for a
temperature of at least 200
o
F, but no higher than 275
o
F. Remember, that some ovens may run hotter
than the set temperature. Place the silica gel in a thick-walled Pyrex dish, no deeper than one inch, and
continue heating until the beads turn deep blue. When drying large quantities (a pound or more), the gel
should be stirred occasionally. The oven drying method takes 1-1/2 hours per quart of gel (with the
oven temperature set at 275
o
F). One quart of silica gel weighs approximately 30 ounces (1.9 pounds).
Microwave-drying method:
The microwave method works much faster but must be monitored more closely to avoid overheating
the gel. Use only a thick-walled Pyrex container for heating the gel. Set the microwave on medium or
medium high and dry for approximately three to five minutes. The color change in the gel can be
monitored through the over door, but the gel should be inspected and stirred at the end of each heating
cycle. If the gel has not dried, heat again for another three to five minutes. Approximate drying time is
eight to twelve minutes per pound of gel, though actual heating time will depend according to the type
of microwave.
Seed Processing and Storage: Principles and Practices
Copyright © 2004 – 2010 by Jeffrey H. McCormack, Ph.D.
Version 1.6 July 21, 2010
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There are some fine points and cautionary notes about using silica gel. When using a Pyrex dish, the
glass should be thick. Do not use plastic microwave containers that will melt on contact with the gel.
Silica gel gets very hot, and the glass container may shatter if it is too thin, of the wrong type, or if
unevenly heated. You may notice a slight odor during heating. This is due to either overheating the
silica gel, or to the evaporation of organic seed volatiles absorbed by the silica gel during the drying
process. Silica gel itself is chemically inert, non-toxic, non-corrosive, and odorless, but breathing the
dust can be hazardous under prolonged and repeated exposure. For that reason, do not use the finely
ground or powdered form.
Instructions for drying seed with silica gel:
To dry seed, determine the weight of the seed to be dried, including the packets or envelopes that
contain the seed. Measure out an equal weight of silica gel and place the seeds and silica gel in an
airtight container for seven days. When drying seed it is important to keep the container size small
in relation to the volume of seeds being dried.
At the end of seven days, remove the packets of seed from the drying container and transfer into
another airtight container, such as a Mason jar, Seed Saver Vial™ or barrier pouch. Because seeds
can re-absorb moisture from the air quickly, they should be transferred quickly.
When used as directed, silica gel dries seed from 12% typical moisture content to a desired moisture
content of approximately 5% for small seeds and 7% for large seeds. As a “rule of thumb”, seeds will not be
damaged provided the drying time doesn’t exceed seven days. This drying time applies to humid climates,
such as the Mid-Atlantic. Longer drying may drop the moisture content below 3% for small seeds and below
5% for large seeds, levels which may damage seeds or force them into dormancy. Legumes, such as beans,
are especially are injured by over drying.
SEED STORAGE
Effect of temperature on seed longevity:
The general effect of temperature on longevity is that longevity increases as temperature decreases. This
is true of “orthodox” seeds: that is, most seeds that follow some general “rules of thumb” regarding longevity
during the storage life of the seeds. The relationship between temperature and seed longevity is that for
each 10
o
F (5.6
o
C) decrease in temperature, longevity doubles (Harrington, 1972). This rule applies to seeds
stored between temperatures of 32
o
F (0
o
C) and 122
o
F (50
o
C). This rule assumes that the moisture content is
a constant. This is a general guideline; in reality the longevity of some vegetable species declines more
rapidly than suggested by the rule, while the longevity of others declines more slowly in relation to storage
temperature.
The longevity of seeds is generally not affected by subfreezing temperatures provided the moisture
content is less than 14% (because ice crystals do not form). This has been established by a number of
published reports as well as my own experience storing and germination testing a wide variety of vegetable,
flower, and herb seeds at 20
o
F (-7
o
C) and below for periods of at least five years. I am still getting excellent
germination from seed stored twenty years, which was dried to approximately five percent moisture content.
This is the ideal way to store seed, especially small seed that doesn’t require much freezer space. One
caveat: seed cycled in and out of the freezer too many times without redrying may cause degradation of
germination
Effect of seed moisture and humidity on seed longevity:
Seed moisture has a greater effect than temperature on seed longevity (as noted in the previous section
titled “principles of seed drying”). Most seeds also follow some “rules of thumb” regarding moisture and
longevity. The general relationship is that for each one percent increase in seed moisture, longevity
decreases by half (Harrington, 1972). This rule applies to seed with moisture content between 5 and 13%.
Above 13% moisture content, seed storage fungi and increased heating due to respiration cause longevity to
decline at a faster rate. Once seed moisture reaches 18 to 20%, increased respiration, and the activity of
microorganisms cause rapid deterioration of the seed. At 30% moisture content, most non-dormant seeds
germinate. At the low end of the moisture range, seed stored at 4 to 5% moisture content is unaffected by
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seed storage fungi, but such seeds have a shorter longevity than seed stored at a slightly higher moisture
content (Bewley and Black, 1985).
Relationship between relative humidity and seed moisture content:
When storing commercially grown seed, it is impractical and too costly to use desiccant to dry the seed
for storage, unless the seed is small and expensive. Commercial seed is usually packaged for short or long-
term storage under conditions of ambient humidity (unless special equipment is used). Because relative
humidity has a significant effect on seed moisture content, it is important to understand the relationship
between humidity and seed moisture.
Regardless of the type of storage conditions,
the moisture content of seed eventually comes
into equilibrium with the moisture in the
surrounding air. The relationship between
atmospheric relative humidity and seed moisture
content is shown at the right in Figure 2.
The curve was derived from measurements of
the average seed moisture content of ten
vegetable species stored at different relative
humidity. Because the curve represents an
average of ten different vegetable crop species,
the response of individual species may vary. For
example, seeds of grains (which contain relatively
high percentages of carbohydrate) will have a
moisture content of 13 to 15% at 75% relative
humidity, whereas seeds rich in oils (such as
peanuts) can have a moisture content of 9 to 11%
at the same humidity.
Figure 2. Relationship of vegetable seed moisture
content to relative humidity. (Nakamura, 1958, as cited
in Justice and Bass, 1978)
Note that once the relative humidity reaches 70%, the moisture content of the seed has reached
approximately 13%, the point at which increased respiration and seed storage fungi become a significant
problem. Above 70% relative humidity the moisture content rises dramatically. In central Virginia, the
relative humidity in a residence or office building during the summer usually averages about 65%, plus or
minus 5%. If air conditioning is used, the relative humidity typically ranges between 50 to 60%, depending on
how often it runs. During the winter heating season, the relative humidity averages about 40%, plus or
minus 5%. If the seed is stored in an open barn, or outbuilding, the relative humidity surrounding the seeds
will be higher than that of a climate-controlled dwelling or office building. By using the values above, and
referring to the chart above, it is possible to get an estimate of the moisture content of stored seed. It is
clear from looking at the chart, that it is important to store seeds in a climate-controlled environment,
especially during the summer.
Determining seed moisture content:
Moisture content of seed is defined by the International Seed Testing Association (ISTA) according to the
following formula:
In order to determine the percent moisture content of fresh seed, a sample of fresh seed is weighed, and
then an equal weight of fresh seed is dried slowly to remove the moisture, and then re-weighed. The
equipment used to do this is a “seed moisture balance”, which has an infrared heating lamp mounted above
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the balance pan. The fresh seed is weighed in the balance pan, and then the lamp is turned on. The weight
of the seed gradually decreases during heating until the weight decreases no further. Once the weight comes
to equilibrium under the heating lamp, the dry weight is recorded. The data is then used in the formula
above. A seed moisture balance is an expensive piece of equipment, but reliable information can be also
obtained by heating the seed slowly in a toaster oven at low heat until dry, and then reweighed. Using this
method it may be necessary to weigh the seed several times during drying to determine if the weight has
come to equilibrium.
Relationship between temperature and moisture on seed longevity:
The effects of temperature, moisture, and relative humidity were discussed above as separate factors
which affect the longevity of stored seed. In reality, the effects of temperature and relative humidity are
highly interdependent in their effect on stored seed. There is a simple method for calculating the combined
effects of relative humidity and temperature on seed longevity, which is as follows: the sum of the storage
temperature (in degrees F), plus the relative humidity (in percent) should not exceed 100. Since seed
moisture is the most important concern, the rule stipulates that no more than half the sum should be
contributed by the temperature (Harrington, 1960).
The majority of crop seeds lose viability quickly when the humidity approaches 80% at temperatures of
77
o
F (25
o
C) to 86 F (30
o
C), but when stored at a relative humidity of 50% or less, and a temperature below
41
o
F (5
o
C), seeds will remain viable for at least ten years (Copeland, 1976).
If seeds are taken from a cold or frozen storage and transferred to room temperate, care must be taken
to prevent condensation on the seeds. If the seeds are in a sealed container, allow them to sit until they
reach room temperature before opening the container. If they are stored in paper, place the seeds into a
plastic bag with the excess air sucked out, seal the bag,, and wait for the temperature to stabilize before
unsealing.
Illumination:
The effects of light on stored seed have been studied (including the effects of different wavelengths of
light): Some studies showed a benefit and some showed a detriment: the results are inconclusive and
controversial.
Seeds stored in glass containers should be stored out of direct sunlight because of the localized
“greenhouse heating effect” on seeds. This might seem an unnecessary and overly obvious cautionary note,
but in my experience, it has happened accidentally more than once. For example, this can happen if jars of
stock seed are taken outside for the purpose of removal of some seeds for planting, or for transporting to
another location. Though the jar may be temporarily stored in the shade, the angle of sunlight may change
quickly and the jar will be in direct sun causing very rapid heating within the jar.
Though some commercially produced seed is dried in direct sunlight (in dry climates), drying seeds in
the sun is a questionable practice in the Mid-Atlantic and South if the air temperature is above 90
o
F (32
o
C).
The air temperature at the seed surface is higher because of the conversion of light energy into heat at the
seed surface, and the heat is “moist heat” (though this wouldn’t be in an issue in dry climates where
evaporative cooling occurs at the surface). I’m not aware of any studies on this issue for our region.
Another concern, also not well documented, is that the ultra-violet light from the sun may have a deleterious
effect on seed longevity while the seeds are drying (Harrington, 1972). Harrington’s suggestion was based on
the known effects of ultra-violet radiation on biological systems (rather than specific data). Whether the
ultra-violet exposure is long enough to cause concern, is unknown.
Respiration and heating:
The largest factor affecting respiration and heating is moisture content, and therefore, at a minimum,
seeds need to be kept dry. Respiration in seed storage has three effects:
1. Depletion of food reserves. Over the life of stored seed, depletion of food reserves is
inconsequential.
2. Release and accumulation of gasses that may affect viability of seeds in storage. The accumulation
of respiratory end products such as carbon dioxide is an advantage of storing seeds in sealed
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containers where carbon dioxide replaces oxygen in the air thereby slowing down respiration and
increasing longevity.
3. Release of energy, mostly in the form of heat. When seeds are stored under favorable conditions,
respiration is of little consequence. When moisture is high, respiration increases, which in turn
increases the production of heat thereby decreasing longevity.
Effects of fungi, bacteria, and pests on seed storage:
The process of seed harvest and cleaning removes most debris and insects, but certain fungi, bacteria,
and insects make their way into stored seed. Fortunately, the same conditions that are favorable to seed
preservation inhibit fungi and bacteria and kill insects.
Bacteria — Bacteria do not have a significant role in seed deterioration because free water is
required for bacterial growth, and if the moisture content of the seed is high enough to support
bacteria, the seed is more likely to succumb to deterioration due to other causes such as fungi,
respiration, heating or premature sprouting.
Fungi — Most seed storage fungi are inhibited when the relative humidity is kept below 65%. At this
relative humidity the moisture content of starchy seeds is about 13%, and oily seeds about 7%. The
major effects of fungi are to:
o decrease viability;
o produce toxins that affect seed viability and germination;
o increase heat production – important in large seed lots; and,
o cause discoloration, mustiness, and caking.
Insects — In hot, humid climates such as the Mid-Atlantic and South, mites, weevils, flour beetles,
and borers can be a serious problem in stored seed, but if the seed is dried to 8% moisture content
and the temperature reduced to (64 to 68
o
F (18 to 20
o
C), insects should not be a problem. At a
moisture content of 15% and a temperature of 86 to 95
o
F (30 to 35
o
C), they can become very
destructive. Mites will not survive when the relative humidity is below 60% (Bewley and Black, 1985).
Variety and harvest conditions:
Different varieties of a particular crop species may have different longevity when stored under the same
conditions. This may be explained by differences in enzyme activity, or differences in the chemical
constituents of unique varieties. For example, sweet corns are rich in simple sugars, whereas dent corns are
high in complex carbohydrates. This explains why dent corn seed keeps longer than seed of sweet corn.
Harvest conditions may also affect longevity of seeds in storage. For example, under poor conditions of
harvest, seed may have more fungi on the seed coat. Differences in harvest conditions are insignificant if
storage conditions are good, but under adverse conditions, harvest differences can be very significant.
(Bewley and Black, 1985).
Seed moisture levels required for long-term sealed storage:
Seed can be stored for long-term sealed storage provided that the seed moisture content is less than 8%,
which means that the relative humidity must be kept below 35% (as shown in Figure 3 below).
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Figure 3. Length of seed storage in relation to seed moisture content and relative
humidity. The data above are based on averages of different vegetable seed crops. Note
that grain crops have higher seed moisture content than do oily seed crops. (adapted from
Copeland, 1976; and Nakamura, 1958; with additions by the author)
For the Mid-Atlantic and South, seed must be dried with a desiccant in order to achieve the desired
moisture level for long-term storage. When drying with silica gel according to the procedure described
above, large seeds such as peas, beans, and corn will be dried to about 6 to 8% moisture content, and smaller
seeds will dry down to about 3 to 5% moisture content, depending on the size of the seed and whether its
primary food reserve is starch or oil. Seeds with a starchy food reserve retain a higher seed moisture content
than oily seeds.
Table 1. Maximum seed moisture content for seeds stored in sealed containers. The
seed moisture percentage of stored seed should not be higher than the values given below
(USDA Federal Seed Act, 1976).
Vegetable
Seed moisture (%)
Vegetable
Seed moisture (%)
Bean, common 7.0 Leek 6.5
Bean, Lima. 7.0 Lettuce 5.5
Beet 7.5 Muskmelon 6.0
Broccoli 5.0 Mustard 5.0
Brussels sprouts 5.0 Onion 6.5
Cabbage 5.0 Onion, Welsh 6.5
Carrot 7.0 Parsley 6.5
Cauliflower 5.0 Parsnip 6.0
Celeriac 7.0 Pea 7.0
Celery 7.0 Pepper 4.5
Chard, Swiss 7.5 Pumpkin 6.0
Chinese Cabbage 5.0 Radish 5.0
Chives 6.5 Rutabaga 5.0
Collards 5.0 Spinach 8.0
Corn, sweet 8.0 Squash 6.0
Cucumber 6.0 Tomato 5.5
Eggplant 6.0 Turnip 5.0
Kale 5.0 Watermelon 6.5
Kohlrabi 5.0 Unlisted 6.0
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Packaging materials and containers for seed storage:
Materials for short-term storage:
There are a wide variety of materials that can be used to store seed for short-term storage. Most of
these are non-rigid materials such as cotton, burlap, paper, and composite materials such as multi-wall
paper and plastic film, or polyethylene bags. Materials used for short-term storage are generally porous.
They adequately contain and protect the seeds from mixing, but do not provide protection from
moisture or loss of seed viability. Such materials are usually used for mechanically separating seed lots,
and for transporting and shipping seed until the seed can be placed in environmentally controlled
conditions for longer-term storage. Each type of packaging material has its own advantages and
disadvantages. Burlap bags have the greatest strength, can be re-used many times, and can be stacked
high without slipping of the stack. The strength of cotton bags is dependent on the thickness of the
weave and thread, and the quality of the seams. Though not as strong or as tear-resistant as burlap,
cotton bags can often be re-used, depending on the quality of the fabric. Bags made of woven plastic
material also are fairly strong but tend to slip when piled high and are harder to close securely after the
bag has been opened. Multi-wall bags are made of several layers of paper in a variety of types of
construction. They have poor bursting strength when piled high or accidentally dropped, and if used
repeatedly, they tend to become brittle along the folds and wear points. Cardboard boxes and cans,
though expensive, are re-usable, good for stacking, and provide some protection against mechanical
injury to seed and to infestation by seed storage insects. Flexible packaging that has a weave, whether it
is burlap, cotton, or plastic offers little protection against seed storage pests such as grain moths.
Consequently, seed stored in such woven bags may have to be inspected if an outbreak is detected in a
particular seed bag. Seed storage insects are very good at locating small openings in bags or containers
that are not well sealed.
For small lots of seed, paper bags such as lunch bags are inexpensive and adequate for storing seed,
but the seams are not always reliable, and when used, the bags should be double or triple bagged to
ensure integrity to prevent bursting. Other materials such cellophane, acetate, and 2- to 4-mil
polyethylene zip lock bags may be used (provided the seed has dried adequately). If using polyethylene,
it is best to use the 4-mil thickness, especially for heavy seeds. Plastics and thin films are not reliable
moisture barriers, though such materials offer better moisture protection than paper.
Materials for long-term storage:
Metal and glass containers, properly sealed to prevent the exchange of moisture and gas, are the
most commonly used containers. They are the only reliable means of protecting seeds against humidity,
insects, rodents, floods, and mechanical damage. Plastic should not be used for long-term storage.
For storing large quantities of seed, metal five-gallon cans fitted with a rubber gasketed lid and
pressure ring are ideal for storing large seeds such as peas, beans, and corn. One-gallon jars are also
excellent, provided that the lid has a gasketed seal. Some one-gallon jars have plastic lids that are
flexible enough to form a tight seal when the lid is screwed tightly, but metal lids need a rubber gasket.
Gaskets can be cut from sheets of neoprene rubber (available at hardware stores) or used automobile
inner tubes. Though glass is breakable, it has the advantage of being transparent so that the contents
can be easily inspected for insect damage (especially useful for detecting bean weevils, grain moths, and
other seed pests). Metal and glass storage containers are available from a number of sources (Supplies
and Equipment section). Glass canning jars are available in sizes ranging from ¼ pint to ½ gallon. One-
gallon glass jars are often available free for the asking at restaurants, fast-food restaurants, and snack
bars that purchase pickles, relish, mayonnaise, and a number of other food ingredients in bulk.
For storing small quantities of seed, ¼ and ½ pint jars are ideal, provided the rubber seal is in good
condition. Baby food jars are often recommended, but again the seal should be in good condition. The
seal can be ruined from seeds getting trapped between the rim of the jar and the lid; therefore jars
should not be packed to the rim. Other materials that can be used are Seed Saver Vials™ which are made
of a high-density polypropylene with a patented inner and outer valve that ensures an airtight seal.
Another material, often used for seed banks, is a heat-sealable barrier pouch. Barrier pouches are a
triple-laminate material made from paper, plastic, and foil. See the Supplies and Equipment section for
suppliers. They are sealed with a heat sealer, or an iron set on “wool” setting.
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When deciding on the type of storage container, it is often helpful to settle on a minimum number of
standard sizes. If using canning jars, it is best to choose a particular brand name and style so that same-
size and same-style containers have the same weight. This is very helpful in inventory control of seeds
being held for long-term storage. Standardization of container style and weight makes it easy to obtain
the weight of stored seed without actually removing the seed from the container and exposing to humid
air. Once the weight of standard containers is known, it is easy to subtract that weight from the
container of seed to obtain the weight of the seed.
Effect of different packaging materials on seeds in sealed storage:
There is somewhat of a general conception among people new to saving seed, that plastic materials
(for example, zip lock bags) are highly moisture impermeable. The conception is only partially true, as
seen in Figure 4.
Figure 4. The effect of different types of seed storage containers on the germination percent of creeping
red fescue seed (Grabe and Isely, 1969). Results do not take into account the thickness of the container.
SEED LONGEVITY
Relationship between viability and vigor:
Seed germination, generally measured by percentage, measures the number of seeds in a lot that can be
expected to germinate and grow healthy plants. Seed vigor is defined by normal seedling morphology and
the rate at which seeds germinate and grow in the early stages. Strong seed vigor has many advantages for
the organic grower, as vigorous seeds are less likely to be overtaken by diseases, weeds, and insects than
weak ones.
During the time that seed is held in storage, there is a gradual decline in germination and vigor. When a
seed company holds seed in storage at ambient temperature, the seed is typically tested every six months to
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a year, depending on the conditions of storage. Seed that is stored at 40
o
F (14
o
C), or at subfreezing
temperatures is tested on a longer cycle depending on how the seed was dried and the temperature at which
it was stored. Every time a germination test is done, the germination percent and test date is recorded on
the stock container. What that label doesn’t show is the degree of seedling vigor. There should be a
germination test number on the label so that the test log can be consulted if there are concerns about vigor.
Loss of seedling vigor is often apparent in the germination test. When the seeds take longer to germinate
than usual, the seedlings are smaller, and sometimes malformed.
Information about the vigor of a variety can be partially inferred from the germination data if you are
familiar with the general relationship between viability and vigor. The relationship between vigor and
viability is illustrated by a sigmoid survival curve as shown in Figure 5. Note that the relationship between
vigor and viability is similar except that vigor declines before viability. There are three distinct phases to this
relationship: (1) the first when germination is approximately 80% or above, when seed is both vigorous and
viable; (2) the second stage when deterioration progresses rapidly; and (3), the third stage when deterioration
slows at approximately 20% or below, and all seeds slowly die. In my own experience from seed testing of
vegetable varieties, when the germination drops to about 75%, I begin to have questions about the vigor of
the seed lot. If I have questions about it, I may review the recency of the test result, the source of the seed
lot, the year it was grown, the storage conditions, and then may consult the seed test log, or re-test the seed
if concerns arise. It should be noted that the curve shown in this graph is a generalized curve, and that
there are significant differences among varieties and types of seeds in regard to viability and vigor. Large
differences in vigor and viability are more likely to show up under substandard storage conditions, especially
when the relative humidity permits the growth of storage molds.
Figure 5. Relationship between the decline in vigor and viability over time (Harrington, 1972).
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Expected life span of stored seed:
The chart below shows the expected lifespan of seed stored under good conditions (seed giving a good
stand of plants at the normal seeding rate at the end of the storage period). The shaded column shows the
total lifespan of seed stored under good conditions. Since the storage conditions are not specified in either
case, the following are general guidelines.
Vegetable Years *
good
germ.
Years **
average
lifespan
Vegetable Years *
good
germ.
Years **
average
lifespan
Asparagus 3 8 Kohlrabi 3 10
Bean 3 10+ Leek 2 9
Beet 4 10+ Lettuce 6 9
Broccoli 3 10+ Muskmelon 5 10+
Brussels sprouts 4 10 Mustard 4 8
Cabbage 4 10 N.Z. spinach 3 8
Cardoon 5 9 Okra 2 10+
Carrot 3 10+ Onion 1 7
Cauliflower 4 10 Parsley 1 9
Celeriac/celery 3 10+ Parsnip 1 4
Chard, Swiss 4 10+ Pea 3 8
Chervil 3 6 Pepper 2 7
Chicory 4 10+ Pumpkin 4 10+
Chinese cabbage 3 7 Radish 5 10+
Collard 5 Rutabaga 4 10+
Corn 2 4 Salsify 1 8
Corn salad 5 10 Scorzonera 2 7
Cress, garden 5 9 Sea kale 1
Cress, water 5 9+ Sorrel 4 7
Cucumber 5 10+ Southern pea 3 8
Dandelion 2 5 Spinach 3 7
Eggplant 4 10 Squash 4 10+
Endive 5 10+ Tomato 4 9
Fennel 4 7 Turnip 4 10+
Kale 4 10 Watermelon 4 10+
* (Lorenz and Maynard, 1980)
** (Vilmorin-Andrieux, 1856)
+ The plus sign indicates that the seed had not lost all its viability before the test was terminated.
SEED DORMANCY
Every type of seed goes through a period of dormancy, which is a mechanism for delaying germination
until the seed is dispersed and exposed to favorable growing conditions. It is not evolutionarily
advantageous for all seeds from the mother plant to germinate at the same time. Environmental conditions,
such as drought, disease, and predation may wipe out a generation of seed. Dormancy provides a
mechanism for seeds to be able to adapt to their environment by germinating at different times and under
different conditions. It is not a mere resting state of a seed, but rather an active physiological state.
Dormancy helps seeds to germinate at the proper time of year, temperature, pH, lighting conditions, nutrient
levels, or other environmental conditions that affect survival. It allows weed seeds to remain ungerminated
in the soil for many years, until the conditions are conducive to growth. Most vegetable crops have a long
history of domestication, a selection process which has eliminated or reduced dormancy in many of our
major food crops. Nevertheless, some of our common crops still exhibit some dormancy, and this poses a
problem for seedsmen. For example, some varieties of brassicas, tomatoes, peppers, especially varieties with
wild ancestry, may not germinate well, or at all, shortly after harvest. A period of dry storage may be
required before the seeds germinate.
The process of artificially breaking dormancy is called conditioning. For 95% of seeds, the basis of
dormancy is biochemical or physiological in nature, whereas for the remainder of dormant seeds, the cause
for dormancy is an impermeable seed coat (Deno, 1993). The mechanism for breaking dormancy varies from
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species to species (and even variety to variety) and most often involves drying (curing), exposure to light,
leaching of chemical inhibitors, exposure to high or low temperature, or alternating temperatures. The
germination rate of many agricultural seeds may be increased if exposed to alternations in daily temperature.
For example, some herb seeds tested in the controlled conditions of a germination chamber do not
germinate as well as seeds germinated outdoors in the same media. In some species, dormancy is broken by
exposure to nitrate ions in the soil, and in others, exposure of seeds to gibberellins (plant hormone)
produced by soil fungi. Seeds that are produced by fleshy fruits have dormancy broken by fermentation and
leaching of the chemical inhibitors from the gelatinous coat surrounding the seed. Preharvest growing
conditions can also affect dormancy. For example, lettuce grown under different conditions of water and
nutrients exhibit differences in the amount of dormancy. Squash and pumpkins have the highest
germination percentage when seed is harvested from fruit that is twenty days past peak ripeness.
In many instances, seed dormancy is released by dry storage. The seeds of crop species that exhibit
dormancy after harvest can be released from dormancy within two to six months after harvest. This is
important for some pepper varieties. If the seeds are frozen, the seeds remain dormant until they are
subjected to above freezing temperatures for several months.
It is important for seed growers and seed purveyors to understand the dormancy characteristics of
crops, especially in regard to germination testing. A seed grower testing the germination of a lot of freshly
harvested seed may be surprised to discover that the tested seed doesn’t germinate. Rather than assuming
that seed is dead and needs to be discarded, it is important to test the possibility that the seed is dormant,
and that the seed may need to be retested after a period of conditioning.
FEDERAL AND STATE SEED LAWS
Seed laws exist for the basic purpose of ensuring that seed is labeled truthfully. They serve to protect
gardeners and farmers, and because seeds move from dealer to dealer they protect seedsmen as well.
Effectively, anyone who handles seed in commerce is afforded protection of the law. When a particular
variety is in short supply, seed dealers may legally sell low-quality seed, but the seed must be labeled as
“below standard”, and additional seed may need to be added to the container to compensate for the low
germination. In most states, seed packets are not required to show the actual percentage germination unless
the package weighs a pound or more. Other labeling requirements exist for some crops, especially grains,
indicating the percentage of weed seed. Some crops have hard seeds - seeds that are impermeable to water
during the usual germination period. When these crops are labeled, the percentage of hard seeds is included
in the germination count. Another protection afforded by labeling is the specification that seeds that have
been treated must be labeled as such. In addition, a dye must be added to the seed so that it can visually be
identified as treated seed. Seed, that has been grown organically, must meet organic certification standards
before the seed can be labeled as organic seed.
When growing seed for sale, you should contact the Department of Agriculture in your state to obtain a
copy of the seed laws for your state. Certain crops such as cotton may require a permit to grow the crop. In
Virginia for example, the state monitors boll weevils by placing traps in the field. Other states control the
sale of seeds, which may be considered noxious weeds, and either forbids their sale or their importation into
the state. State seed laws contain fairly similar requirements, so if you are meeting the requirements of the
Federal Seed Act, very likely you are in compliance with state seed laws. Some states have different
germination minimums for crops, but these differences pertain more to herbs and flowers than they do to
vegetable seed.
Federal germination standards are specified in the Federal Seed Act. Some crops, such as peppers have a
minimum germination standard of only 55%. Though this seems low, when dealing with varieties of peppers
that have wild genes in their ancestry, germination in these varieties is lower and dormancy tends to be an
issue. Over the years I discovered that many heirloom beans seem to germinate better than modern varieties
of beans. In fact, the 1966 Virginia Seed Law gives a standard of 75% germination for a list of older varieties
and a standard of 70% for newer varieties. Seed companies generally require their growers to produce seed
with higher germination rate than that of federal standards because they need seeds with a long shelf life. A
listing of the germination standards set for growers by Southern Exposure Seed Exchange and Garden
Medicinals and Culinaries is found in Appendix B.
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SEED TREATMENTS
The ideal seed treatment should be: (1) very effective against seed-borne pathogens, (2) relatively non-
toxic to animals and plants, even if misused, (3) effective for a long time during seed storage, (4) easy to use,
(5) acceptable according to the OMRI list of practices, and (6) economical. The hot water treatment method
meets many of these criteria. Though not as easy to use as chemical treatments, it can be more effective and
is non-toxic.
The purpose of the hot water treatment method is to submerse seed in water hot enough to kill the
pathogen without damaging the seed embryo. When properly done the procedure is very effective, but there
are some precautions to follow: (1) use only new, vigorous, high germination seed, (2) control the
temperature of the process very carefully, since this is fundamental to success, (3) test the germination of
the seed lot before and after treating the seeds, and (4) practice first on a small sample before treating a
large batch of seeds. Although the germination of properly treated seed will not be significantly affected, the
storage life of the seed may be shorter.
Equipment need for this process is as follows: (1) large metal pot and several smaller containers; (2) bags
for seed made of muslin, cotton, or nylon (panty hose will do); (3) an accurate thermometer from a science
supply house, and (4) sieve, colander, vegetable steamer, or hardware cloth cut in the same diameter as the
inside of the pot.
The procedure is as follows:
1. Place the seeds loosely in a fabric bag, filling no more than half full, and then tie securely. Add a
small weight, such as a bolt to help sink the seeds.
2. Set up the treatment container so that the bag of seed doesn’t touch the bottom of the pot.
There are several ways to keep the bag off the bottom: (a) suspend the seed bag on a vegetable
steamer, (b) set the bag in a colander, or (c), cut a circle of hardware cloth and suspend it at
least ½” off the bottom by using small stones.
3. Fill the treatment container about half full, and using an accurate thermometer, bring the water
to the treatment temperature.
4. Pre-soak the seeds in water, between 100 to 105
o
F (38 to 41
o
C) for five to ten minutes to pre-
warm the seeds.
5. Once the temperature in the treatment container is constant, add the bag of seed. While treating
the seed, stir the water slowly and constantly, while keeping an eye on the temperature and the
clock. An accurate thermometer is essential because temperatures one or two degrees too high
can injure the seed, and one or two degrees too low will fail to kill the pathogen. To help adjust
the temperature, cold or hot water can be added while stirring.
6. Remove the seed, and cool by dipping the bag briefly in lukewarm water, and then in cool water.
7. Open the bag and spread out the seed to dry on a hard surface or fine screen. Dry at room
temperature using a fan for supplemental ventilation, and stir the seeds several times daily until
dry.
For hot water treatment times for vegetable seed, see Appendix A.
GERMINATION TESTING
Basic procedure for doing your own tests:
Most growers prefer to sell seed wholesale and leave germination testing to the seed company that has
the equipment and supplies necessary to do accurate testing. You can perform your own germination tests
for the purpose of testing the germination of old seed lots, or testing new seed lots that you may want to
store for some period of time. Tests that you do on your own may not necessarily be legal for the seed trade,
but they will be accurate enough for your own use.
For supplies you will need blotters or thick paper towels, tweezers or forceps for picking up or placing
seed, zip lock bags, a misting bottle, and perhaps plastic boxes used for honeycomb (for light-dependent
germinators. As a substitute for a germination chamber you can place the seeds on top of a refrigerator or
hot water heater.
Seed Processing and Storage: Principles and Practices
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Version 1.6 July 21, 2010
20
To build a simple germination chamber, obtain an old student refrigerator (approximately two cubic feet
capacity) and remove the compressor. Alternatively, an insulated plywood box can be used as a chamber.
Cover any holes with caulk, duct tape, or other suitable material. For monitoring temperature, drill a hole in
the side or top to insert the probe of a thermometer. A maximum-minimum thermometer is recommended
because it records the temperature history and is useful for determining if the temperature has exceeded the
desired range. For a heat source, use a 40-watt bulb in a porcelain fixture and wire it in series with a
computer fan and dimmer switch to control the heating rate. The dimmer shouldn’t be run below 75% of
capacity because the computer fan may stall or not start up when the current is switched on. The electrical
supply cord from the chamber is connected to a timer to give a 16-hour warm period at 86
o
F (30
o
C) and an 8-
hour cool period (ambient room temperature).
To perform a simple germination test, count out 100 seeds and place them somewhat equidistant from
each other, usually not closer than ½”, spreading them out on the upper half of the moist blotter or paper
towel. The bottom half of the blotter is folded over the top, and then the towel is folded over with a fold
about every two to three inches and held in place loosely by a rubber band. The towels or blotters are then
placed vertically in plastic trays refrigerator containers, which are placed in the germination chamber or
suitable location. The trays or containers should be covered loosely with zip-lock or plastic bags to keep the
moisture in the towels, but they should not be completely closed because seeds need air. Towels are
inspected twice a day and misted as necessary. Most vegetable seeds germinate in about 7 to 14 days.
During the first count remove the seeds that have germinated and when the final count is done, the
remaining seeds are scored and the paper towel discarded. Seeds that require light are placed on blotter
paper within plastic honey boxes. The light from the germination chamber will be sufficient to satisfy the
light requirement. Some cool-weather crops such as lettuce should be germinated at room temperature.
Most varieties of lettuce will not germinate above 80
o
F (27
o
C). Although a test of 100 seeds will give you a
fairly reliable germination test, a test of 400 seeds will remove most of the statistical error and sampling
error in testing.
Testing by laboratories:
For an accurate germination test, you should contact your state seed-testing laboratory to find out what
is required in terms of fees (if any) and sample size for testing. Virginia requires a minimum 1000 seeds for
a test. State seed testing laboratories and seed companies follow a strict protocol for testing each type of
seed. Rules for testing seeds are available from the Association of Official Seed Analysts, Inc. (PMB #411,
1763 East University Blvd., Las Cruces, NM 88001). Their Web site is http://www.aosaseed.com.
GERMINATION ENHANCEMENT TECHNIQUES
There are several techniques that can be used to improve the germination of seed lots. These are
especially useful when it is necessary to bring a lot of seed above the Federal germination standard.
Mixing seed lots of the same variety:
The easiest way to enhance germination is to combine two lots of germination tested seed, one having a
high germination score and the other having a lower germination score. The two lots are thoroughly mixed
and the new germination percentage is calculated. For example, mixing 600 grams of 90% germination seed
with 400 grams of 70% germination seed yields 1000 grams of 82% germination seed. This technique might
be used by a seed company in an effort to meet market demand when the seed is in short supply or to avoid
discarding seed. If this technique is used, the seed lot with the lower germination should not be too low in
vigor or germination, otherwise there will be too much variability in the stand of plants produced from such
seed. This technique should not be used if one of the seed lots is significantly below Federal standard. The
other concern is that organic growers should make certain that both lots are certified organic. The mixed
seed is given a new lot identification number, but a record must be kept indicating the origin and
germination data of the original seed lots.
Additional seed processing:
Types of seed that have been cleaned using winnowing as part of the seed cleaning process are
candidates for these methods. Seeds of pumpkins, squash, melons, cucumbers, sunflowers, lettuce, and
flower seed can be aggressively re-winnowed to remove lighter, immature seed that is not well filled out.
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Version 1.6 July 21, 2010
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Such seed will be lighter than the good seed, which is denser and more resistant to being carried away by the
air current during the cleaning process.
Brief microwave treatment:
When seeds are subjected to a brief exposure of microwaves they respond with enhanced germination.
This treatment must be carefully controlled, is appropriate only for small seed lots, and is mainly used to
help rescue low germination seed, especially seed of endangered varieties. I have successfully used this
treatment on a lot of low-germination pepper seed, using an exposure time of 15 seconds to increase the
germination by about 25%. Longer treatment times were not attempted. This method is not recommended
for large seed lots because of the difficulty in obtaining equal microwave exposure in all areas of the treated
seed lot. The procedure involves placing a thin layer of seed in the center of the microwave oven and
microwaving for approximately 10 to 30 seconds depending on the size and water content of the seed.
Sunflower seed, when subjected to 30 seconds of microwave treatment, germinates faster than non-
microwaved seeds, but when microwaved for 60 seconds they fail to germinate. Microwave radiation
selectively heats the water within the seed. In addition, fats and oils are heated, though to a lesser extent
(Bloomfield, 2004). Fava beans microwaved for 30 seconds show a seven-fold increase in certain nutrients
(Randhir and Shetty, 2003). The heating process speeds up metabolism and there is some speculation that
the heating may affect the permeability of the lipid-containing membranes, thereby causing cellular changes
leading to enhanced germination. If the seed is heated too rapidly or too long, certain biological molecules
(proteins) such as enzymes are denatured, losing their enzymatic activity, and the seed is killed. Seeds that
have a low seed moisture content may need longer exposures than seeds with a high seed moisture content.
For most seed treatments the initial exposure should be anywhere from 10 to 15 seconds, with additional
trials at 10 or 15 second increments. If this procedure is used for germination enhancement, some
preliminary experiments should be done to determine the optimum exposure for germination enhancement
for a particular type of seed. The effect of microwaves on seed germination is not well documented, so
reports of any experiments are welcome and should be forwarded to the author. Because microwaving seed
constitutes a seed treatment, check with your organic certifier before planting the seed or using it in
commerce. In any case, microwaved seed needs to be retested after treatment.
Other techniques:
There are a number of other techniques that can be used to enhance seed germination. These include
use of seed coatings, sodium hypochlorite treatment, and in the case of dormant seeds use of scarification
and stratification. These techniques are beyond the scope of this publication, but it is worth noting that
certain recently harvested vegetables seeds, especially pepper seed, can exhibit dormancy or slow
germination. Post-harvest dormancy in most peppers can be overcome by curing the seed for at least six
weeks at a temperature of 70 to 80
o
F (21 to 27
o
C).
SEED STATISTICS
Information about seeds per ounce, sowing rates, germination data, and Federal germination standards
for vegetable crops are found in Appendix B. It is important to note that the measure of seeds per ounce,
and the sowing rate can be quite variable, depending on (1) variety characteristics; (2) pre-harvest growing
conditions, and (3), seed processing procedures. Some types of vegetables, notably squash, beans, corn and
beets, exhibit considerable variation in seed size that is genetically determined. Seed size is further
influenced by pre-harvest growing conditions. Considering that 95% of the dry weight of seed is storage
material, conditions during growth may influence the amount of storage material produced. Seed crops
harvested too early may have a higher percentage of immature seeds, which are smaller in size. Seed
processing procedures also affect seed size. For example, the screening and winnowing process may leave
behind a disproportionate number of large or small seeds depending on how the seeds were processed.
Special seed processing procedures such as de-fuzzing tomato seed will have a large effect on the seed count
per ounce.
Seed Processing and Storage: Principles and Practices
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Version 1.6 July 21, 2010
22
LABELING AND RECORD KEEPING
Labeling:
During the growing season, through planting, harvest, and processing it is essential that all crops be
properly labeled. This is especially important once the crop is harvested, as many seeds may look alike.
Labels should be on every container and should travel through every step of process and cleaning. While this
may seem obvious, it is very easy to think that you will remember what was in a particular container, but it is
surprisingly easy for several hours to slip by, and then to get confused about the identity of a particular seed
lot. Also, when seed is harvested at different times from the same variety, it is also useful to put a date on
the label. This can be important later on if there are questions about purity due to improper isolation,
disease, or other issues that arise retrospectively.
Record keeping:
You should keep a log of most everything you do with your crops, including information about planting
times and location for each variety, lot number and source of seed planted, germination percent or
comments about germination and vigor, number of plants transplanted (useful for calculating seed yields
later), maturity data, yield data, insect problems, organic fertilizers and amendments, and other data which
may become useful later. Your garden or farm log is also important for organic certification.
Detailed records should be kept in a card file or computer database regarding variety characteristics and
performance. Such data should include an accession number and information about seed source, lot
number, year last grown, maturity data, plant characteristics (growth habit, yield, productivity, color and
shape of fruit), disease resistance or susceptibility, and taste comparisons with other varieties. Record
keeping can be time consuming, so from the perspective of time management it is important to keep in mind
that the real value of information is information re-use.
SHIPPING SEEDS
Prior to shipment:
All seed must be thoroughly dry prior to shipment. Typically the minimum recommended time between
harvest and shipment is six weeks. During the last three weeks of this period, the seed should be stored in a
climate-controlled environment to allow the seed to cure. This means that seeds shipped in the late summer
should be cured in an air-conditioned environment, and seeds shipped in winter should be stored in a heated
environment prior to shipment.
Containers for shipping:
Small lots of seeds can be packed in ordinary envelopes, provided that all edges of the envelope are
sealed with tape, and the envelope placed inside a second envelope. The seams of the inner envelope must
be sealed so that seeds do not sift out of the envelope during shipment. Zip-lock bags are available in
various sizes, but it is best to use 4 mil thick bags. Zip-lock bags are fine to use, but if the bag is to hold
several ounces of seed, especially heavy or sharp seed (for example, cucumber seed) it is best to double bag
these. The zipped edge should be folded over and taped or banded to help prevent accidental opening of the
zipped edge. All air should be sucked out of the bags to avoid popping. The seeds should then be packed in
a cardboard box or a padded mailer.
Large seeds, such as peas, beans, and corn can be packed in grocery bags, but must be triple-bagged, and
taped securely to prevent breakage of the bag during shipment. The rule of thumb I use for shipping is that
the container and contents should be able to survive a ten-foot drop without damage to the container or
contents. This is about the approximate maximum distance a package might be thrown by a package
handler. Remember: if bags break during shipment it may be impossible to tell one variety from another and
the seed will be worthless. There should be enough padding inside the container so that there is no rattling
or shifting of contents. During the process of delivery, the packing material will become compacted.
Newspaper tends to compact too much, so stiffer materials such as crumpled grocery bags are
Seed Processing and Storage: Principles and Practices
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Version 1.6 July 21, 2010
23
recommended. One of the most enlightening lessons of running a mail-order business is opening a package
that has been shipped across country and back. That provides great feedback about your skill in packing.
Mark each seed container with your name and the variety name on a slip of paper, both inside the
container and on the outside of the seed container. Mark on the outside of the shipping package in large
letters "Perishable - Live Seeds Keep Cool and Dry." Include shipping address and a packing list with the
package. Address labels should always be covered with clear packing tape; otherwise the package conveyor
belt may snag the edge of the label and rip it off the package. All seams need to be securely taped.
Sometimes packages can be left outside in the rain by the delivery person, so you don’t want water entering
an open seam.
Appendix A: Hot water treatment times (in minutes) for vegetable seed.
Temperature of water treatment bath
Vegetable
118
120
122
125
Broccoli 20’
Brussel sprouts 25’
Cabbage 25’
Cabbage, Chinese 20’
Carrot 15’ – 20’
Cauliflower 20’
Celery, celeriac 30’
Collard 20’
Cucumber 20’
Eggplant 25’
Garlic (cloves) 20’
Kale 20’
Kohlrabi 20’
Lettuce 30’
Mustard 15’
Pepper 30’
Radish 15’
Rutabaga 20’
Spinach 25’
Tomato 25’
Turnip 20’
Note: There is some variation in the literature regarding the recommended treatment temperature
and times. This raises the question of whether the recommended temperature and time differences can
be explained by differences in elevation where the original research was performed. (For example, it
takes longer to boil an egg at 5,000 feet than it does at sea level.) If treatments are being performed at
high elevations, perhaps over 1,000 feet, it may be necessary to raise the treatment temperature by one
or two degrees, or increasing the treatment time (but testing may be required to check for damage to
seed treated at higher temperatures or greater times).
Seed Processing and Storage: Principles and Practices
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Version 1.6 July 21, 2010
24
APPENDIX B: Vegetable seed data, germination information, and federal standards
Germination
Vegetable
Seeds per ounce
a
Seeding rate
(lb/acre)
b
Time at
68 - 86oF
(days)
a
Temp.
range
opt.
o
F
c
Temp.
optimum
o
F
c
Federal
standard
(%)
d
Grower
standard
(%)
e
Asparagus 700 2 – 3 7 - 21 60 – 85 75 70 75
Bean, asparagus 225 20 – 40 5 - 8 75 80
Bean, broad 20 – 50 60 - 80 4 - 14 75 80
Bean, Lima bush 25 – 75 40 – 60 5 - 9 65 – 85 85 70 80
Bean, Lima pole 25 - 75 30 – 40 5 – 9 65 – 85 85 70 80
Bean, runner 25 - 30 50 - 70 5 – 9 75 80
Bean, snap bush 100 - 125 70 – 90 5 – 8 60 – 85 80 70 80
Bean, snap pole 100 - 125 20 – 40 5 – 8 60 – 85 80 70 80
Beet 1,600 6 - 10 3 – 14 55 – 85 85 65 75
Broccoli 9,000 ½ - 1-½ 3 – 10 75 80
Brussels sprouts 9,000 ½ - 1-½ 3 – 10 70 80
Cabbage 9,000 ½ - 1-½ 3 – 10 75 80
Cabbage, Chinese 9,000 1 - 2 3 – 7 75 80
Cardoon 700 4 - 5 7 – 21 60 70
Carrot 23,000 2 - 4 6 – 21 45 - 85 80 55 65
Cauliflower 9,000 ½ - 1-½ 3 – 10 45 – 85 80 75 80
Celeriac/celery 72,000 1 - 2 10 - 21 60 – 70 70 55 70
Chard, Swiss 1,600 6 - 8 3 – 14 50 – 85 85 65 75
Chicory 27,000 3 - 5 5 – 14 65 75
Collards 9,000 2 – 4 3 – 10 80 85
Corn, sweet 120 - 180 10 – 15 4 – 7 60 - 95 95 75 85
Cucumber 1,100 2 - 3 3 – 7 60 - 95 95 80 85
Dandelion 35,000 2 7 – 21 60 60
Eggplant 6,500 2 7 – 14 75 – 90 85 60 70
Endive 27,000 3 – 4 5 – 14 70 75
Fennel, Florence 7,000 3 60 60
Kale 9,000 2 – 4 3 – 10 75 80
Kohlrabi 9,000 3 – 5 3 – 10 75 80
Leek 11,000 4 6 – 14 60 70
Lettuce 25,000 1 – 3 7 40 - 80 75 80 85
Muskmelon 1,300 2 – 4 4 – 10 75 – 95 90 75 80
Mustard 15,000 3 – 5 3 – 7 75 80
New Zealand spinach 350 15 5 – 28 40 60
Okra 500 6 – 8 4 – 10 70 - 95 95 50 75
Onion 8,500 3 – 4 6 – 10 50 – 95 75 70 75
Parsley 18,500 3 – 4 11 – 28 50 – 85 75 60 65
Parsnip 12,000 3 – 5 6 – 28 50 – 70 65 60 70
Pea 90 – 175 90 – 220 5 – 8 40 – 75 75 80 85
Pepper 4,500 2 – 4 6 – 14 65 – 95 85 55 75
Pumpkin 100 - 300 1 4 – 7 70 – 95 95 75 80
Radish 2,500 10 – 20 4 – 6 45 – 90 80 75 80
Rutabaga 12,000 1 – 2 3 – 14 75 80
Salsify 1,900 8 – 10 5 – 10 75 80
Sorrel 30,000 2 – 3 3 – 14 65 75
Southern pea 225 20 – 40 5 – 8 75 80
Soybean 175 - 350 20 –40 5 – 8 75 80
Spinach 2,800 10 –15 7 – 21 45 – 75 70 60 70
Squash 120 - 400 2 – 6 4 – 7 70 - 95 95 75 80
Tomato 11,500 ½ - 1-½ 5 – 14 60 – 85 85 75 80
Turnip 15,000 1 – 2 3 – 7 60 –105 85 80 85
Watermelon 200 - 300 1 - 3 4 –14 70 - 95 95 70 80
References: a (Stefferud, 1961), b (Lorenz and Maynard, 1980), c (Harrington, 1954),
d (USDA Federal Seed Act, 1976), e Suggested minimum for growers (McCormack, J.H.).
Seed Processing and Storage: Principles and Practices
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Version 1.6 July 21, 2010
25
Appendix C: Basic seed cleaning equipment
The following is a list of basic and more specialized equipment for seed growers and seed savers. The
following items will suffice for just about any seed cleaning needs. Seed-cleaning screens should measure
12” x 12” outside dimension so that they can nest or be placed on top of each other to be used in
combination.
Various sizes of hardware cloth are used to make the most essential screens. The 1/8” and 3/8” sizes
are a little harder to find but some hardware stores carry these sizes.
Specialty seed cleaning screens can be ordered from specialty companies. See also the Supplies and
Equipment section. Specialty screens are noted with an asterisk in the table below.
Parts of electronic cabinets or housing are very useful. These are the cabinets on certain air-cooled or
ventilated electronic equipment. Parts of the cabinet may have round or oblong holes. Also various types of
strainers, grease-splatter screens, that keep oil from splattering from a frying pan, are useful. Visit the house
wares department of stores and antique shops where you’ll find an assortment of useful shapes and sizes.
Framed seed screens (12" x 12")
& seed cleaning equipment
Comments
Cloth-covered wood frame
Useful for cleaning beets, chard and other seeds which roll and leave
chaff behind
* Copper wire 20x22 Useful for lettuce and some herbs and other small seeds
* Copper wire 24x24 Useful for lettuce and some herbs and other small seeds
* Copper wire 4 x18 Useful for small seeds
* Copper wire 6x18 Useful for small seeds
* Copper wire 6x20 Useful for small seeds
Diamond metal lath 1/2" x 3/8"
Can be used in place of ¼” hardware cloth, for beans but hardware cloth
is best
Hardware cloth 1/2" Good all-purpose screen for large seeds for removing large rough chaff
Hardware cloth 3/8" Good all-purpose screen for large seeds for removing large rough chaff
Hardware cloth 1/4"
Good all-purpose screen for large seeds, especially beans, and pre-
screening
Hardware cloth 1/8" Good all-purpose screen for medium and small seeds. Very useful size
Round #12 (3/16") deep tray Electronic parts cabinet, good for sizing and cleaning beans and corn
* Round 10/64 Round holes are very useful.
* Round 5.5/64 Round holes are very useful.
Round electronic 3/16”
Electronic parts cabinet –very useful size for brassica and other small
round seeds
* Slotted 3x5/16 Good for oblong and long seed – very useful size
Window screen
Handy and common but not used much since other specialty screens
do the trick
Mixing bowls, set of various sizes, stainless steel
An assortment of 6 or more sizes from 4” to 16” diameter with
varying depth/slopes
Vacuum "Hoover"
Inexpensive hand-held models. Hose can be reversed to blow air.
Good for beans, corn, and cucurbits.
Grain mill with custom stand Excellent for processing hot pepper seeds.
Squeezo® strainer and accessories Can be used for processing tomato seed if you want to save the juice
Hair dryer
Just about any will do, look for at least two speed options and cool
or warm air only. Not necessary to remove heating element unless
there is only a hot air setting.
Fan or squirrel cage blower Fan useful for winnowing if a vacuum is not available.
Seed Processing and Storage: Principles and Practices
Copyright © 2004 – 2010 by Jeffrey H. McCormack, Ph.D.
Version 1.6 July 21, 2010
26
SOURCES OF SUPPLIES AND EQUIPMENT
The following list of suppliers is provided because some of the equipment and supplies mentioned in
this manual may be difficult to locate.
Germination papers and blotters:
Anchor Paper Company, 480 Broadway Street, St. Paul, MN 55101 (651) 298-1311
www.anchorpaper.com
Pollination bags:
Lawson Bag Company, P.O. Box 8577, Northfield, IL 60093 (847) 446-8812
Seed germination testing manuals:
Association of Official Seed Analysts, Inc., MB #411, 1763 East University Blvd., Las Cruces, NM 88001
www.aosaseed.com.
Seed screens:
Abundant Life Seed Foundation, P.O. Box 772, Port Townsend, WA 98368 (360) 385-5660
www.abundantlifeseed.com
Seedburo Equipment Company, 1022 W. Jackson Blvd., Chicago, IL 60607 (312) 738-3700)
www.seedburo.com
Seed saving containers, supplies and equipment:
Garden Medicinals and Culinaries, P.O. Box 320, Earlysville, VA 22936 (434) 964-9113
www.gardenmedicinals.com
Southern Exposure Seed Exchange, P.O. Box 460, Mineral, VA 23117 (540) 894-9480
www.southernexposure.com
SELECTED BIBLIOGRAPHY AND LITERATURE CITED
Bewley, J. D. and M. Black. 1985. Seeds: Physiology of Development and Germination. Plenum Press, NY.
Bloomfield, L.A. 2004. How Things Work: The Physics of Everyday Life. http://howthingswork.virginia.edu
(June 1)
Copeland, L. O. 1976. Principles of Seed Science and Technology. Burgess Publishing Co., Minneapolis, MN.
Deno, N. 1993. Seed Germination Theory and Practice (2
nd
ed.). (self-published) State College, PA.
Grabe, D. F. and D. Isely. 1969. “Seed storage in moisture-resistant packages” Seed World (Jan 24.) p. 4.
Harrington, J.F. 1954. Vegetable seed germination temperatures. Agricultural Extension leaflet, University of
California, Davis.
Harrington, J. F. 1960. “Drying, storing, and packaging seed to maintain germination, vigor” Seedman’s
Digest 11(1): 16 56-57, 64, 66, 68.
Harrington, J. F. 1972. Seed storage and longevity. In Kozlowski, T. T., Seed Biology 3: 145-245. NY.
Justin, O. L. and L. N. Bass. 1978. Principles and Practices of Seed Storage. Agriculture Handbook Number
506, U.S.D.A. Agricultural Research Service, Washington, DC.
Seed Processing and Storage: Principles and Practices
Copyright © 2004 – 2010 by Jeffrey H. McCormack, Ph.D.
Version 1.6 July 21, 2010
27
Logsdon, Gene. 1977 Small-Scale Grain Raising. Rodale Press, Emmaus, PA.
Lorenz, O. A. and D. N. Maynard. 1980. Knott’s Handbook for Vegetable Growers (2nd edition). John Wiley
& Sons, NY.
Navazio, J. 2003. “Seed-born diseases” (lecture handout). Organic Seed Alliance. Port Townsend, WA.
Randhir, R.O. and K. Shetty. 2003. Microwave-elicited pentose phosphate pathway for stimulation of L-
DOPA, phenolics and antioxidant activity in fava bean (Vicia faba) sprouts for Parkinson’s diet.
http://ift.confex.com/ift/2003/techprogram/paper_17111.htm.
Stefferud, A. (ed.) 1961. Seeds: The Yearbook of Agriculture. U.S. Department of Agriculture, Washington,
DC.
Vilmorin-Andrieux, M. M. 1856. The Vegetable Garden: Illustrations, Descriptions, and Culture. John Murray
(Publishers), Ltd., London.
Weber, L.E. et al. 1986. Amaranth Grain Production Guide. Rodale Research Center, Rodale Press, Kutztown,
PA.
CREDITS
Front matter illustration is from page 310 (Stefferud, 1961)
Seed Processing and Storage: Principles and Practices
Copyright © 2004 – 2010 by Jeffrey H. McCormack, Ph.D.
Version 1.6 July 21, 2010
28
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... This supports the findings of Schmidt (2007) who mentioned that seeds of tropical trees, stored into a low oxygen levels, reduce the rate at which their seeds deteriorate and age [23]. Moreover, McCormack (2004) opined that in general, storage for long or short term is improved under ambient humidity if the seed is well packaged. Safe storage of seeds depends primarily on its moisture percentage, temperature and storage duration [24]. ...
... Moreover, McCormack (2004) opined that in general, storage for long or short term is improved under ambient humidity if the seed is well packaged. Safe storage of seeds depends primarily on its moisture percentage, temperature and storage duration [24]. Sastryet al., (2007) reported that low moisture content reduces respiration and deterioration and thereby enhances the quality of stored seeds [25]. ...
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A study was carried out to determine the effects of storage materials and storage duration on physiological seed quality an important indigenous forest tree species (Sterculiarhinopetala). The experiment started from December, 2015 to June, 2016. Seed collection was done at the Bobiri Forest Reserve, Kumasi, Ghana. The experimental design used was 3 x 6 factorial arrangements in Completely Randomized Design (CRD) with three replications. Storage materials used were (jute, paper, nylon, airtight bottle, ziplock bag and no packaging) with three storage durations (0 day, 90 days and 180 days). Physiological and biochemical seed quality parameters were studied. Results revealed that germination percentage, seed vigour, 1000 seed weight, moisture content, carbohydrate, protein and oil contents were maintained when seeds were stored in airtight and ziplock bags for 90 days and 180 days without any deleterious effect on seed quality. The initial germination percentage (95.4%)had reduced slightly to 93.33% and 92.60% when stored in airtight bottle for 90 and 180 days respectively. Seeds packaged in jute, nylon and paper bags had significant reduction in germination percentage after 180 days of storage. The study concluded that storing seeds in airtight bottle and the ziplock materials improved storability than in jute, paper and nylon bags.
... Alternatively, meat grinder can also be used to cut the fruits into small pieces and the seeds collected by water extraction as reported by Tay (1989). The use of wet processing with a 24-hour fermentation period was suggested by McCormack (2004). The fermentation serves several purposes; it loosens bits of placenta and other debris from the seed coat, gives cleaner seed by breaking down plant material making the separation and washing easier and gives higher seed germination when the seed is properly washed. ...
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The effects of three seed extraction methods on seed quality characters in cayenne pepper genotypes were investigated at the central laboratory of the National Horticultural Research Institute (NIHORT) Ibadan. The study was carried out using completely randomized design (Factorial). Fresh, mature, red-ripe pepper fruits harvested from sixteen cayenne pepper genotypes were subjected to three extraction methods in two cropping years. Seeds obtained from each of the extraction process from each genotype were evaluated for seed quality characters. Seed output and quantity of seed damage were also determined. Data collected were subjected to analysis of variance and mean separation was carried out with Fisher's Least Significant Difference. Pearson correlation analysis was also carried out among the parameters. The result showed that seed quality parameters, seed output and quantity of damage seeds varied significantly among the cayenne pepper genotypes. Similarly, extraction methods impacted on the seed quality parameters, seed output and seed damage. Among the genotypes, Cayenne slim, Ata wewe and Bird eye 11 were of the best seed quality traits and good seed output. Seed extraction using maceration method gave the best seed quality traits, seed output and lowest seed damage. Quantity of damage seed in wet extraction (mortal/pestle) was 69.1% and 15.2 % higher than extraction by maceration and blender methods respectively. All seed quality traits positively and significantly correlate with one another. Seed output and seed damage also correlate positively and significantly with each other. Seed extraction by maceration method is therefore recommended if maximum seed quality, seed output and lowest seed damage is desired. However, further modification/improvement of wet extraction (mortal/pestle) method in order to ensure reduction in quantity of seed damaged, increase high seed output and improve physiological quality will enable easy adoption of the method to process large quantity of pepper seeds by resource limited seed producers
... In the study of McCormack (2004), low seed moisture content is a pre-requisite for long-term storage, and is the most important factor affecting longevity. Seeds lose viability and vigor during processing and storage mainly because of high seed moisture content. ...
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In the Philippines, native corn substitutes rice during rice scarcity which makes it the second most important staple crop. Japanese snail is considered an economically important mollusk pest but is known to address plant growth problems when used as amino acid fertilizer. The study compared seaweed residue, bamboo shoot residue, Japanese snail residue and their combination with organic pesticide, Guduchi, to commercially available compost fertilizer (10t/ha). These were done in three replications arranged in Randomized Complete Block Design. Photosynthetic parameters were measured using LI-6800 Portable Photosynthesis System. Application of Japanese snail residue improved height from 4th to 8th week by 67.53 cm. Bamboo shoot residue also showed effectiveness in improving the plant height on the 8th week at 139.13±6.28SD cm. On leaf length, Japanese snail residue outperformed the commercial basal organic fertilizer from the second up to the eighth week by 38.5 cm. On reproductive stage, Japanese snail showed the highest weight of ears at 86.6± 20.36SD g and is comparable with seaweed residue in the weight of kernels per ear at 69.60± 1.48SD g. However, in the computed yield, Japanese snail fertilizer has the lowest at 2.13±09SD g but the combination of pesticide and Japanese snail obtained the highest at 3.33±04SD g. Seaweed residue alone attained the highest transpiration rate and the lowest assimilation rate at 1604168655.67±191.66SD µmol H2O m-2 s-1 and 13.16±0.69SD µmol CO2 m-2 s-1, respectively while the combination of Japanese snail and Guduchi was the one that improved stomatal conductance to CO2 and the total conductance to CO2 at 478.6350±34.82SD μmol mol¯ and 47.5083±3.55SD Pa. Japanese snail residue has the lowest vapor pressure deficit at leaf temperature at 4.54±0.02SD kPa which showed significantly strong correlation with increase leaf length and plant height. Hence, Japanese snail residue fertilizer has the potential to be used as solid organic fertilizer for the growth of indigenous corn.
... Berdasarkan hasil uji perkecambahan, benih S. glutinosa dengan media dan waktu simpan yang berbeda menunjukkan bahwa benih S. glutinosa disarankan untuk disimpan pada media penyimpanan kertas, aluminium foil maupun botol kaca bersilika pada suhu ruang. Menurut(McCormack, 2004), kadar air benih yang terlalu tinggi (lebih dari 18 persen) akan menyebabkan benih kehilangan perkecambahannya. Sehingga mempertahankan kadar air benih dalam kondisi yang ideal sangat penting dilakukan. ...
... However, unfavourable storage conditions, particularly high temperature and relative humidity contribute to accelerating seed deterioration [10]. High relative humidity and temperature cause high moisture content in seeds and result in low germination at the end of storage [11]. Duration of seed storage can be another factor causing seed deterioration due to natural seed aging event that takes place. ...
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Okra (Abelmoschus esculentus) is an important crop grown mainly for its pods which are used either fresh, canned or dried and ground as powder. It is a good source of vitamins A, B, C and also rich in protein, minerals and iodine. However, conservation of okra seed for long term use especially for the purpose of crop improvement in Nigeria is still a big challenge. The objective of this study was to carry out preliminary investigation on the influence of storage conditions and duration on seed germination of okra seeds. Freshly harvested and processed seeds of three okra accessions (NGAE-96-011, NGAE-96-0060 and NGAE-96-0062-1) were packaged inside well-covered plastic containers and kept in three storage environments namely, ambient (control), short and medium term chambers. The laboratory experiment was conducted at the seed testing laboratory of NACGRAB between February and November 2015. One hundred seeds of each accession were subjected to germination test in three replicates at three-month intervals. The experiment was arranged in 3 x 3 x 3 factorial using completely randomized design (CRD). The results of analysis of variance (ANOVA) revealed that effects of accession and storage duration as well as interactive effect of storage conditions by duration were significant (P<.01) on germination of okra seeds indicating that there were differential germination response of okra seeds to accession and storage duration. Moreover, the significant interactive effect of storage conditions and duration indicates that duration of okra seed in storage and storage conditions should be given prime consideration in determining germination status of okra seeds in order to avoid wrong conclusion on okra seeds germination potential.
... Mechanized planting requires seed of a uniform size, which can be obtained by more precise sizing and grading techniques according to (Creemer etal., 1990). Cleaning methods divided into two depending on the types of crops according to (McCormack, 2004). ...
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Seed storage plays a vital role for the production of high yield, longevity, vigor, and quality. Mungbean seed decreased its viability if it is not stored properly. The respondents of the study considered 100 mungbean farmers that were randomly selected. Data were collected using a structured questionnaire and focus group discussion. The data revealed that farmers are still doin g manual harvesting and sun drying and majority of the respondents are using mechanical thresher. Findings also revealed that majority practiced the traditional way of storing seeds. The farmers must be provided well-suited methods or technology to assure quality and condition of the crop.
Chapter
The seed is a rudimentary embryonic form of a plant present in a capsulated form that has an ability to grow up into a mature plant. Also the seed is the basic unit of production in agriculture, and therefore, seed health is an important factor in agriculture. Moreover, seeds with superior potency make contributions almost to 30% of the excessive harvest. However, factors such as seed characteristics, pests, microbial contamination, geographical locations, moisture, temperature, and storage have a direct influence on seed viability, germinations, and quality. Therefore, it is important to monitor the series of the process starting from seed collection to seed testing and storage. In the absence of high-quality seeds, farmers continue to use their own seeds. Ideally, this retained produce cannot be substituted for high-quality seeds because it lacks genetic vigour and has poor germination. Thus, the availability of certified seeds of right varieties has become crucial. In this chapter, we elaborate about current agronomic practices, technological advancements in seed processing, treatment, qualitative parameters necessary for a healthy seed, seed storage practices, and testing parameters for certified seeds.
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Quality seed is the foundation block of vegetable production system. Proper post-harvest processing of seed is important to maximize yield, longevity, vigour, and overall quality. The engineering properties determination is the first and foremost step for the design of any seed processing, storage and handling machine or equipment. Engineering properties like geometric, gravimetric, frictional and aerodynamic properties of bottle gourd, sponge gourd, garden pea, and radish seeds have been determined at different levels of moisture content (7.5 % to 19 % dry basis). Results revealed a quadratic increase in all properties with moisture content except for gravimetric properties. The properties like sphericity (0.83), bulk density (710.61 kg/m3), true density (1109.79 kg/m3), angle of repose (26.41°), terminal velocity (10.13 m/s) and drag coefficient (0.92) were highest for garden pea seeds. Whereas, bottle gourd seeds exhibited maximum values for size (7.17 mm), projected area (115.68 mm2), porosity (53.4%) and coefficient of friction (0.727).
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