Content uploaded by Glenn J. Evans
Author content
All content in this area was uploaded by Glenn J. Evans on Mar 11, 2016
Content may be subject to copyright.
Weed Management—Techniques
Herbicidal Effects of Vinegar and a Clove Oil Product on Redroot Pigweed
(Amaranthus retroflexus) and Velvetleaf (Abutilon theophrasti)
Glenn J. Evans, Robin R. Bellinder, and Martin C. Goffinet*
Weed management can be difficult and expensive in organic agricultural systems. Because of the potentially high cost of the
natural product herbicides vinegar and clove oil, their efficacy with regard to weed species growth stages needs to be
determined. A further objective was to identify anatomical and morphological features of redroot pigweed and velvetleaf
that influence the effectiveness of vinegar and clove oil. Research was conducted on greenhouse-grown cotyledon, two-leaf,
and four-leaf redroot pigweed and velvetleaf. Dose–response treatments for vinegar included 150-, 200-, 250-, and 300-
grain vinegar at 318 L/ha and at 636 L/ha. Clove oil treatments included 1.7, 3.4, 5.1, and 6.8% (v/v) dilutions of a clove
oil product in water (318 L/ha), and a 1.7% (v/v) dilution in 200-grain vinegar (318 L/ha). An untreated control was
included. Separate plantings of velvetleaf and pigweed were treated with vinegar or clove oil and were used to study
anatomical and morphological differences between the two species. Redroot pigweed was easier to control with both
products than velvetleaf. Whereas 200-grain vinegar applied at 636 L/ha provided 100% control (6 d after treatment
[DAT]) and mortality (9 DAT) of two-leaf redroot pigweed, this same treatment on two-leaf velvetleaf provided only 73%
control and 18% mortality. The obtuse leaf blade angle in velvetleaf moved product away from the shoot tip, whereas in
pigweed, the acute leaf blade angle, deep central leaf vein, and groove on the upper side of the leaf petiole facilitated
product movement toward the stem axis and shoot tip. For both species, and at all application timings, 150-grain vinegar
at 636 L/ha provided control equal to that of 300-grain vinegar at 318 L/ha. As growth stage advanced, control and
biomass reduction decreased and survival increased. Application timing will be critical to maximizing weed control with
vinegar and clove oil.
Nomenclature: Vinegar, acetic acid; redroot pigweed, Amaranthus retroflexus L.; velvetleaf, Abutilon theophrasti Medic.
Key words: Anatomy, leaf structure, natural product herbicides, organic, weed management.
Organic farmers rely on multiple management tactics that
include good husbandry and cultural, physical, and mechan-
ical techniques (Marshall 1992). Weed management can be a
difficult and expensive part of low-input agricultural systems
(Ryan et al. 2007; Walz 2004). Redroot pigweed and
velvetleaf are major annual broadleaf weeds in organically
grown vegetable crops. It might be possible to control these
weeds with vinegar or clove oil, two natural products.
A concentrated vinegar herbicide
1
has recently been
registered for noncrop uses (Susan Lewis, personal commu-
nication). Vinegar is made from aerobic bacterial oxidation of
ethanol in fermented grains, fruit juice, or nearly any other
liquid containing alcohol. Vinegar concentration is measured
by the percent acetic acid in the vinegar; for instance, 200-
grain vinegar contains 20% acetic acid. Clove oil is a distillate
of the herbaceous parts of the clove tree [Eugenia aromatica
(L.) Baill.] (Briozzo et al. 1989). One clove oil–based
herbicide
2
contains 34% (v/v) clove oil. The active compound
in clove oil is eugenol, a volatile phenol. Applications of both
vinegar and the clove oil herbicide would, similar to
conventional herbicides, require the use of protective
equipment (Anonymous 2008a,b).
Redroot pigweed is a principal weed in many row crops,
and its control at early growth stages is essential to preventing
significant yield reductions (Weaver 2001). Boydston (2004)
found that 200-grain vinegar applied at 187 and 374 L/ha to
cotyledon to two-leaf redroot pigweed and to Powell amaranth
(Amaranthus powellii S. Wats.) provided 99% control at 7 d
after treatment (DAT). Smooth pigweed (Amaranthus hybridus
L.) treated with a clove oil product at 66 and 131 L/ha was
controlled 73 to 83% (Curran et al. 2004).
Velvetleaf is a strong competitor; it is difficult to control,
and it is a problem in numerous row crops (Warwick and
Black 1988). Plants that emerge before, or with, the crop are
most detrimental to yield (Hock et al. 2005). Glacial acetic
acid applied at 234 L/ha to 15- to 20-cm velvetleaf, at
concentrations equivalent to 50-, 100-, and 200-grain vinegar,
provided 0, 24, and 59% control (2 DAT), respectively
(Doohan et al. 2005). Velvetleaf treated with 66 L/ha of clove
oil in an application volume of 562 L/ha was controlled 61 to
66% (7 DAT; Curran et al. 2005).
Anatomical and morphological differences in weeds could
influence the effectiveness of vinegar and clove oil. Optimal
postemergence applications should maximize the delivery of
product to leaves and shoot tips. Differential susceptibility can
be due to variations in spray retention and herbicide
absorption or to leaf surface characteristics such as cuticle
composition, leaf development and age, leaf position and
angle, and the number of trichomes and stomata (Anderson
1996; Bainard et al. 2006; Hess 1987; Hull et al. 1982;
McWhorter 1985; Wanamarta and Penner 1989).
DOI: 10.1614/WT-08-158.1
* First and second authors: Graduate Student and Professor, Department of
Horticulture, Cornell University, Ithaca, NY 14853; Third author: Senior
Research Associate, Department of Horticultural Sciences, New York State
Agricultural Experiment Station, Geneva, NY 14456. Corresponding author’s
E-mail: gje2@cornell.edu
Weed Technology 2009 23:292–299
292 N Weed Technology 23, April–June 2009
The volume of spray intercepted depends on leaf area and
leaf arrangement with respect to overlapping of leaves and leaf
angle (Radosevich et al. 1997). Horizontally oriented leaves
intercept and retain more spray than do upright leaves
(Radosevich et al. 1997; Rao 1999). Plants with a large
number of stomata or large-sized stomata, particularly on the
adaxial surface, could absorb more herbicide (Rao 1999).
Trichomes could restrict herbicide spread and retention
because spray droplets are held off the leaf surface by these
hairs (Hess 1987; Wanamarta and Penner 1989).
Efficacy levels for vinegar and clove oil need to be clarified,
and morphological factors that could contribute to differences in
natural product efficacy between species need to be identified.
Researchers have reported varying levels of weed control with
these products (Coffman et al. 2005; Curran et al. 2004;
Ferguson 2004; Georgis 2003; Radhakrishnan et al. 2002).
Characterizing the sensitivity of redroot pigweed and velvetleaf
to vinegar or clove oil is difficult because of variation in the
volumes applied and in the growth stages of the targeted weeds.
The study objectives were to determine effective product
concentrations and volumes of vinegar and clove oil on
multiple growth stages of these weeds and to describe
morphological and anatomical features of weeds that could
affect product effectiveness.
Materials and Methods
Dose–Response Studies. Studies were conducted in the fall
and winter of 2004 to 2005 and 2005 to 2006 at the
Guterman Greenhouse Complex in Ithaca, NY. Locally
collected seeds of redroot pigweed and velvetleaf were planted
in a standard soilless potting mix
3
in separate Styrofoam trays
measuring 30.5 by 10.2 by 6.4 cm deep. Approximately 50
seeds were sown in a single lengthwise row in each tray to
ensure simultaneous germination of at least 20 seeds per tray.
Three to five days after seedling emergence, trays were thinned
to 10 evenly spaced plants of uniform size. All trays of each
weed species were planted at the same time. Trays were kept
in greenhouses under supplemental sodium vapor lights at
16/8 h day/night, with an average midday light intensity of
1,000
mmol/m
2
/s photon flux. Daytime temperatures were
held between 21 and 29 C. Plants were watered and fertilized
routinely.
Experimental treatments included 150-, 200-, 250-, and
300-grain vinegar at 318 and 636 L/ha; 1.7, 3.4, 5.1, and
6.8% (v/v) of a clove oil–based product
2
in water (318 L/ha);
a 1.7% (v/v) dilution of the clove oil product in 200-grain
vinegar (318 L/ha); and an untreated control. All treatments
were applied to separate trays of cotyledon, two-leaf, and four-
leaf redroot pigweed and velvetleaf. After treatments had been
applied, the trays were not watered for 6 h. White distilled
vinegar
4
of a 200- and 300-grain strength and the clove oil
product were obtained directly from the manufacturers. The
200- and 300-grain vinegar treatments were applied undiluted,
and the remaining vinegar and clove oil treatments were
prepared by dilution with tap water. Vinegar treatments
included 0.1% yucca extract (v/v),
5
and each clove oil treatment
included 2.5% humasol (v/v).
6
These adjuvants were selected for
use on the basis of the results of a trial conducted in 2005 that
suggested that they improved the efficacy of these natural
products (unpublished preliminary data).
Treatments were broadcast with an air-driven, single-nozzle
greenhouse track sprayer
7
equipped with a flat fan nozzle.
8
The sprayer was calibrated to deliver 318 L/ha of product in a
single pass, with all four replications of a given treatment
timing and weed species treated in a single pass. Vinegar
treatments that were applied at 636 L/ha required two passes
of the sprayer; the second application was applied immedi-
ately after the first application.
Visual weed control ratings were taken 6 DAT. Control
was estimated on a scale of 0 (no injury) to 100% (plant
death). At 9 DAT, visual percent mortality ratings were taken
(on the basis of the presence or absence of new growth), and
all remaining biomass in each tray was cut at the soil surface,
oven dried, and weighed. The trial was repeated once in a
completely randomized design, and all treatments were
replicated four times for each species. S-plus statistical
software
9
was chosen for data analyses. Data were subjected
to ANOVA. Means were separated with Fisher’s Protected
LSD tests with significance values set at P # 0.05.
The percent dry weight reduction relative to the untreated
control was used as the response variable for linear regression
modeling. Predictors of the percent weight reduction included
treatment, species, growth stage, and study year. Residual
plots were used to assess model assumptions of linearity, equal
variance, and normality. The percent reduction in dry weight
was log transformed to improve model fit. Year was not a
significant factor within the full model (P 5 0.13); therefore,
data were combined across years. The final reduced model
included the effect of treatment, weed species, and weed size
on the log of the percent weed weight reduction. The reduced
model explained 97% (R
2
value) of the variance observed in
weed weight reduction.
Anatomical and Morphological Investigations. Work was
conducted in the spring of 2006 at the New York State
Agricultural Experiment Station in Geneva, NY. Velvetleaf
and redroot pigweed seeds were planted into a standard
soilless potting mix in separate square pots measuring 10 cm
wide and 15 cm deep. Four days after emergence, plants were
thinned to four to six evenly spaced plants per pot. Plants were
subjected to the same growing conditions as those described
for the prior study.
For each species, one pot was left untreated, one was treated
with 200-grain vinegar, and one was treated with 3.4% clove
oil; no adjuvants were included. Treatments were broadcast
over each weed species by the same track sprayer previously
described. The sprayer was calibrated to deliver 318 L/ha per
pass, with both species receiving a given treatment at the same
time. All plants were treated at the two-leaf stage. One day
after treatment, visibly injured leaves of pigweed (first true
leaf) and velvetleaf (cotyledon) were cross-sectioned by hand,
and the sections were mounted on slides in a drop of water
and viewed with a compound microscope. Cross sections were
selective samples of injured leaf sections, and not proportion-
ally related to the injury of the entire plant. Eight cross
sections of injured leaf areas were taken from each pot of each
species. Representative visual examples of cellular injury were
photographed.
Evans et al.: Vinegar and a clove oil product N 293
Separate plantings of velvetleaf and redroot pigweed were
grown to the four-leaf stage in the same size pots and under
the same growing conditions as the dose–response studies.
Individually marked leaf areas of each species were sprayed
with 200-grain vinegar (271 L/ha) or 3.4% clove oil (271 L/
ha) with a handheld spray bottle. The nozzle tip was held
13 cm above the targeted foliage and 30 cm to the side of the
foliage. From this single fixed position, the spray was spritzed
from the bottle twice over each pot to achieve a precalibrated leaf
application volume of 271 L/ha. This application strategy
allowed for even distribution of product on the targeted leaf
area. Entire leaf surfaces were viewed with a dissecting scope at 5-
min intervals between 0 and 120 min, without leaf removal from
the plant. Injury was visually documented as time progressed.
The outcome of the anatomical and morphological work
was not statistically quantified. The intent of this work was to
illustrate the influence of these two natural products on a
cellular and structural level and to highlight visually such
differences that were indicative of dissimilarities between the
control of redroot pigweed and velvetleaf.
Results and Discussion
Redroot Pigweed. Redroot pigweed control was greatest (83 to
100%) when treated at the two-leaf stage when vinegar was
applied at both 318 and 636 L/ha (Table 1). Pigweed at the
two-leaf stage had enough leaf surface area to maximize spray
interception without having multiple leaf layers to penetrate or
multiple meristematic regions to control. Vinegar concentration
influenced control at the 318 L/ha volume to a greater extent
than at the 636 L/ha volume. Pigweed control ranged from 84
to 100% when treated with 636 L/ha vinegar, regardless of
growth stage, whereas control at 318 L/ha was inconsistent,
ranging from 23 to 100% (Table 1). Curran et al. (2004) also
reported that smooth pigweed control increased when 200-grain
vinegar was applied at 561 rather than 281 L/ha.
Redroot pigweed mortality at 318 L/ha of vinegar was
greatest with 300-grain vinegar (96%). In contrast, only 1%
of the plants died when 150-grain vinegar was applied to
cotyledon-stage weeds (Table 1). Plant mortality increased
with the higher volume vinegar applications. No plants
survived the 636 L/ha treatments when applied at the two-leaf
stage. Targeting pigweed at this stage would be most
efficacious.
Redroot pigweed mortality was similar when 300-grain
vinegar was applied at 318 L/ha or when 150-grain vinegar
was applied at 636 L/ha (Table 1). Vinegar of 150 grain
applied at 636 L/ha was the same as 300-grain vinegar applied
at 318 L/ha (i.e., diluted to half strength by doubling the
water content). The use of a lower concentration vinegar
product would reduce safety and handling concerns. Weeds of
differing sizes are likely to be present in a field situation;
therefore, reliable control of pigweed with vinegar across a
range of sizes might necessitate applications at 636 L/ha.
Clove oil at a 3.4% dilution controlled redroot pigweed at
least 94% (Table 1). Clove oil at the 1.7% dilution was
marginally effective. When 200-grain vinegar was added to a
1.7% dilution of clove oil, pigweed control was 98 to 100%,6
DAT. Although this combination enhanced control relative to
200-grain vinegar (318 L/ha) or 1.7% clove oil applied alone,
there was no synergistic effect. By altering product concentration
and volume, both vinegar and clove oil can effectively control
redroot pigweed from the cotyledon to the four-leaf stage.
Velvetleaf. Velvetleaf was more difficult to control than
pigweed, regardless of treatment (Table 2). Control 6 DAT
with 318 L/ha vinegar was , 72%, and declined as
concentration decreased and as plant size increased. Increasing
vinegar application volume (636 L/ha) improved control to
71 to 86%. Radhakrishnan et al. (2002) found . 95%
control of velvetleaf when treated with 200-grain vinegar
applied to the point of run-off.
Table 1. Effect of vinegar concentration and volume and of clove oil concentration on greenhouse-grown redroot pigweed.
a,b,c
Treatment
Control (6 DAT) Mortality (9 DAT)
Reduction in dry weight relative to the
untreated control
Coty. 2lf. 4lf. Coty. 2lf. 4lf. Coty. 2lf. 4lf.
------------------------------------------------------------------------------------------------------------- % ------------------------------------------------------------------------------------------------------------
Untreated 0 0 0 0 0 0 — — —
150-grain vinegar, 318 L/ha 23 e 83 c 62 c 1 e 36 c 15 d 29.7 c 86.8 b 61.8 c
200-grain vinegar, 318 L/ha 61 d 96 b 82 b 13 de 81 b 59 c 77.0 b 98.0 a 89.1 a–c
250-grain vinegar, 318 L/ha 68 d 99 ab 94 a 19 d 95 a 86 ab 77.3 b 99.3 a 96.9 a
300-grain vinegar, 318 L/ha 81 c 100 a 94 a 46 c 96 a 83 ab 91.3 a 99.5 a 94.5 ab
150-grain vinegar, 636 L/ha 84 bc 100 a 96 a 51 c 100 a 89 ab 94.0 a 100 a 97.5 a
200-grain vinegar, 636 L/ha 92 ab 100 a 99 a 69 b 100 a 99 a 97.7 a 100 a 99.9 a
250-grain vinegar, 636 L/ha 99 a 100 a 99 a 94 a 100 a 99 a 99.7 a 100 a 99.2 a
300-grain vinegar, 636 L/ha 100 a 100 a 96 a 98 a 100 a 100 a 99.5 a 100 a 100 a
1.7% clove oil, 318 L/ha 85 bc 83 c 79 b 45 c 34 c 18 d 90.3 a 89.5 b 83.2 b
3.4% clove oil, 318 L/ha 96 a 98 ab 94 a 91 a 93 ab 56 c 99.0 a 99.2 a 92.8 ab
5.1% clove oil, 318 L/ha 99 a 100 a 96 a 98 a 100 a 75 bc 100 a 100 a 98.4 a
6.8% clove oil, 318 L/ha 100 a 100 a 96 a 99 a 100 a 73 bc 100 a 100 a 97.2 a
1.7% clove oil diluted in 200-grain vinegar, 318 L/ha 98 a 99 ab 100 a 93 a 96 a 100 a 98.5 a 99.1 a 100 a
Standard deviation 10 4 10 17 12 21 5.4 1.7 6.4
a
Means within columns followed by the same letter did not differ significantly (Fisher’s Protected LSD, P # 0.05).
b
Vinegar treatments included 0.1% (v/v) yucca extract, and clove oil treatments included 2.5% (v/v) humasol.
c
Abbreviations: DAT, days after treatment; Coty., cotyledon-stage application; 2lf., two-leaf stage application; 4lf., four-leaf stage application.
294 N Weed Technology 23, April–June 2009
A large percentage of velvetleaf plants regrew, particularly
when the treatments were applied at the four-leaf stage.
Mortality was 8% or less in these treatments (Table 2). In the
low-volume vinegar treatments, mortality did not increase
notably when plants were treated at the cotyledon or two-leaf
stage relative to application at the four-leaf stage. Increasing
vinegar volume to 636 L/ha increased velvetleaf mortality,
although in all cases, more than half of the plants regrew. Thus,
velvetleaf control with vinegar is unlikely to be successful.
Biomass reduction with vinegar was greatest when velvetleaf
was sprayed at the cotyledon stage, with up to a 90% reduction
in dry weight relative to the control (Table 2). Vinegar applied
at the four-leaf stage gave 48 to 76% dry weight reduction across
all treatments. Curran et al. (2005) also found that by the time
velvetleaf reached 11 cm, biomass reduction 7 DAT with 200-
grain vinegar (562 L/ha) was , 29%.
Clove oil at a 1.7% dilution was also ineffective on
velvetleaf (33 to 48% control and , 5% mortality; Table 2).
Clove oil at 3.4, 5.1, and 6.8% dilutions provided increased
control (60 to 95%). In each case, control declined visibly as
plant size increased. Curran et al. (2005) found comparably
poor control of 9- to 13-cm velvetleaf 7 DAT with both 200-
grain vinegar (, 47%) and 66 L/ha clove oil (, 66%).
Marginal control of velvetleaf will require high concentra-
tions and volumes of vinegar and clove oil, even with
cotyledon-stage applications. A high percentage of regrowth
will occur. Sufficient velvetleaf control will require early-stage
applications, repeat applications, or the integration of other
strategies to reduce velvetleaf competition.
Defining Weed Control Potential. Redroot pigweed was more
susceptible to vinegar and clove oil than velvetleaf. However, for
both species, control and mortality decreased and dry weights
increased with increased plant size at the time of application.
Miller and Libbey (2004) also noted a decline in the efficacy of
vinegar on weeds having greater than three leaves. Timing with
regard to product application is critical for maximizing weed
control. Because all weeds were grown under greenhouse
conditions, the tested plants might not have been as hardened
(having thinner leaves, stems, and cuticles) as field plants might
be. Therefore, the greenhouse-grown weeds could have been
more susceptible to control by these products.
Increasing vinegar from 318 L/ha to 636 L/ha improved
weed control. Curran et al. (2004) likewise found better weed
control with 561 L/ha applications of 200-grain vinegar and
clove oil relative to applications at 281 L/ha. In contrast,
Boydston (2004) did not observe an increase in the visual
control of field-grown pigweed when 200-grain vinegar was
applied at 374 L/ha, as opposed to 187 L/ha. At the lower
volumes Boydston tested, visual differences in weed control
between volumes might have been less apparent.
Vinegar concentration and volume appear to be flexible
determinants of weed control potential where an increase in
concentration might allow a reduction in application volume
and where a decrease in concentration could be compensated for
by an increase in volume. This might fail to work at vinegar
concentrations so low that they are unable to penetrate leaf
surfaces, or at volumes so low that coverage becomes inadequate.
Weed density might also affect control. Plants in the
greenhouse were spaced at a constant density of 10 plants per
30.5-cm linear row. In the field, spray coverage to crowded
weed flushes might not penetrate or cover weeds as well,
leading to poorer control. An uneven seedbed, with clods or
rocks on the surface, could limit spray contact of some weeds
(Bond and Grundy 2001). Weeds missed or minimally
injured by this effect would need to be controlled by other
methods. Neither vinegar nor clove oil has any pre-emergence
or residual weed control effects.
Erratic germination of weeds in the field environment can
lead to different weed sizes and species at any given time,
Table 2. Effect of vinegar concentration and volume and of clove oil concentration on greenhouse-grown velvetleaf.
a,b,c
Treatment
Control (6 DAT) Mortality (9 DAT)
Reduction in dry weight relative to the
untreated control
Coty. 2lf. 4lf. Coty. 2lf. 4lf. Coty. 2lf. 4lf.
---------------------------------------------------------------------------------------------------------------------- % --------------------------------------------------------------------------------------------------------------------
Untreated 0 0 0 0 0 0 — — —
150-grain vinegar, 318 L/ha 53 f 37 g 51 c 0 f 0 e 0 c 66.0 f 39.4 bc 47.6 g
200-grain vinegar, 318 L/ha 69 e 48 f 59 bc 0 f 1 e 0 c 78.2 e 59.3 ab 60.8 f
250-grain vinegar, 318 L/ha 72 de 56 ef 68 ab 6 ef 4 e 1 bc 80.4 de 60.3 ab 66.3 de
300-grain vinegar, 318 L/ha 70 de 64 de 69 ab 4 ef 8 de 0 c 75.7 e 69.1 ab 61.8 ef
150-grain vinegar, 636 L/ha 74 de 72 cd 71 ab 13 de 16 cde 3 abc 80.2 de 72.1 ab 66.9 de
200-grain vinegar, 636 L/ha 78 cd 73 bcd 75 a 13 de 18 cde 8 a 80.2 de 70.4 ab 73.1 bc
250-grain vinegar, 636 L/ha 79 cd 81 abc 77 a 20 cd 34 abc 5 abc 80.2 de 81.4 a 71.4bc
300-grain vinegar, 636 L/ha 86 abc 86 a 79 a 49 b 39 ab 6 ab 90.2 ab 86.1 a 76.4 ab
1.7% clove oil, 318 L/ha 45 f 48 f 33 d 4 ef 3 e 0 c 61.9 f 56.5 ab 28.2 h
3.4% clove oil, 318 L/ha 84 bc 72 cd 60 bc 14 cde 8 de 0 c 84.5 cd 82.0 a 49.9 g
5.1% clove oil, 318 L/ha 93 ab 82 ab 72 ab 41b 16 cde 0 c 89.7 abc 89.6 a 75.4 ab
6.8% clove oil, 318 L/ha 95 a 88 a 81 a 64 a 44 a 6 ab 93.5 a 91.6 a 80.2 a
1.7% clove oil diluted in 200-grain vinegar,
318 L/ha 86 bc 84 a 77 a 24 c 24 bcd 6 ab 85.2 bcd 87.4 a 67.7 cd
Standard deviation 9 10 14 11 19 6 2.7 19.0 2.6
a
Means within columns followed by the same letter did not differ significantly (Fisher’s Protected LSD, P # 0.05).
b
Vinegar treatments included 0.1% (v/v) yucca extract, and clove oil treatments included 2.5% (v/v) humasol.
c
Abbreviations: DAT, days after treatment; Coty., cotyledon-stage application; 2lf., two-leaf stage application; 4lf., four-leaf stage application.
Evans et al.: Vinegar and a clove oil product N 295
Figure 1. (Top) redroot pigweed leaf area showing leaf venation and angle, and a magnification of the few hairs present on a pigweed shoot tip. (Bottom) Velvetleaf leaf
area showing leaf venation and angle, and a magnification of the many hairs covering the shoot tip.
296 N Weed Technology 23, April–June 2009
which compounds the issue of application timing. Multiple
applications would be ideal, although product cost, crop
injury effects, and continual weed flushes over the growing
season limit such use. Vinegar, in an unregistered form, is
comparatively less expensive than clove oil. Product costs
could be considerably reduced with banded or spot spray
applications, or by integrating product usage with cultivation,
plastic mulch or other weed management strategies. Vinegar
and clove oil, when timed appropriately and applied over
susceptible weed species at an adequate volume, do have the
potential for use as natural product herbicides.
Leaf Surface Observations. Time-lapse observations from 0 to
120 min after treatment of velvetleaf and redroot pigweed leaves
with vinegar and clove oil indicated morphological differences
between the two species that could affect control. Redroot
pigweed has alternately placed, simple, ovate leaves. Leaf veins
are deeply cut into the adaxial leaf surface, with hairs only on the
abaxial side. Leaves of younger plants generally angle toward the
plant axis (Figure 1). Velvetleaf has cordate, alternately placed
leaves, with each leaf having a tapered apex and fine-toothed
margins. Hairs on adaxial and abaxial surfaces are sometimes
gland-tipped and star-shaped. Leaf blades generally angle away
and downward from the plant’s central axis (Figure 1).
Leaf angle, leaf vein pattern and petiole morphology can
influence movement of liquids across leaf surfaces. An ideal
application of vinegar or clove oil would spread uniformly over
the leaf surfaces and contact shoot apices. When a seedling-stage
weed has a single shoot apex, penetration of product close
enough to damage that apex will kill the weed. Product
applications that contact only the larger leaf surfaces will burn
foliage and reduce plant vigor, although regrowth from the
shoot apex or axillary buds is probable. When either vinegar or
clove oil is applied to the leaf surface, leaf blade angle determines
the direction of spray solution movement. With pigweed, an
acute blade angle allows excess product to flow toward the
central shoot apex. Additionally, a deep central leaf vein and a
channel that runs down the adaxial side of the petiole facilitate
product movement directly to the shoot apex and axillary buds
(Figure 1). The outcome is increased injury of these meriste-
matic regions (Figure 2), leading to increased mortality.
Velvetleaf has an obtuse leaf blade angle and leaves are
subject to diurnal movements that make that angle increas-
ingly negative. Product moving off the leaves would be shed
away from the shoot apex and axillary buds. Velvetleaf leaf
venation is complex and netted, minimizing any channeling
effect and further reducing foliar movement. Also, leaves of
velvetleaf often shield the shoot apex to some degree. Thus,
minimal product is deposited on the shoot apex, and despite
injury to outer or upper leaf surfaces, shoot regrowth is
possible at even high application volumes (Figure 2). The
volume of spray that contacts leaf surfaces might also be
reduced as leaf angle becomes more obtuse. Peterson and Al-
r
Figure 2. (Top) injury to a redroot pigweed plant treated with 200-grain vinegar
(636 L/ha) at the four-leaf stage, 3 d after treatment. The apical meristem has
been killed (the plant will not regrow). (Bottom) Injury to velvetleaf plants treated
with 200-grain vinegar (636 L/ha) at the cotyledon stage, 3 d after treatment.
New leaf growth is occurring from the shoot tip.
Evans et al.: Vinegar and a clove oil product N 297
Khatib (1999) found that Palmer amaranth (Amaranthus
palmeri L.) and velvetleaf control was greater with glyphosate
applied at 10:00 A.M., 1:30 P.M., and 5:30 P.M. than when
applied at 6:00 A.M. and 9:00 P.M. One of the attributing
reasons for this finding was diurnal leaf movement; the leaf
angle of both weeds was most negative at the 6:00 A.M. and
9:00 P.M. applications.
Product deposited on the youngest emerging leaves could be
affected by leaf hairs. Magnification of a velvetleaf shoot tip seen
from above shows hairs over the entire surface (Figure 1).
Mature velvetleaf leaves have star-shaped trichomes. In contrast,
the shoot tip of pigweed has only a few fine hairs (Figure 1), and
mature leaves generally do not have hairs on the adaxial surface.
Spray droplets contacting large trichomes can shatter and
bounce off (Boize et al. 1976). Leaf hairs also reduce wetting and
spread of droplets (Hull et al. 1982). Hess et al. (1974)
suggested that closely spaced trichomes could create air pockets
beneath droplets, which would prevent leaf surface contact. Leaf
hair interference might explain the reduced natural product
efficacy in velvetleaf, particularly in limiting apical injury.
Anatomical Observations. Cellular damage was obvious in
both species 1 DAT. Cellular injury to pigweed leaves is
shown in Figure 3. Vinegar damage was widespread through-
out the leaf tissues, with cellular contents lysed, including the
upper and lower epidermal cells. Cellular damage was also
evident in the clove oil treatment, with the most injury
appearing in the mesophyll cells. These cells appeared as
darkened areas in cross sections where cell contents and turgor
had been lost and the cells compacted together. Epidermal
cells appeared less affected by clove oil than by vinegar. The
differential permeability of these products could explain
differences in injury symptomology.
Vinegar caused the pigments contained in the abaxial
epidermal cells of pigweed to diffuse out as cell membranes
ruptured (Figure 3). Amaranthus pigments are water-soluble
betacyanins with a reddish-violet color (Cai et al. 1998;
Turakhozhaev et al. 1998). Being water soluble, dispersal of
these pigments could have increased with penetration of the
water-based solvent, vinegar. When treated with clove oil, the
abaxial cells of pigweed did not generally lyse (Figure 3).
Betacyanin remained in the epidermal cells, although it
appeared brighter and more condensed than that within
untreated epidermal cells. This could be a result of decreased
cell size (loss of turgor) forcing the condensation of pigment.
Anatomical and morphological differences between weeds
affect natural product effectiveness. Effective vinegar and clove
oil applications will require adequate surface coverage. Once
plant size increases, a greater number of apices and a denser
leaf canopy will reduce coverage and product effectiveness.
For weeds with diurnal leaf movement, spraying at times
when the leaf angles are least negative might increase bud
injury. With the high initial cost of these products and the
selective nature of their control, minimizing cost and deciding
r
Figure 3. Magnified redroot pigweed leaf cross sections from top to bottom:
untreated; 1 DAT with 3.4% clove oil (318 L/ha); and 1 DAT with 200-grain
vinegar (318 L/ha). Adaxial surfaces are to the top of each photo; abaxial surfaces
to the bottom.
298 N Weed Technology 23, April–June 2009
between the use of these products and tillage, flame weeding,
hand-weeding, or another weed control method might be
determined by the weed composition of the field.
Sources of Materials
1
WeedPharm, Pharm Solutions Inc., 2023 E. Sims Way, Suite
358, Port Townsend, WA 98368.
2
Matran IIH, EcoSMART Technologies Inc., 318 Seaboard
Lane, Suite 208, Franklin, TN 37067.
3
Cornell Mix, a soilless potting mixture including peat moss,
vermiculite, and nutrients, Cornell University, Ithaca, NY.
4
Vinegar, Fleischmann’s Vinegar Co. Inc., 12604 Hiddencreek
Way, Suite A, Cerritos, CA 90703.
5
Yucca extract, Pharm Solutions Inc., 2023 E. Sims Way, Suite
358, Port Townsend, WA 98368.
6
Humasol, Agricare Inc., P.O. Box 399, Amity, OR 97101.
7
Allen Track Sprayer, Allen Machine Works, 607 East Miller
Road, Midland, MI 48640.
8
Teejet 8003VS spray nozzle, Teejet Spraying Systems Co., P.O.
Box 7900, Wheaton, IL 60189-7900.
9
S-PLUS Version 7.0. 2005. Insightful Corp., Seattle, WA.
Literature Cited
Anderson, W. P. 1996. Weed Science: Principles and Applications. 3rd ed. St
Paul, MN: West Publishing. Pp. 108–124.
Anonymous. 2008a. Matran ECH Herbicide MSDS. http://www.biconet.com/
lawn/infosheets/MatranMSDS.pdf. Accessed: January 26, 2009.
Anonymous. 2008b. Weed PharmH Herbicide MSDS. http://pharmsolutions.
com/dealerAssets/MSDS/MSDS_Weed_Pharm_07.pdf. Accessed: January 26,
2009.
Bainard, L. D., M. B. Isman, and M. K. Upadhyaya. 2006. Phytotoxicity of clove
oil and its primary constituent eugenol and the role of leaf epicuticular wax in
the susceptibility to these essential oils. Weed Sci. 54:833–837.
Boize, L., C. Gudin, and C. Purdue. 1976. The influence of leaf surface
roughness on the spreading of oil spray drops. Ann. Appl. Biol. 84:205–211.
Bond, W. and A. C. Grundy. 2001. Non-chemical weed management in organic
farming systems. Weed Res. 41:385.
Boydston, R. 2004. Weed Control in Potato with 200 Grain Vinegar Applied
Postemergence. Prosser, WA: U.S. Department of Agriculture Agricultural
Research Service Laboratory Research Summary. Pp. 1–3.
Briozzo, J., L. Nunez, L. J. Chirife, L. Herszage, and M. D’Aquino. 1989.
Antimicrobial activity of clove oil dispersed in a concentrated sugar solution. J.
Appl. Bacteriol. 66:69–75.
Cai, Y., M. Sun, H. Wu, R. Huang, and H. Corke. 1998. Characterization and
quantification of betacyanin pigments from diverse Amaranthus species. J.
Agric. Food Chem. 46:2063–2070.
Coffman, C. B., J. Radhakrishnan, and J. R. Teasdale. 2005. Corn and soybean
responses to basal applications of vinegar. Proc. Northeast. Weed Sci. Soc.
59:79. [Abstract]
Curran, W. S., D. D. Lingenfelter, and C. B. Muse. 2004. Vinegar and clove oil
for non-selective control of annual weeds. Proc. Northeast. Weed Sci. Soc.
58:21. [Abstract]
Curran, W. S., D. D. Lingenfelter, and C. B. Muse. 2005. Effectiveness of
vinegar and clove oil for control of annual weeds. [Poster] University Park, PA:
Penn State University, http://weeds.cas.psu.edu/pdf/WSSA05poster.pdf. Ac-
cessed: January 26, 2009.
Doohan, D., J. Felix, and T. Koch. 2005. Weed control with acetic acid and
surfactants in organic vegetables. Pages 269–273 in Weed Management in
Horticultural Crops. Columbus: Ohio State University Extension Horticulture
and Crop Series 745.
Ferguson, J. J. 2004. Evaluation of organic herbicides. Hortscience 39:876.
Georgis, R. 2003. Efficacy and speed of control of Matran 2 with humasol and
stylet oil against eight weed species under field conditions. Elmhurst, IL:
AgroSci Advanced Agricultural Technologies Rep. 866. Pp. 2–6.
Hess, F. D. 1987. Relationship of plant morphology to herbicide application and
absorption. Pages 19–35 in C. G. McWhorter and M. R. Gebhardt, eds.
Methods of Applying Herbicides. Chapter 3. Monograph Series of WSSA
Number 4. Champaign, IL: Weed Science Society of America.
Hess, F. D., D. E. Bayer, and R. H. Falk. 1974. Herbicide dispersal patterns as a
function of leaf surface. Weed Sci. 22:394–401.
Hock, S. M., S. Z. Knezevic, A. R. Martin, and J. L. Lindquist. 2005. Influence
of soybean row width and velvetleaf emergence time on velvetleaf (Abutilon
theophrasti). Weed Sci. 53:160–165.
Hull, H. M., D. G. Davis, and G. E. Stoltenberg. 1982. Actions of adjuvants on
plant surfaces. Pages 26–67 in Adjuvants for Herbicides. Lawrence, KS: Weed
Science Society of America.
Marshall, T. 1992. Weed control in organic farming systems. Pages 311–314 in
Proceedings of the First International Weed Control Congress. Melbourne,
Australia: Weed Science Society of Victoria.
McWhorter, C. G. 1985. The physiological effects of adjuvants on plants. Pages
141–158 in S. O. Duke, ed. Weed Physiology. Volume II. Herbicide
Physiology. Boca Raton, FL: CRC.
Miller, T. and C. Libbey. 2004. Effect of postemergence organic products.
Mount Vernon, WA: Washington State University Northwestern Washington
Research and Extension Center. Pp. 1–2.
Peterson, D. E. and K. Al-Khatib. 1999. The influence of application time of day
on glyphosate efficacy. Proc. North Central Weed Sci. Soc. 54:17–18.
Radhakrishnan, J., J. R. Teasdale, and C. B. Coffman. 2002. Vinegar as a
potential herbicide for organic agriculture. Proc. Northeast. Weed Sci. Soc.
56:100. [Abstract]
Radosevich, S. R., J. S. Holt, and C. Ghersa. 1997. Weed Ecology. New York:
Wiley. Pp. 413–424.
Rao, V. S. 1999. Principles of Weed Science. Enfield, NH: Science Publishers.
Pp. 63–65.
Ryan, M., S. Duh, D. Wilson, and P. Hepperly. 2007. The Skinny on a Big
Problem … Weeds. http://www.newfarm.org/depts/weeds/features/1007/
survey.shtml. Accessed: April 3, 2009.
Turakhozhaev, M. T., M. A. Khodzhaeva, T. D. Kasymova, and M. A.
Abduazimov. 1998. Pigment components of Amaranthaceae. Chem. Natur.
Compd. 34:42–43.
Walz, E. 2004. Final Results of the Fourth Biennial National Organic Farmers’
Survey. Santa Cruz, CA: Organic Farming Research Foundation. 69 p.
Wanamarta, G. and D. Penner. 1989. Foliar absorption of herbicides. Rev. Weed
Sci. 4:215–231.
Warwick, S. I. and L. D. Black. 1988. The biology of Canadian weeds: Abutilon
theophrasti. Can. J. Plant Sci. 68:1069–1085.
Weaver, S. 2001. Pigweeds (Redroot, Green and Smooth). OMAF Factsheet. http://
www.omafra.gov.on.ca/english/crops/facts/01-009. Accessed: April 6,2007.
Received October 29, 2008, and approved February 27, 2009.
Evans et al.: Vinegar and a clove oil product N
299
