The Effects of Black and White Plastic Mulch on Soil Temperature and Yield of Crisphead Lettuce in Southern New England

Abstract

Abstract
Annually, Americans consume an average of 24.5 lb of lettuce (Lactuca sativa) per capita, more than half of which is head lettuce. This study examined the impacts of using black and white-on-black polyethylene mulches on three crisphead lettuce cultivars for spring production in the open field, with data collected on the soil temperature and lettuce yields. Black polyethylene, white-on-black polyethylene, and bare ground were compared for effects on soil temperature, lettuce yields, and lettuce head height and diameter. Mean soil temperatures at a 5 cm depth were 18.9 °C under black polyethylene, 17.7 °C under white-on-black polyethylene, and 17.1 °C in bare ground plots. Changes in the lettuce canopy size presented a similar trend over the growing season in all treatments. Both mulch type and cultivar significantly (P < 0.01) affected the canopy growth in head lettuce. Lettuce on black polyethylene mulch grew significantly (P < 0.01) faster than lettuce on white-on-black polyethylene or bare ground. However, the black and white-on-black mulches produced similar yields, averaging 5.76 and 5.71 kg·mˉ2, respectively. Meanwhile, bare ground plot yields were significantly (P < 0.01) lower at 4.57 kg·mˉ2. Cultivar rank order was consistent across treatments, and Crispino and Garmsir at 5.82 and 5.47 kg·mˉ2 fresh weight had significantly higher yields than Nevada at 4.75 kg·mˉ2 (P < 0.01).

Full text

The Effects of Black and White Plastic Mulch on
Soil Temperature and Yield of Crisphead
Lettuce in Southern New England
Rahmatallah Gheshm
1
and Rebecca Nelson Brown
1
ADDITIONAL INDEX WORDS. black polyethylene mulch, canopy growth, head size,
Lactuca sativa, white-on-black mulch
SUMMARY. Annually, Americans consume an average of 24.5 lb of lettuce (Lactuca
sativa) per capita, more than half of which is head lettuce. This study examined the
impacts of using black and white-on-black polyethylene mulches on three crisphead
lettuce cultivars for spring production in the open field, with data collected on the
soil temperature and lettuce yields. Black polyethylene, white-on-black poly-
ethylene, and bare ground were compared for effects on soil temperature, lettuce
yields, and lettuce head height and diameter. Mean soil temperatures at a 5 cm depth
were 18.9 C under black polyethylene, 17.7 C under white-on-black poly-
ethylene, and 17.1 C in bare ground plots. Changes in the lettuce canopy size
presented a similar trend over the growing season in all treatments. Both mulch type
and cultivar significantly (P<0.01) affected the canopy growth in head lettuce.
Lettuce on black polyethylene mulch grew significantly (P<0.01) faster than lettuce
on white-on-black polyethylene or bare ground. However, the black and white-on-
black mulches produced similar yields, averaging 5.76 and 5.71 kg

mˉ
2
, re-
spectively. Meanwhile, bare ground plot yields were significantly (P<0.01) lower at
4.57 kg

mˉ
2
. Cultivar rank order was consistent across treatments, and Crispino and
Garmsir at 5.82 and 5.47 kg

mˉ
2
fresh weight had significantly higher yields than
Nevada at 4.75 kg

mˉ
2
(P<0.01).
Lettuce (Lactuca sativa) is one
of the most important com-
mercial leafy vegetables in the
United States. Annually, Americans
eat an average of around 24.5 lb of
lettuce per capita, and more than 50%
of the lettuce consumed in 2018 was
head lettuce (12.3 out of 24.5 lb)
[U.S. Department of Agriculture
(USDA), 2019a]. In 2017, about
147,396 acres of crisphead lettuce
was grown in the United States, pri-
marily in California and Arizona
(USDA, 2019b). New England is
the national leader in per capita veg-
etable consumption (Produce for
Better Health Foundation, 2015).
Two important issues for consumers
are the freshness of vegetables and the
price (Martinez et al., 2010). Urban
areas of the northeastern United
States create a large market for let-
tuce, and local production can signif-
icantly reduce the carbon footprint of
supplying these markets (Plawecki
et al., 2014). The increasing cost of
shipping and consumers who value
quality and freshness have increased
demand for locally grown vegetables
(Bloom et al., 2018) and have made
lettuce a viable crop for the eastern
United States, despite higher costs of
production. The northeastern U.S.
region, which includes New England,
ranked second for both the number of
farms with commodity sales direct to
consumers and the value of sales
(USDA, 2016). In 2017, nearly 650
farms in New England reported
growing head lettuce, for a total of
almost 300 acres harvested (USDA,
2019b). These values likely underes-
timate total production, as many
small farms in New England grow
head lettuce in plantings of less than
one tenth of an acre as part of di-
versified vegetable production, and
underreporting is a known problem.
The crisphead lettuce market class
includes two subtypes, iceberg and
Batavia (Simko et al., 2014). The
iceberg subtype is grown by the Cal-
ifornia and Arizona lettuce industries,
and it dominates the wholesale mar-
ket in the United States (Simko et al.,
2014; USDA, 2004). However, most
crisphead lettuce grown for local mar-
kets in the United States, and for all
markets in other regions, is more like
the Batavia subtype (Ryder, 1999).
Lettuce is a cool-season vegeta-
ble crop with an optimal air temper-
ature of 23 C and an optimal root
temperature of 19 C (Fazil Elahi
et al., 2017). Daily mean air temper-
atures above 28 C can cause bitter-
ness, bolting, poor heading, and tip
burn of lettuce (Turini et al., 2011;
Whitaker et al., 1974), but damage
can be prevented if temperatures in
the root zone are near optimal (Jie
and Kong, 1998). In warm and hu-
mid environments, the full expression
of lettuce yield potential is not achiev-
able (Vargas et al., 2018). In general,
heading lettuces, such as crisphead,
are more susceptible to both tip burn
and frost injury than loose-leaf let-
tuces. Climate change is extending the
frost-free period in the northeastern
Units
To convert U.S. to SI,
multiply by U.S. unit SI unit
To convert SI to U.S.,
multiply by
0.4047 acre(s) ha 2.4711
0.3048 Ft m 3.2808
0.0929 ft
2
m
2
10.7639
0.0283 ft
3
m
3
35.3147
3.7854 gal L 0.2642
2.54 inch(es) cm 0.3937
16.3871 inch
3
cm
3
0.0610
0.0418 langley(s) MJm
–2
23.9006
0.4536 lb kg 2.2046
1.1209 lb/acre kgha
–1
0.8922
4.8824 lb/ft
2
kgm
–2
0.2048
0.0254 mil(s) mm 39.3701
305.1517 oz/ft
2
gm
–2
0.0033
(F – 32) O1.8 FC(C·1.8) + 32
Received for publication 16 June 2020. Accepted for
publication 6 Oct. 2020.
Published online 9 November 2020.
1
Department of Plant Sciences and Entomology,
University of Rhode Island, Kingston, RI 02881
We acknowledge the Rhode Island Agricultural Ex-
periment Station for financial support.
We thank Timothy Sherman for help with fieldwork.
R.G. is a Post doctoral Fellow.
R.N.B. is the corresponding author. E-mail: brownreb@
uri.edu.
This is an open access article distributed under the CC
BY-NC-ND license (https://creativecommons.org/
licenses/by-nc-nd/4.0/).
https://doi.org/10.21273/HORTTECH04674-20
•December 2020 30(6) 781

United States, lengthening the harvest
season for lettuce. However, pro-
longed periods of spring rains delay
planting and complicate the produc-
tion of spring head lettuce (Wolfe
et al., 2018). Head lettuces are the
slowest to mature (Lee et al., 2015),
increasing the risks that delayed spring
planting will result in exposure to
above-optimum temperatures before
the heads reach harvestable size. Risks
are increased in areas such as north-
eastern North America, where above-
optimum temperatures occur during
periods where daylength exceeds 13 h
(Rappaport and Wittwer, 1956). Cul-
tivars with reduced vigor, high head-
ing tendency, and tolerance to heat
stress are recommended (Waycott and
Ryder, 1992). Modern Imperial-type
iceberg lettuce cultivars were devel-
oped for the cool-climate production
areas of California and Arizona (Ryder,
1999) and are more susceptible to bolt-
ing and development of puffy heads
than the older Great Lakes- and Batavia-
type cultivars (Jenni and Yan, 2009).
Crisphead lettuces grown outside the
primary production areas are more like
the ‘Great Lakes’ and ‘Batavia’ types
than to the ‘Imperial’ type.
Lettuce is typically grown on
bare soil in New England, but adopt-
ing raised beds covered with plastic
mulch would allow farmers to create
beds in the fall for spring planting,
avoiding delays from spring rains.
Plastic mulches provide multiple ben-
efits, including serving as a physical
barrier against soil erosion, reduc-
ing water evaporation from the soil,
buffering against temperature fluc-
tuations, suppressing weeds, and im-
proving foliage quality (Lamont,
2005; Vargas et al., 2018). Mulch
films alter the flow of thermal energy
between the air and the rhizosphere
(Rosenberg et al., 1983; Tarara,
2000) By modifying the radiation
budget, plastic mulches directly affect
the rhizosphere microclimate. The
soil temperature under plastic mulch
depends on the optical properties of
the mulch material, and each material
has a different reflectivity and absorp-
tivity (Pramanik et al., 2015). Dark-
colored mulches will absorb more
solar radiation and emit longwave
radiation toward both the air and soil.
Plastic mulches, regardless of color,
warm the soil quickly and increase
early plant development in the colder
months. The soil under white and
reflective films remains cooler than
soil under black, dark, and clear plas-
tic films (Snyder et al., 2015). The
warming effects of mulch films are
beneficial in the spring when soil
temperatures are below optimum,
but these effects could be hazardous
to plant growth later in the season
(Snyder et al., 2015).
Plastic mulch has the potential to
improve yields and quality of spring
lettuce, and to facilitate early spring
planting; but choosing the type of
plastic mulch is complicated by the
need to balance the risks of below-
optimum temperatures early in the
season with those of above-optimum
temperatures during heading. The
objective of this research was to ex-
amine the effects of black and white
polyethylene mulches on root zone
temperatures and the growth of three
crisphead lettuce cultivars during the
spring production season in a north-
eastern U.S. coastal climate.
Materials and methods
The experiment was conducted
as a spring planting at the University
of Rhode Island’s Gardener Crops
Research Center in Kingston. The
field soil is Bridgehampton silt loam,
with a pH of 5.9 and 3.6% organic
matter. In the growing season, air
temperatures ranged from a minimum
of 6.9 C in May to a maximum of
26.6 C in June, with an average
daytime temperature of 22.7 C and
an average night temperature of
10.4 C. Average solar radiation was
19.5 MJm
–2
d
–1
. Cumulative solar
radiation over the 56 d of the exper-
iment was 1096 MJm
–2
. Precipita-
tion during the experimental period
was 7.1 inches. The experimental field
was irrigated four times, each time for
6 h. Total amount of irrigation was
18.5 m
3
, equivalent to an additional
6.7 inches of precipitation.
On 23 Mar. 2017, lettuce seeds
of three crisphead lettuce cultivars,
Nevada, Crispino, and Garmsir, were
seeded into 102-cell plug trays with
a cell volume of 22 cm
3
(Landmark
Plastics, Akron, OH) filled with
a bark- and peatmoss-based green-
house medium with starter nutrients
(Metromix 510; Sun Gro Horticul-
ture, Agawam, MA). ‘Nevada’ is
a green summer crisp or Batavian
lettuce, introduced by Vilmorin
(Paris, France) in 1991 and devel-
oped for heat tolerance (Holmes
et al., 2019) ‘Crispino’ was released
before 1985, probably in Europe
(Reppenhorst, 1986). It is sold as an
iceberg-type crisphead lettuce to
home gardeners and market farmers
in the United States, but it is not used
in the primary wholesale production
areas (Simko et al., 2014). ‘Crispino’
is described in seed catalogs as adapt-
able and well suited to climates that
are not ideal for the Imperial-type
cultivars. Seeds of ‘Nevada’ and
‘Crispino’ were obtained from
Johnny’s Selected Seeds (Albion,
ME). ‘Garmsir’ is a crisphead lettuce
adapted to arid climates; seeds were
obtained from the Falat Iran Com-
pany (Tehran, Iran). The planted
trays were kept for 10 d in the green-
house with a constant temperature of
25 C, then transplants were grown in
uncontrolled conditions in a high
tunnel for 30 d. On 8 May, plants
were transplanted into raised beds on
5-ft centers. Each bed had two rows
of lettuce 40 cm apart. In-row spac-
ing between plants was 30 cm (8.3
plants/m
2
). Fertilizers were broad-
cast and incorporated during bed
formation to provide 96 lb/acre ni-
trogen, 35 lb/acre phosphorous, and
58 lb/acre potassium using urea
(46N–0P–0K), potassium sulfate
(0N–0P–44.9K), and blended fertil-
izer (12N–10.5P–10.0K; Southern
States Cooperative, Richmond, VA).
Each bed had two lines of drip tape
with 12-inch emitter spacing and
a 0.25-gal/h flow rate (Chapin BTF;
Jain Irrigation, Watertown, NY),
spaced 10 inches apart beneath the
mulch. Standard 1.0-mil microem-
bossed black polyethylene mulch
and 1.5-mil polyethylene white-on-
black film (Berry Plastics, Evansville,
IN) were laid using a mulch layer
(2550; Rain-Flo Irrigation, East Earl,
PA). Sixteen data loggers with inte-
gral sensors (iButton; Maxim Inte-
grated, San Jose, CA) were placed
between plants in the center of main
plots at 2 inches below the soil sur-
face. Air temperature loggers (also
iButton) were installed in three dif-
ferent points of the field at a height of
10 inches above the bed surface and
shaded from direct sunlight. Air and
soil temperatures were recorded at
4-h intervals. Plants were watered
immediately after transplanting and
fertigated on 12 and 23 May 2017
with a water-soluble fertilizer (Aqua-
trols, Paulsboro, NJ) to provide a total
782 •December 2020 30(6)

of 25.3 lb/acre nitrogen, 9 lb/acre
phosphorus, and 18.9 lb/acre potas-
sium. Weeds in the bare ground plots
were controlled by hand on 13 and 24
May. Weeds in other treatments were
controlled by plastic mulch; alleys and
buffers were mulched with locally
sourcedagedhardwoodchips.Lettuce
was harvested on 5 July when heads
had fully formed (Fig. 1).
Our experimental design was
a split-plot with four replications.
Fig. 1. Three head lettuce cultivars (Nevada, Crispino, and Garmsir) grown as a spring-to-summer crop on raised beds covered
by black polyethylene mulch, white-on-black polyethylene mulch, and bare ground as a control in Rhode Island in Spring
2017. Top: Black plastic mulch (left), white-on-black plastic mulch (center), bare ground (right). Bottom: Overview of
experiment field. Photos were taken on 22 June 2017, 51 d after transplanting.
•December 2020 30(6) 783

Mulch treatments were applied as main
plots, and cultivars applied as the sub-
plots. The main plot treatments were
completely randomized. The main
plots were 6 m long; subplots were
2 m long. Raised bed width and height
were 0.75 and 0.2 m, respectively.
Themainplotswereseparatedbyan
unplanted buffer 60 cm long. The
main plot treatments were bare ground,
black polyethylene mulch, and white-
on-black polyethylene film with the
white layer facing upward.
Data were collected on soil tem-
perature, canopy growth, yield, head
height (vertical external diameter),
head diameter, and plant dry weight
(shoot only). Canopy growth was es-
timated using a variation of the
method of Bumgarner et al. (2012).
Every 10 d during vegetative growth,
digital images were taken using a white
quadrat (35 ·35 cm) placed around
two randomly selected plants in the
middle of each subplot. Images were
taken from a height of 60 cm above
the quadrat, perpendicular to the
ground. The percentage of the green
cover was calculated using Canopeo
(Patrignani and Ochsner, 2015) and
converted to an area based on the
quadrat area. The selected plants were
also used to determine dry weights.
Yield measurements were based on
theentiresubplot.Atharvest,five
trimmed heads were randomly se-
lected from each subplot and used
to determine head height and head
diameter. Height was measured from
the trimmed core base to the top of
the head, while diameter was mea-
sured perpendicular to the core at the
widest point. Measurements were
made on intact heads. To measure
dry weight, two heads per subplot
were cut into small pieces and then
dried in a forced-air dryer at 43 C.
Yield and plant size data were
analyzed using the analysis of variance
(ANOVA) and Duncan’s multiple
range tests in SAS (version 9.4; SAS
Institute, Cary, NC). Canopy growth
data were analyzed using repeated-
measures ANOVA (PROC MIXED)
and pairwise comparison of least
squares means. Soil temperature data
were analyzed as a completely ran-
domized design (main plots only)
and means separated with Tukey’s test.
Graphs were drawn in Microsoft Of-
fice Excel (version 2016; Microsoft
Corp., Redmond, WA) and Veusz
3.1 (Sanders, 2019).
Results and discussion
EFFECTS OF MULCHES ON SOIL
TEMPERATURES.The mean soil tem-
perature was 18.9 C in black poly-
ethylene mulched plots, which was
significantly (P<0.01) warmer than
soil temperature in plots mulched
with white-on-black polyethylene
(17.7 C) and bare ground plots
(17.1 C). Daily mean temperatures
ranged from 11.0 to 24.7 C under
black mulch and from 10.2 to 23.4 C
under white-on-black mulch. Bare
ground soil temperatures ranged
from 9.9 to 23.4 C. (Fig. 2) Mean
daily high temperatures were 21.3 C
for black mulch, 20.2 C for white-
on-black mulch, and 20.6 C for bare
ground. Mean daily low temperatures
were 16.5 C for black mulch,
15.5 C for white-on-black mulch,
and 15.1 C for bare ground. These
Fig. 2. Daily mean air and soil temperatures in head lettuce plots mulched with
black or white-on-black polyethylene film, and bare ground during Spring 2017.
Air temperature was measured at 10 inches (25.4 cm) above the bed surface. Soil
temperature was measured at 2 inches (5.1 cm) in depth. Temperature was
measured every 4 h from 8 May to 5 July; readings from each 24-h period were
averaged to calculate daily means. Lines represent the average of three plots.
Differentials between soil temperatures in the three treatments were calculated as
the difference between the grand mean across all treatments and the treatment
means for each date. Bands represent standard error for each treatment mean;
(1.8 ·C) D32 =F.
784 •December 2020 30(6)

values suggest that the mulches did
provide some degree of buffering,
although the differences are not sta-
tistically significant, and variation
around the means was homogeneous.
Many studies have found that black
plastic mulch increases soil tempera-
tures (Lamont, 2005), and it is widely
used to enhance the growth of heat-
loving crops. For crops such as head
lettuce, which are vulnerable to heat
stress, the white-on-black film is gen-
erally recommended to avoid harmful
soil warming (Snyder et al., 2015;
Vargas et al., 2018). However, Jenni
et al. (2003) found that infrared (IR)
transmitting mulch increased yields
over both bare ground and reflective
mulch in spring-to-summer head let-
tuce in Ontario, Canada. Lettuce
growth rate is highly correlated
with both accumulated growing de-
gree days (GDD) and accumulated
photosynthetically active radiation
(PAR), especially in the heading stage
where the growth rate is exponential
(Salomez and Hofman, 2007). How-
ever, the root zone temperature effect
in lettuce is also substantial because of
the soil temperature effects on assim-
ilation partitioning in addition to
water and nutrient uptake (Fazil Elahi
et al., 2017; Sun et al., 2016). Fazil
Elahi et al. (2017) demonstrated that
the optimal ambient root zone tem-
perature for head lettuce is 19 C,
similar to the average soil temperature
under black polyethylene mulch in
this experiment. While the average
temperature over the experiment as
a whole in the black mulch treatment
did not exceed the optimal tempera-
ture for lettuce, the 24-h average
exceeded the optimal temperature
on day 7 (Fig. 2). The effects of black
and white-on-black film on root zone
temperature is qualitatively similar,
but values of root zone temperature
achieved are lower for white-on-black
film (Fig. 2). White-on-black plastic
resulted in root zone temperatures
like those in bare ground plots until
24 June, when precipitation de-
creased the temperature of the bare
ground plots. Over the entire study,
nightime low temperatures tended to
be slightly warmer under white-on-
black mulch than in bare ground
plots, while daytime high tempera-
tures were slightly cooler.
EFFECTS OF MULCHES AND
CULTIVARS ON LETTUCE CANOPY SIZE.
Mulch type significantly (P<0.01)
affected canopy growth, and there
were significant (P<0.01) differences
in the growth rate between cultivars.
Pairwise comparison of least squares
means showed that lettuce on black
mulch grew significantly (P<0.01)
faster than lettuce on white-on-black
mulch or bare ground. White-on-
black mulch affected growth rate sim-
ilarly to bare ground (Fig. 3). Jenni
et al. (2003) similarly found that
crisphead lettuce grown on IR trans-
mitting mulch reached marketable
size in fewer days after transplanting
than the same cultivar grown on
white-on-black mulch or bare
ground. Mulch effects were consis-
tent across all cultivars. The cultivar
Nevada demonstrated significantly
slower growth (P<0.01) than Crisp-
ino or Garmsir, which were similar
(Fig. 4). At the last measurement on
27 June, the lettuce canopy covered
more than 74% of the soil surface in
black mulch plots. At the same time,
only 67% and 64% of the soil surface
was covered in the white-on-black
mulch and bare ground plots, respec-
tively. Higher soil temperature ap-
pears to have increased canopy
growth. Dufault et al. (2009) found
a similar connection between soil
temperature and growth rate in ro-
maine lettuce. Brault et al. (2002)
reported that coated paper mulches
and white-on-black polyethylene
mulch significantly increased growth
rate of head lettuce relative to the bare
soil control, and leaves started to cup
faster under mulched treatments. In-
terestingly, they found that all inor-
ganic mulches tested had this effect,
while we found that only black poly-
ethylene mulch increased growth
rate.
THE EFFECTS OF MULCHES AND
CULTIVARS ON LETTUCE YIELDS AND
HEAD SIZE.Both black mulch and
white-on-black mulch significantly
(P<0.01) increased lettuce yields in
Fig. 3. Change in canopy size of head lettuce over time for three treatments of soil
cover, including black plastic mulch, white-on-black plastic mulch, and bare
ground as a control, grown in the field in Kingston, RI during Spring 2017. Data
are averages across four replications. Means were significantly different (P<0.01).
Bands indicate 95% confidence intervals.
•December 2020 30(6) 785

comparison with bare ground (Table
1). Dry weight and head diameter
were unaffected by the mulch treat-
ments (Table 1). Jenni et al. (2003)
also found that IR transmitting and
white-on-black mulches increased
yields relative to bare ground; but in
their study, mulch did significantly
increase head diameter. Toth et al.
(2008) reported that marketable yields
of head lettuce grown on polyethylene
film were significantly greater than
yields of lettuce grown with hay, straw,
or paper mulches or in uncovered
soil. Using mulches and rowcovers on
lettuce increased yields in Iowa, but
plant dry weight was unaffected by
increasing the soil and air temperature
(Nair and Hovlovic, 2013). Brault
et al. (2002) reported that inorganic
mulches including plastics and de-
gradable paper mulches significantly
increased lettuce yield in comparison
with bare ground. Siwek et al. (2007)
reported that regardless of mulch
color, the marketable yield of lettuce
heads cultivated on mulch was signifi-
cantly higher than in the bare ground.
Our results suggest that the increases
in yield found in this and other studies
are not a result of increased average soil
temperatures, although increased soil
temperatures do correlate with in-
creased yields in romaine lettuce
(Gheshm and Brown, 2020). In-
creased yields may be due to increased
nutrient availability, particularly ni-
trate, as plastic mulches prevent leach-
ing following heavy rainfall, and
lettuce has been shown to increase
yields when nitrate leaching is reduced
(Verdial et al., 2001). Other possible
explanations for increased yield in
polyethylene-mulched plots include
reduced root stress from soil saturation
following heavy rainfall events, re-
duced soil compaction, and increased
carbon dioxide (CO
2
) levels in the
lettuce canopy because the imperme-
able plastic film causes all CO
2
released
from the soil to be vented through the
planting holes, directly into the canopy
(Lamont, 1993).
CULTIVAR EFFECTS.All cultivars
in this experiment were developed for
full-sized head production and re-
quired similar time to maturity.
ANOVA showed that ‘Crispino’ and
‘Garmsir’ yielded significantly more
than ‘Nevada’ [P<0.01 (Table 2)].
‘Garmsir’ yield did not differ signifi-
cantly from ‘Crispino’. Results also
showed that the dry weight and head
diameter did not differ among culti-
vars (Table 2).
Results obtained from the
ANOVA showed no significant inter-
actions between cultivars and mulch
for yield, dry weight, and head di-
ameter. However, head height (verti-
cal diameter) was affected by the
interaction between mulch and culti-
var. Mulch type had no effect on head
height in ‘Nevada’ and ‘Garmsir’, but
it significantly affected head height in
‘Crispino’, where heads from plants
grown in black mulch were signifi-
cantly taller than those from plants
grown in white-on-black mulch or
bare ground (Table 3). Cultivars did
not differ significantly in vertical
diameter of trimmed heads when
grown on either black or white-on-
black mulches, but Crispino heads
were significantly shorter than the
other cultivars when grown on bare
ground (Table 3). Head height in
Fig. 4. Change in canopy size, measured as covered soil percentage, over time for
three head lettuce cultivars (Nevada, Crispino, and Garmsir) grown in the field in
Kingston, RI during Spring 2017. Data are averages across four replications.
Means were significantly different (P<0.01). Bands indicate 95% confidence
intervals.
Table 1. The effect of color of polyethylene mulch on the yield, dry weight, and
external head diameter of crisphead lettuce grown in the field in Kingston, RI
during Spring 2017. Lettuce was planted at a density of 8.3 plants/m
2
(0.77
plant/ft
2
).
Mulch
Yield
(kg

m
L2
)
z
Dry wt
(g

m
L2
)
z
Head diam
(cm)
z
Black polyethylene mulch 5.76 a
y
170 a 18.78 a
White-on-black polyethylene mulch 5.71 a 150 a 19.22 a
Bare ground 4.57 b 160 a 19.33 a
z
1kgm
–2
= 0.2048 lb/ft
2
;1gm
–2
= 0.0033 oz/ft
2
; 1 cm = 0.3937 inch.
y
Different letters indicate significant difference at P<0.01 according to Duncan’s multiple range test.
786 •December 2020 30(6)

‘Crispino’ varied by 4.6 cm across
treatments, as compared with only
1.4 cm in ‘Nevada’ and ‘Garmsir’.
We did not directly measure stem
length inside the lettuce heads, but
changes in the rate of stem elongation
are the most likely causes of differ-
ences in vertical head diameter within
a single cultivar. The rate of stem
elongation in lettuce has been shown
to be very sensitive to root zone
temperature, with stem elongation
increasing as temperatures increase
(Al Said et al., 2018). This stem
elongation is independent of bolting,
and genotypes are known to differ in
response to temperature (Jenni and
Yan, 2009). Our results suggest that
rate of stem elongation in ‘Crispino’
may be more sensitive to changes in
root zone temperature than in the
other cultivars in this study.
Conclusions
Using polyethylene mulch sig-
nificantly increased lettuce yields rel-
ative to growing in bare ground.
Black mulch significantly increased
soil temperature, while temperatures
under white-on-black mulch were
like those in bare ground plots. How-
ever, the increased temperatures un-
der black mulch did not translate to
increased yields relative to white-on-
black mulch, suggesting that yield
increases were caused by some other
effect of the mulch. This study was
not designed to measure other ef-
fects, but one likely possibility is that
the plastic film prevented soils in the
lettuce root zone from becoming
saturated following rain events, re-
ducing root stress and nitrate leach-
ing (Lamont, 1993; Verdial et al.,
2001). The lack of harmful effects
on lettuce yield from polyethylene
mulch show that growing lettuce on
mulch-covered beds is a viable prac-
tice in New England. The use of
mulch would allow growers to pre-
pare beds in the late fall, facilitating
planting in the spring.
In this study polyethylene mulch
increased yields by just over 1 kgmˉ
2
relative to bare ground. Mulches also
eliminate the need for hand weeding,
reducing weed-control costs. Using
mulch increases bed preparation costs
by $0.08/m
2
for black polyethylene,
and $0.10/m
2
for white-on-black
polyethylene, but saves $0.36/m
2
in
weed control costs [estimate based on
hand cultivation]. Mulch removal and
disposal is estimated to cost $0.08/
m
2
(Chen et al., 2018). At a retail
price of $2/lb, using black mulch
would increase grower net profits by
$5.44/m
2
, while using white-on-
black would increase net profits by
$5.20/m
2
. Black plastic mulch would
be most profitable unless the grower
planned to follow the spring-lettuce
crop with a summer-to-fall cool-season
crop that would benefit from the soil
cooling provided by white-on-black
plastic mulch.
Heat tolerance is often a primary
consideration when selecting lettuce
cultivars for the northeastern United
States (Jenni and Yan, 2009). Bolting
is a major cause of yield loss. The lack
of mulch effect on head height in
Nevada and Garmsir suggests that
these cultivars are less likely to bolt.
Yield is a primary consideration when
selecting cultivars for all environ-
ments, and Crispino and Garmsir
yielded significantly more than
Nevada across all mulch treatments.
When all cultivar traits are considered,
Garmsir appears to be the best choice.
Unfortunately this Iranian cultivar is
not currently available in the United
States. Thus we recommend that
growers in the northeastern United
States use ‘Crispino’ in combination
with white-on-black mulch to maxi-
mize yields while minimizing stem
elongation.
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