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HORTSCIENCE 52(12):1700–1706. 2017. doi: 10.21273/HORTSCI12422-17
Plastic Biodegradable Mulches Reduce
Weeds and Promote Crop Growth
in Day-neutral Strawberry in
Western Washington
Lisa W. DeVetter
1,5
, Huan Zhang
2
, Shuresh Ghimire
2
, Sean Watkinson
3
,
and Carol A. Miles
4
Department of Horticulture, Northwestern Washington Research and
Extension Center, Washington State University, 16650 State Route 536,
Mount Vernon, WA 98273
Additional index words. Fragaria 3ananassa, plasticulture, plastic mulch, paper mulch,
growth analysis
Abstract. Day-neutral strawberry (Fragaria 3ananassa) is typically grown in plasticul-
ture production systems that use black polyethylene (PE) mulch for weed management
and promotion of crop growth and yield. The objectives of this research were to evaluate
several commercial plastic and paper biodegradable mulch (BDM) products [Bio360,
Experimental Prototype (Exp. Prototype), and WeedGuardPlus] in comparison with
standard black PE mulch and bare ground cultivation in day-neutral strawberry grown
in an annual system in northwestern Washington. Mulch performance [as percent visual
cover (PVC)], weed suppression, marketable yield, plant biomass, and fruit quality were
evaluated in ‘Albion’ and ‘Seascape’ strawberry grown in 2014 and 2015. PVC measured
at the end of the production season was lowest for the Exp. Prototype (8%) in 2014 and
was greatest for Bio360 (90%), WeedGuardPlus (90%), and PE (98%). In 2015, PVC at
the end of the production season was again lowest for Exp. Prototype (62%), followed by
WeedGuardPlus (64%), Bio360 (93%), and PE mulch (97%). Overall, weed pressure was
higher in 2015 relative to 2014 and was greatest in the bare ground treatment in both
years of the study. By the end of the 2015 season, weed cover in the bare ground treatment
was 95%, followed by WeedGuardPlus (50%), Exp. Prototype (34%), PE (25%), and
Bio360 (15%). Yield showed year and cultivar effects and was higher in mulched
treatments. Plant biomass showed varying effects; root biomass was lowest in ‘Seascape’
in 2015 under the bare ground treatment and greatest under Bio360, which was similar to
PE mulch and WeedGuardPlus. Leaf biomass was lowest in the bare ground treatment
and highest in mulched treatments (except in 2015, when leaf biomass was intermediate
for plants grown with WeedGuardPlus). Crown biomass showed a similar trend and was
overall greater for plants grown in mulched treatments except for Bio360 in 2014, which
was the same as the bare ground treatment. Overall, fruit quality was maintained among
strawberry grown with BDMs, with soluble solids concentration (SSC, %) and titratable
acidity (TA) being the only variables to show treatment effects. SCC tended to be lower in
fruit from bare ground plots. TA was different for ‘Seascape’ in 2015 with fruit from
bare ground and Exp. Prototype treatments having higher TA than the PE treatment.
This study demonstrates that BDMs can be comparable to PE mulch in terms of
performance and impacts on crop productivity in day-neutral strawberry, suggesting
that BDMs could be a viable alternative to PE mulch for strawberry growers in the
Pacific Northwest.
Total U.S. strawberry (Fragaria ·ananassa)
production was estimated to be 1.59 million t
from 21,245 ha in 2016 [National Agricul-
tural Statistics Service (NASS), 2017]. About
79% of the U.S. strawberry industry is
concentrated in California, which predomi-
nately uses remontant/perpetual-flowering
(i.e., day neutral) cultivars grown in annual
plasticulture systems (California Strawberry
Commission, 2017). Strawberry production
in the Pacific Northwest (PNW), specifically
Oregon and Washington, ranks fourth and
fifth in national strawberry production, gen-
erating 5233 and 3810 t in 2016, respectively
(NASS, 2017). The PNW strawberry industry
traditionally produced fruit for processing
and grew June-bearing cultivars in matted-
row production systems without plastic
mulch. This regional industry is currently
undergoing a transition as acreage and
production of processing strawberry declines
because processors are increasingly buying
cheaper strawberries from California. Con-
sumption and value of fresh-market day-
neutral strawberry is increasing, however,
with the value of fresh market strawberry
production in Washington and Oregon in-
creasing 43% between the years 2000 and
2016 (NASS, 2001, 2017). Consequently,
many PNW growers are exploring plasticul-
ture production using day-neutral cultivars
targeting the fresh market.
Black PE mulch is extensively used in
plasticulture because of its low cost and
ability to manage weeds, conserve soil mois-
ture, modify soil temperatures, increase crop
yields and quality, and promote on-farm
profitability (Fernandez et al., 2001; Freeman
and Gnayem, 2005; Garwood, 1998; Lament,
1993; Miles et al., 2012). These benefits
extend to both conventional and organic
production systems, as PE mulches may be
used for weed management in organic agri-
culture as long as the mulch is completely
removed from the field once the growing or
harvest season is complete [§ 205.206
(United States Department of Agriculture;
USDA, 2014a) and § 205.601 (b)(2)(i–ii)
(USDA, 2014b)]. Presently, plastic BDMs
are not allowed in certified organic produc-
tion, but may be a tool to enhance sustain-
ability in nonorganic production systems
(Miles et al., 2017).
Despite the horticultural and economic
benefits of PE mulch, removal, and disposal
imposes both financial and environmental
problems. Removal and disposal of PE
mulch can be costly (Galinato and Walters,
2012; Galinato et al., 2012; Ghimire and
Miles, 2016; Lucas et al., 2008) and was
estimated to be $1100 per hectare in
strawberry systems in western Washington.
These costs are expected to increase as the
cost of labor increases (R. Sakuma, per-
sonal communication). To avoid these
costs, some growers resort to stockpiling,
landfilling, burying, or burning removed
mulches, which causes hazards to the envi-
ronment and human-health (Garthe and
Kowal, 1993; Hakkarainen and Albertsson,
2004; Levitan, 2005). While mulch recy-
cling is available in some regions, it is
limited in the PNW and this adds to
growers’ disposal costs for transport and
cleaning of plastic mulch (G. Jones, per-
sonal communication). The significant
transportation and labor costs needed to
recycle PE mulch impede the adoption of
plastic mulch recycling and the overall
sustainability of plasticulture specialty
crop production.
BDMs may minimize some of the eco-
nomic, environmental, and human health
impacts associated with PE mulches and their
associated disposal (Kasirajan and Ngouajio,
2012; Miles et al., 2017). These materials are
manufactured from feedstocks derived from
fossil fuels plus natural materials (e.g., starch
polysaccharides and cellulose, up to 20% of
the BDM) (Jamshidian et al., 2010; Miles
et al., 2017). BDMs are engineered to completely
Received for publication 22 Aug. 2017. Accepted
for publication 10 Oct. 2017.
This work was funded by Washington State Uni-
versity’s Emerging Research Issues program.
We gratefully acknowledge the assistance of Curtis
Faustich, Rachel Rudolph, Rachel Weber, Matt
Arrington, and China Moss for project assistance.
1
Assistant Professor.
2
Graduate Research Assistants.
3
Scientific Assistant.
4
Professor.
5
Corresponding author. E-mail: lisa.devetter@wsu.
edu.
1700 HORTSCIENCE VOL. 52(12) DECEMBER 2017
biodegrade within 2 years, with 90% of their
mass released as CO
2
and water and the
remaining 10% residing in the soil as micro-
bial biomass (according to International Or-
ganization for Standardization 17556 and
ASTM D5988). BDMs are applied using the
same field equipment as PE mulch and are
designed to be functionally similar to PE
mulch. BDMs have undergone extensive
testing in vegetable production systems.
Depending on the specific product, BDMs
have been found to completely deteriorate
within soils after 13 months of incorporation
and produce yields comparable to crops
grown with PE mulch (Cowan et al., 2013;
Haapala et al., 2014; Li et al., 2014; Miles
et al., 2012).
BDMs have not been tested widely in
strawberry and could be a suitable alternative
to PE mulch in expanding plasticulture pro-
duction systems. Bilck et al. (2010) found
white and black BDMs made from blends of
cassava (Manihot esculenta) starch and poly-
butylene adipate-co-terephthalate (PBAT)
maintained yield and berry quality relative
to PE mulch in short-day ‘Ventana’ straw-
berry grown in Brazil. Mechanical properties
measured 8 weeks after application showed
BDMs had reduced tensile strength and
elongation at break, whereas film rigidity
was higher relative to PE mulch. However,
the BDM films were found to provide
adequate mulch functionality in terms of
groundcover and maintenance of yield in
this production system. Yield and fruit qual-
ity were similar to PE mulch in short-day
‘Honor’ and ‘Camarosa’ strawberry grown
with five BDMs made from Mater-Bi
(Novamont S.p.A, Novara, Italy) in Portugal
(Costa et al., 2014). BDMs have also been
studied as a tool to suppress weeds and
enhance establishment of short-day ‘Jewel’
and ‘Honeoye’ strawberry grown in a peren-
nial matted-row system (Weber, 2003).
Black polymer (IP40 Black) and paper
mulch (Planters paper; Ken-Bar, Inc.,
Reading, MA) were effective at reducing
weeds, but limited runner production neces-
sary for matted-row establishment.
Modification of soil temperature by
mulches is an important aspect of strawberry
production systems. White-on-black BDMs
M2 and M3 (Mater-Bi) and M1 (Biomind;
Polivouga, Albergaria-a-Velha, Portugal) in
autumn–winter strawberry production
showed 70%, 86%, and 20% soil coverage,
respectively, at the end of the crop cycle,
whereas PE mulch had 100% soil coverage
(Andrade et al., 2014). Soil covered with
these BDMs had 0.02 to 2.78 C higher soil
temperatures in the summer period at 15 cm
depth than soils covered with PE mulch,
which the authors of the study inferred was
a key contributing factor to the reduced yields
observed across all BDM-treated plots rela-
tive to PE. In other studies, soil treated with
black BDMs (Ecoflex, BASF, Florham Park,
NJ) overall showed a slightly lower soil
temperature than low-density PE (LDPE,
Pliant Corp., Schaumburg, IL) mulch at
a depth of 1 cm in the soil (Ngouajio et al.,
2008). These studies show that different
BDMs have different effects on soil temper-
ature; the impact of BDMs on soil tempera-
ture will be an important factor in how well
BDMs fit in strawberry production.
The objectives of this research were to
evaluate several commercial plastic and pa-
per BDM products and to compare them with
standard black PE mulch and bare ground
cultivation in day-neutral strawberry grown
in an annual system in western Washington.
Mulch performance, including deterioration
and weed suppression ability, as well as
impacts on plant growth, yield, and fruit
quality were measured to assess mulch per-
formance and suitability for commercial pro-
duction systems.
Materials and Methods
Plot establishment and maintenance. The
study was conducted at the Washington State
University (WSU) Northwestern Washington
Research and Extension Center in Mount
Vernon, WA (lat. 4826#28.9$N, long.
12223#44.1$W). Soil is a field silt loam,
characterized as a mixed, nonacid, and mesic
Aquic Xerofluvent (USDA, 2017). The exper-
imental design was a randomized complete
block split-plot with five mulch treatments and
two strawberry cultivars replicated four times.
The main plot treatment consisted of three
BDMs, PE mulch, and bare ground (Table 1).
The subplot treatments consisted of day-
neutral strawberry cultivars, Albion and Sea-
scape. The experiment was carried out in2014
andrepeatedin2015inanadjacentfield(soil
type was still a field silt loam). Main plot
treatments were hand-applied to 6.1 m-long
raised bed plots in May 2014 and June 2015.
Raised beds were formed using a mechanical
bed shaper (Rain-Flo 2600; Rain-Flo Irriga-
tion LLC., East Earl, PN). Resultant beds
were 0.61 m wide, 0.25 m tall, spaced 4.3 m
apart on the center; plots within a row were
separated by 0.9 m. Before mulch applica-
tion, pressure-compensating drip tape (20 cm
emitter spacing, 1.3 LPH, Aqua-Traxx; Toro,
Bloomington, MN) was centered on the
raised beds. Tensiometers (Irrometer, River-
side, CA) were installed in the third replicate
(row) of the bare ground and PE treatments
at depths of 30.5 and 45.7 cm and were used
to schedule irrigation. All plots were irrigated
when tensiometers averaged –30 kpa (Hoashi-
Erhardt and Walters, 2014).
After mulch application in May 2014 and
June 2015, bare-root ‘Albion’ and ‘Seascape’
strawberries were planted in all treatments in
holes that were punctured with a 41-cm long
dibble bulb and seed planter (DeWit Tools,
TDI Brands, Jasper, IN). Resultant holes were
7.6 cm in diameter and 15.2 cm deep. The
dibble was found to tear the paper mulch, so
a knife with a 10-cm long blade was first used
to cut an ‘‘X’’ shape in the mulch, which
created four flaps that were folded and the
dibble was subsequently used to create the
planting holes. Planting holes were arranged
in staggered double rows, with 25.4 cm
between each twin row and 30 cm between
plants within a row. Each plot contained 38
plants total, with 19 plants each of the cultivars
Albion and Seascape. Any plants that failedto
grow within 2 weeks after planting were
replaced. All runners were removed from
plants throughout the duration of the experi-
ment and blossoms were removed for 6 weeks
before being allowed to form fruit.
The site was managed according to the
recommended guidelines for day-neutral
strawberry grown in western Washington
(Hoashi-Erhardt and Walters, 2014). A cus-
tom blend preplant fertilizer (6N–8.7P–
16.6K; Wilbur-Elis Co., Burlington, WA)
was broadcast at a rate of 20 kg·ha
–1
N2
weeks before bed formation and planting in
both years of the study. Supplemental fertil-
izer (20N–8.7P–16.6K; Plant Marvel Labo-
ratories, Inc., Chicago Heights, IL) dissolved
in water and injected through the irrigation
system was provided beginning in July of
both years at a rate of 5.6 kg·ha
–1
N per week.
On 24 July 2015, 56 kg·ha
–1
of MgSO
4
(Magriculture; Premier Magnesia, LLC.,
Waynesville, NC) was applied through ferti-
gation, and on 3 and 10 Aug. 2015, foliar
applications of 8% CaCl
2
(Phyta-Cal QC; CA
Organic Fertilizers, Inc., Hanford, CA) were
applied at 4.7 L·ha
–1
. Applications of MgSO
4
and CaCl
2
were made based on visual obser-
vations of symptoms indicating these nutri-
ents were lacking and production guidelines
(Hoashi-Erhardt and Walters, 2014).
Data collection. Soil temperature was
recorded (HOBO U12 logger; Onset Com-
puter, Bourne, MA) every 15 min throughout
the growing season in 2014 and 2015. Probes
Table 1. Mulch treatments evaluated in ‘Albion’ and ‘Seascape’ strawberry grown in Mount Vernon, WA,
2014–15.
Mulch product Company Composition
z
Bio360 Dubois Agrinovation, Saint-Remi,
Quebec, Canada
Mater-Bi + PBAT; biodegradable
and compostable; black; 20 mm
WeedGuardPlus Sunshine Paper Co. LLC, Aurora,
CO
Cellulosic; biodegradable and
compostable; maroon; 230 mm
Experimental prototype
y
Custom Bioplastics, Burlington,
WA
TPS and PHA; biodegradable and
compostable; black; 20 mm
Polyethylene Poly Expert, Laval, Quebec,
Canada
Standard agricultural PE mulch;
nondegradable; black; 20 mm
z
PBAT = polybutylene adipate terephthalate; TPS = thermoplastic starch; PHA = polyhydroxyalkanoate;
PE = polyethylene.
y
Exp. Prototype was reformulated to provide longer coverage in 2015; the specifics of the reformulation
were not disclosed, as this was proprietary information of the mulch manufacturer.
HORTSCIENCE VOL. 52(12) DECEMBER 2017 1701
were installed in one replicate block to a depth
of 10 cm and were 10 cm away from a plant
crown. Mean air temperature, relative hu-
midity, precipitation, and soil temperature
(recorded at 5 cm depth) were collected every
15 min from a weather station located 0.7 km
away and evaluated monthly from June
through September of each year (WSU
AgWeatherNet, 2017).
To assess mulch deterioration, visual
observations of rips, tears, and holes were
observed and recorded as PVC of the soil on
the 15th and 30th of each month during the
experiment. The percentage of weed cover
was also recorded to assess weed suppression
by the mulch treatment. PVC and weed cover
were determined in a permanently designated
0.6 ·0.9 m area in the center of each split
plot. A fully intact mulch film received
a PCV rating of 100%. For both mulch PVC
and weed cover measurements, a 1% mea-
surement interval was used up to 5%, and
after that point a 5% measurement interval
was used. All plots were hand-weeded after
PVC and weed cover assessments were com-
pleted on 17 July 2014 and 23 and 24 July
2015, which returned all treatments to 0%
weed cover.
Harvest was from 14 Aug. to 23 Sept.
2014 and from 17 Aug. to 5 Oct. 2015 (6 and
7 weeks, respectively). Ripe fruits were
harvested by subplot three times per week;
fruit were picked by hand and transported to
a laboratory for sorting and weighing. Total
marketable and unmarketable berry weight
and percentage unmarketable fruit per sub-
plot were recorded within 48 h after harvest-
ing, with fruit being stored at 1.7 C
overnight as needed. Fruit were culled into
the unmarketable category based on size
(fruit #21 mm were culled), physical or
developmental defects, and pest and disease
damage. Each week during harvest, a 20-
berry subsample was collected per subplot
and frozen at –23 C until later fruit quality
analysis. Percent SSC, juice pH, and TA (as
percent citric acid) were determined from the
subsample of fruit from each harvest week in
2014 and 2015; all fruit quality data were
performed in triplicate and were averaged by
treatment. Berries were thawed at room
temperature (21 C), blended in a 10-speed
1.25-L capacity blender (Model #6883 by
Oster, Boca Raton, FL) for 30–60 s and then
strained through a kitchen-grade fine mesh
strainer followed by straining through two
layers of cheesecloth to obtain a solids-free
juice solution. SSC of juice was measured
with a hand-held refractometer (REF-113;
Index Instruments U.S., Inc., Kissimmee,
FL). Juice pH and TA were determined
simultaneously using a digital titrator that
measured initial juice pH (HI-84532; Hanna
Instruments, Woonsocket, RI) and titrated to
an endpoint of pH 8.1 using a solution of 0.1
N sodium hydroxide.
Dry biomass of strawberry plants was
determined the week after the conclusion of
harvest. In each year, four plants per subplot
were harvested and separated by roots, leaves
(petioles included), and crowns; remaining
flowers and fruit were discarded and not
included in biomass analyses. Plant material
was dried at 70 C for 5 d or until constant
weight was achieved.
Statistical analysis. All data were sub-
jected to the generalized linear mixed model
procedure in SAS (Statistical Analysis Sys-
tem software, Ver. 9.3; SAS Institute, Inc.,
Cary, NC). The assumptions of normality and
homogeneity of variances were assessed
using the Shapiro–Wilk test (W> 0.80) and
the Levene’s test (a= 0.05), respectively.
Blocks were treated as random effects with
mulch treatment as the fixed effect. Data
were analyzed as a one-way analysis of
variance with a least-squares mean option;
a Tukey–Kramer adjustment for multiple
comparisons was used for estimates and tests
of significance (a= 0.05). For PVC and weed
data, no transformation satisfied the assump-
tions of normality and equality of variances;
therefore, those data were nonparametrically
analyzed using mixed models on rank trans-
formed data. Untransformed means are re-
ported. Cultivar and year effects were also
assessed. Data are presented by cultivar when
there was a cultivar effect and by year
when analyses revealed a treatment by year
interaction.
Results
Environmental data. Weather conditions
from June through September were slightly
warmer and drier in 2015 relative to 2014
(Table 2). Compared with 2014, average air
temperature was 2 and 1 C higher in June
and July 2015, respectively, similar in Au-
gust for both years, and 1.8 C higher in Sept.
2015. Relative humidity was generally lower
throughout the growing season in 2015
(76.5%) than in 2014 (83.1%). Precipitation
from June through September was 10.5 mm
lower in 2015 (37.8 mm) than in 2014 (27.3
mm), with precipitation deficits occurring in
June and July of 2015.
Soil temperature was higher in 2015 than
2014; however, the data logger in the bare
ground treatment consistently malfunctioned
and data for Sept. 2014 and all of 2015 are not
reported for that treatment. Similarly, the
data loggers malfunctioned in Sept. 2015
across all treatments so those data are like-
wise not reported. A few notable differences
occurred with regard to soil temperature.
Between June and Aug. 2014, soil tempera-
tures averaged 19.8 and 19.7 C in the
WeedGuardPlus and PE treatments, respec-
tively. Average soil temperatures for the
same time period were 20.0, 20.3, and
20.3 C for the Bio360, Exp. Prototype, and
bare ground treatments, respectively. This
trend of soil temperatures among the mulch
treatments was not observed in 2015. Soil
temperature from June through August in
2015 was highest under PE (21.7 C) and
Bio360 (21.5 C) and slightly lower under
WeedGuardPlus (20.4 C) and the new
formulation of the Exp. Prototype (20.2
C). Average soil temperatures between
June and Aug. 2014 and 2015 at 5 cm depth
from an adjacent area with mixed species
grass groundcover were 19.2 and 19.3 C,
respectively.
PVC and weed pressure. PVC differed
due to treatment (P< 0.0001), but did not
differ due to year or cultivar (P= 0.62 and P=
0.77, respectively). There was an interaction
between treatment and year (P= 0.003). By
the end of the growing season in 2014, PVC
reached 8% for the Exp. Prototype, whereas
PVC was 90% for both Bio360 and WeedG-
uardPlus and 98% for PE (Fig. 1). By the end
of the season in 2015, PVC reached 62% for
the Exp. Prototype, 64% for WeedGuardPlus,
93% for Bio360, and 97% for PE.
In 2014, there were minimal weeds across
all of the mulched treatments throughout the
growing season except bare ground plots,
which averaged 47% weed cover by the last
sampling time point on 30 Sept. (data not
presented). Percent weed cover in 2015
Table 2. Environmental and soil temperature data for day-neutral strawberry grown with different mulch
treatments in Mount Vernon, WA, 2014 and 2015.
Variables
z
2014 2015
June July Aug. Sept. June July Aug. Sept.
Air temperature (C) 15.1 17.8 18 15.8 17.1 18.8 17.9 14
Relative humidity (%) 82.3 80.7 84.0 85.3 76.2 75.0 72.7 81.9
Precipitation (mm) 29.0 32.8 22.4 67.1 15.5 2.8 37.1 53.6
Soil temperature (C)
y
Bio360 19.1 20.8 20.2 16.9 21.8 22.4 20.2 15.6
WeedGuardPlus 19.3 20.5 19.5 16.2 20.9 20.8 19.6 —
Exp. Prototype 18.8 21.5 20.6 17.3 20.5 20.6 19.6 —
Plastic (PE) 18.1 20.8 20.1 16.6 22.0 22.6 20.6 —
Bare ground (control) 19.2 21.4 20.2 —
x
——— —
Nontreatment (5 cm) 18.3 19.8 19.6 16.1 18.8 19.9 19.2 16.6
z
Environmental data (air temperature, relative humidity, precipitation, and nontreatment soil temperature)
were collected from a weather station located 0.7 km from the experimental sites and were summarized
every 15 min from June through September of both years. Data courtesy of WSU AgWeatherNet.
y
Soil temperature under the mulches [Bio360, WeedGuardPlus, Experimental prototype, and polyethylene
(PE)] and bare-ground control were recorded every 15 min from probes installed in one replicate block to
a depth of 10 cm, and were 10 cm away from a plant; nontreated soil measurements were collected from the
WSU AgWeatherNet station at a depth of 5 cm (soil surface had a mixed species grass groundcover).
x
Soil temperature data for the bare ground treatment in Sept. 2014 and in 2015 are not presented due to
a logger malfunction; Sept. 2015 data also not reported due to a logger malfunction in the WeedGuardPlus,
Experimental Prototype, and PE treatments.
1702 HORTSCIENCE VOL. 52(12) DECEMBER 2017
differed because of treatment across all sam-
pling dates (P= 0.0008) and weeds signifi-
cantly increased from strawberry planting to
the end of the last sampling date in all
treatments (P< 0.0001) (Fig. 2). By the end
of the season, weed cover was greatest in the
bare ground treatment (95%) followed by
WeedGuardPlus (50%). Percent weed cover
was intermediate in the Exp. Prototype (34%)
and PE (25%), whereas Bio360 had the
lowest weed cover (15%) (Fig. 2).
Fruit yield. Marketable yield and fruit
number differed by cultivar, treatment, and
year (Table 3). In 2014, ‘Albion’ marketable
yield was the same across all mulch treat-
ments and was higher than yield from the
bare ground treatment (P< 0.0001). ‘Albion’
marketable yield in 2015 was lower than
2014, but was highest from plots treated with
PE and plastic BDMs (Exp. Prototype and
Bio360), intermediate for WeedGuardPlus,
and lowest for bare ground plots (P<
0.0001). Marketable fruit number in ‘Albion’
was greater for all mulch treatments than the
bare ground treatment in 2014. Bare ground
plots of ‘Albion’ also had the lowest market-
able fruit number in 2015, whereas PE-
treated plots produced the most marketable
fruit number with Bio360 having similar
results. Marketable fruit number in Bio360-
treated plots was similar to the Exp. Pro-
totype, but greater than WeedGuardPlus.
‘Seascape’ marketable yield was greatest
from plants treated with the Exp. Prototype
and lowest in the bare ground plots in 2014,
with the remaining treatments being similar
(P= 0.05). In 2015, marketable yield was
greatest in the PE treatment followed by
Bio360 and the Exp. Prototype, WeedGuard-
Plus, and the bare ground. ‘Seascape’ mar-
ketable fruit number only responded to
treatments in 2015, where the pattern was
the same as 2015 marketable yield. Unmar-
ketable berry weight showed a year effect and
was not different in 2014, averaging 49 and
65 g/plot for ‘Albion’ and ‘Seascape’, re-
spectively (Pvalue = 0.56 for ‘Albion’ and
0.675 for ‘Seascape’). In 2015, ‘Albion’ un-
marketable berry weight was greatest in the PE
treatment (46 g/plot) followed by WeedGuard-
Plus (31 g/plot), with Bio360 and Exp. Prototype
being similar to PE and WeedGuardPlus (39 and
37 g/plot, respectively); unmarketable berry
weight was lowest in the bare ground treatment
(12 g/plot) (P< 0.0001). The trend of differences
in unmarketable berry weight was similar in
‘Seascape’, with unmarketable berry weight
greatest in the PE treatment (59 g/plot) followed
by WeedGuardPlus (43 g/plot), with Bio360 and
Exp. Prototype being similar to PE and WeedG-
uardPlus (50 g/plot for both treatments); un-
marketable berry weight was lowest in the bare
ground treatment (15 g/plot) (P< 0.0001).
Dry biomass. Plant biomass differed
because of cultivar, year, and treatment
(Table 4). There was a year and cultivar
effect for root biomass and a treatment effect
only in 2015 for ‘Seascape’. ‘Seascape’ root
biomass was greatest under Bio360 mulch
followed by WeedGuardPlus and PE, Exp.
Prototype, and the bare ground control.
There was no cultivar effect in both years
of the study for leaf biomass, but there were
effects due to year and treatment. Leaf bio-
mass was higher across all treatments com-
pared with bare ground control in 2014. In
2015, plants treated with plastic BDMs
(Bio360 and Exp. Prototype) responded
similarly to the PE treatment, whereas plants
treated with WeedGuardPlus had a lower
leaf biomass than other mulch treatments.
Leaf biomass was lowest in the bare ground
control. Crown biomass did not differ due to
the cultivar in 2014 and was higher in the PE
and Exp. Prototype treatments than the bare
ground and Bio360 treatments, but was
similar to WeedGuardPlus. In 2015, crown
biomass differed by cultivar. ‘Albion’ treat-
ed with PE had the highest crown biomass
and was similar to Bio360 and the Exp.
Prototype, whereas crown biomass was simi-
lar among WeedGuardPlus, Bio360, and Exp.
Prototype treatment plots. ‘Seascape’ crown
biomass in 2015 was less responsive than
‘Albion’, with plants from all mulched plots
producing higher biomass than bare ground.
Fruit quality. Only SSC showed a cultivar,
year, and treatment difference, whereas fruit
pH only differed due to year, and TA differed
due to treatment only in ‘Seascape’ in 2015
(Table 5). ‘Albion’ SSC only differed in 2015
(P= 0.05), where it was higher in fruit
harvested from the PE treatment when
Fig. 1. Percent visual cover (PVC) of different mulch treatments applied to strawberry in Mount Vernon,
WA, 2014–15.
Fig. 2. Percent weed cover in strawberry grown
with biodegradable and black polyethylene
(PE) plastic mulches in Mount Vernon, WA,
2015.
HORTSCIENCE VOL. 52(12) DECEMBER 2017 1703
compared with the bare ground control, but
was similar to the BDM treatments (Bio360,
WeedGuardPlus, and Exp. Prototype). In
2014, ‘Seascape’ SSC was greatest in fruit
harvested from the PE plots, but was similar
to Bio360 and WeedGuardPlus. ‘Seascape’
SSC was lowest in the Exp. Prototype and
bare ground control when compared with
PE-treated plots. However, in 2015, ‘Sea-
scape’ SSC was the same in fruit from all
mulch treatments but higher than the bare
ground control (P= 0.0002). Fruit pH was
slightly higher in 2014 than 2015 (P=0.03),
but there were no cultivar nor treatment
effects. TA only differed in 2015 for ‘Sea-
scape’, where it was higher in fruit harvested
from the Exp. Prototype and bare ground
treatments compared with the PE treatment;
both treatments were similar to Bio360 and
WeedGuardPlus.
Discussion
Yield, plant biomass, and fruit quality of
day-neutral strawberry grown with plastic
BDMs were overall comparable to plants
grown with PE mulch, whereas many of these
variables were reduced for plants grown
without mulch and were intermediate for
plants grown with the paper BDM (WeedG-
uardPlus). Marketable yield and fruit number
were greater in 2014 relative to 2015 because
of the warmer, drier conditions during the
2015 June and July production period, which
were less favorable for day-neutral straw-
berry production. Unmarketable berry weight
was only different in 2015, but was overall
high across both years because of tarnished
plant bug (Lygus lineolaris) feeding and
rigorous culling for size. However, unmar-
ketable berry weight was greater for all
mulched treatments among both cultivars in
2015, which may be due to microclimate
effects impacting crop development and is an
area recommended for further study. Never-
theless, bare ground plots consistently had
the lowest marketable yield and fruit number,
which was consistent with previous studies
that show productivity of strawberry and
other horticultural crops are enhanced
through mulch application because of re-
duced competition from weeds and a modified
soil environment that is more favor-
able to plant growth and development
(Anzalone et al., 2010; Diaz-Perez et al.,
2005; Forcella et al., 2003; Moore, 1963;
Touchaleaume et al., 2016; Waterer, 2010).
Other studies comparing yield of strawberry
grown with plastic BDMs show similar re-
sults and demonstrate that plants grown with
plastic BDMs can produce yields equal to
those grown with PE mulch (Bilck et al.,
2010; Costa et al., 2014).
The current study showed cultivars can
respond differently to BDM treatments. In
general, ‘Albion’ appeared less sensitive to
type of mulch application, with marketable
yield and fruit number from plants treated
with plastic BDMs being equal to those
grown with PE mulch. Reduced yield from
plants treated with the Exp. Prototype plastic
BDM in 2015 can be attributed to greater
weed pressure and resultant plant competi-
tion that may have been exacerbated by the
warmer and drier environmental conditions
that year. ‘Seascape’ had less consistent
trends with regard to marketable yield and
fruit number across the years of the study.
Marketable yield was similar for all mulch
treatments in 2014 but was strongly impacted
by mulch treatment in 2015 when yield was
greatest in plants treated with PE mulch,
followed by the plastic BDM treatments,
paper BDM, and was lowest in nonmulched
plots. Taken together, these results suggest
cultivar genetics may influence how day-
neutral strawberry plants respond to different
mulch treatments and that some cultivars
may be more adaptable to systems that use
BDMs. Similar to the yield results, plant
biomass tended to be the lowest in bare
ground, intermediate in the paper BDM
treatment, and the greatest in PE and plastic
BDM treatments. ‘Albion’ appeared less
sensitive to mulch treatments with regard to
root biomass, whereas ‘Seascape’ appeared
less sensitive in regard to crown biomass, and
both cultivars showed a similar response for
leaf biomass. These results highlight how
cultivar genetics lead to different develop-
mental responses after mulch application and
that environmental and weed pressure condi-
tions of 2015 elicited more plant growth
responses to mulch treatments when con-
trasted to 2014.
Root and leaf biomass were overall great-
er in 2015, which may have been an adaptive
Table 3. Marketable fruit yield and number in ‘Albion’ and ‘Seascape’ strawberry grown with different
mulch treatments in Mount Vernon, WA in 2014 and 2015.
Treatment
Albion Seascape
Marketable
yield/plot (g)
z
Marketable fruit
number/plot
Marketable
yield/plot (g)
Marketable fruit
number/plot
2014 2015 2014 2015 2014 2015 2014 2015
Bio360 253 a
y
181 a 19 a 15 ab 311 ab 190 b 32 18 b
WeedGuardPlus 259 a 137.5 b 17 a 11 c 316 ab 144 c 30 13 c
Exp. prototype 281 a 181.9 a 20 a 14 b 340 a 202 b 32 18 b
Plastic (PE) 295 a 200.7 a 22 a 17 a 322 ab 239 a 32 22 a
Bare ground (control) 126 b 80.0 c 9 b 6 d 257 b 96 d 25 8 d
Pvalue <0.0001 <0.0001 <0.0001 <0.0001 0.05 <0.0001 0.47 <0.0001
z
Marketable yield and fruit number determined from annual means of harvested fruit from 1.8 m
2
subplots.
y
Means with the same letter within a column are not different due to treatment at a= 0.05; a Tukey–
Kramer adjustment was used for multiple comparisons.
Table 4. Dry biomass of ‘Albion’ and ‘Seascape’ strawberry grown with different mulch treatments in
Mount Vernon, WA in 2014 and 2015. Data are presented by year and cultivar when there was an
interaction due to year or cultivar.
Treatment
Roots (g) Crown (g)
Albion Seascape Leaves (g) 2015
2014 2015 2014 2015 2014 2015 2014 Albion Seascape
Bio360 2.3 8.1 2.2 6.5 a
z
23.4 a 32.2 a 4.7 b 3.7 abc 2.7 a
WeedGuardPlus 3.2 7.4 2.2 6.0 ab 25.1 a 26.9 b 4.8 ab 3.3 bc 3.0 a
Exp. prototype 2.8 7.4 2.5 5.2 b 26.1 a 32.9 a 5.5 a 4.1 ab 3.1 a
Plastic (PE) 3.0 7.5 2.5 5.8 ab 27.1 a 34.2 a 5.6 a 4.4 a 2.7 a
Bare ground (control) 3.1 5 2.9 3.9 c 16.8 b 6.5 c 4.2 b 2.9 c 2.0 b
Pvalue 0.34 0.19 0.63 0.002 <0.001 <0.001 0.003 0.006 0.023
z
Biomass determined from four plants per cultivar subplot replicated four times; Means with the same
letter within a column are not different due to treatment at a= 0.05; a Tukey–Kramer adjustment was used
for multiple comparisons.
PE = polyethylene.
Table 5. Total soluble solids concentration (SSC, %), juice pH, and titratable acidity (as percent citric acid;
CA, %) of ‘Albion’ and ‘Seascape’ strawberry grown with different mulch treatments in Mount
Vernon, WA in 2014 and 2015.
Treatment
SSC (%) CA (%)
Albion Seascape pH 2015
2014 2015 2014 2015 2014 2015 2014 Albion Seascape
Bio360 10
z
8.0 ab
y
8.8 ab 8.1 a 3.6 3.4 0.72 0.66 0.65 ab
WeedGuardPlus 9.3 8.1 ab 8.7 ab 7.9 a 3.6 3.5 0.68 0.64 0.66 ab
Exp. prototype 9.3 8.3 ab 8.1 b 7.6 a 3.6 3.5 0.67 0.67 0.71 a
Plastic (PE) 10.1 8.6 a 9.0 a 7.7 a 3.5 3.4 0.7 0.62 0.64 b
Bare ground 9 7.5 b 8.1 b 6.3 b 3.5 3.5 0.68 0.62 0.71 a
Pvalue 0.28 0.05 0.05 0.0002 0.55 0.4 0.45 0.67 0.05
z
SSC (%), pH, and CA (%) determined from the mean of weekly 20-berry samples per subplot, performed
in triplicate.
y
Means with the same letter within a column are not different due to treatment at a= 0.05; year and/or
cultivar were combined when they were not statistically different; a Tukey–Kramer adjustment was used
for multiple comparisons.
CA = citric acid; PE = polyethylene.
1704 HORTSCIENCE VOL. 52(12) DECEMBER 2017
response to the environmental conditions and
come at the cost of decreased fruit produc-
tion. Perennial plants augment growth and
development during periods of environmen-
tal stress, such as temperature and moisture
stress, and mulching can alleviate plant stress
by suppressing weeds, enhancing soil mois-
ture, and improving soil temperatures.
Kumar and Dey (2011) found that ‘Chandler’
strawberry plants grown with PE mulch,
straw mulch, or bare ground had higher root
growth when plants received irrigation
(100%, 80% and 60% of volume of water)
and lower root growth under rainfed condi-
tions in a subtemperate climate in India. Soil
moisture content at 0–25 cm was 2.8% to
12.8% higher under PE mulch than bare
ground. In a separate study by Taparauskien _
e
and Miseckait_
e (2014) in a subhumid area of
Lithuania, soil moisture content at 0–40 cm
depth was higher under straw mulch (18.0%)
than under PE mulch (16.5%) or bare ground
(16.2%), whereas strawberry fruit yield was
60% higher from plants grown with PE
mulch than bare ground and 56% greater
relative to plants grown with straw mulch.
Several studies have found that fruit
quality is maintained or unchanged as a result
of plastic BDM use (Bilck et al., 2010; Costa
et al., 2014). However, Morra et al. (2015)
found increased production of secondary
metabolites (anthocyanins, flavanols, and
polyphenols) and greater antioxidant activity
in strawberry grown with plastic BDM (with
Mater-Bi as the primary feedstock) relative to
those grown with PE mulch. While the
current study did not measure these second-
ary metabolites, there was an overall trend of
lower SSC in fruits harvested from the bare
ground treatment relative to mulched treat-
ments. This difference may be due to the
diurnal absorption and reradiation of heat
energy from the dark-colored mulches, which
could impact soil and canopy temperatures.
Mulches may also change the quality and
quantity of light energy being reradiated into
the plant canopy, which may impact plant
development and resultant fruit quality. Re-
search has shown that mulch color can impact
strawberry crop development and quality
attributes of fruit. For example, in strawberry
production in central Colombia, silver color
mulch resulted in lower fruit pH, SSC, TA,
and dry biomass compared with red, blue,
yellow, green, and black mulch treatments;
the ratio of SSC to TA was highest with black
mulch; fresh fruit weight and fruit length
were highest with red mulch, which might be
the result of absorbing more red and far-red
light by phytochrome (Casierra-Posada et al.,
2011; Kasperbauer et al., 2001). Wang et al.
(1998) found that fruit of ‘Northeaster’
strawberry grown with red mulch had higher
TA than fruit grown with black mulch.
All mulches reduced weed pressure rela-
tive to bare ground both years in the current
study. This result is consistent with other
studies that show plastic BDMs can provide
efficacious weed suppression depending on
their formulation and compatibility with the
cropping system (Anzalone et al., 2010;
Cowan et al., 2014; Miles et al., 2012).
Despite rapid changes in PVC of the Exp.
Prototype in 2014, weed cover was compa-
rable to the other mulched treatments and
indicates weed suppression was adequate for
the experimental conditions. Because of the
rapid degradation of the Exp. Prototype in
2014, the mulch manufacturer re-formulated
the mulch product, and consequently, the
Exp. Prototype deteriorated less rapidly in
2015 (the specifics of the reformulation were
not disclosed, as this was proprietary infor-
mation of the mulch manufacturer). Exp.
Prototype is made up of thermoplastic starch
(TPS) and polyhydroxyalkanoate, which
have been shown to have comparably high
rates of above-ground disintegration (Cowan
et al., 2013). Despite this, TPS has been
successfully used in BDMs in strawberry
production in Brazil (Bilck et al., 2010).
Bio360 (formerly BioTelo) was observed to
have high rates of PVC in both years of the
experiment and was comparable to the PE
control. Percent weed cover was low for
Bio360 and was comparable to PE. This
result was similar to a study with tomato
(Lypocersicum esculentum) in Washington
that included BioAgri mulch, which has
a similar formulation as Bio360 (Miles et al.,
2012). The starch in Bio360 is characterized
as highly biodegradable while the PBAT
component is characterized as having a low
to moderate rate of biodegradation in soil
(Brodhagen et al., 2015).
For WeedGuardPlus, PVC was reduced
and percent weed cover increased in 2015
due to the occurrence of several windy days
in July 2015 (maximum wind gusts surpassed
12.9 km·h
–1
, WSU AgWeathernet, 2017),
which caused the WeedGuardPlus mulch to
rip and tear along the buried edges of the
raised bed, creating more exposed surfaces
for degradation. Weber (2003) reported sim-
ilar experiences when using Planters Paper in
establishing matted-row strawberry in New
York, where degradation occurred rapidly
along buried edges of the planting beds and
allowed wind to rip fragments of the mulch
off the beds. Similarly, studies with WeedG-
uardPlus in tomato grown in Washington
confirm this material is susceptible to ripping
and has low rates of PVC later in the growing
season (Cowan et al., 2014; Miles et al.,
2012). Performance of WeedGuardPlus,
however, varied based on location and pro-
duction system, and was found to have higher
deterioration in open-field production in
Washington relative to Texas, and lower
deterioration in a high tunnel environment
relative to open-field (Li et al., 2014). The
primary feedstock in WeedGuardPlus is cel-
lulose, a plant-derived polysaccharide that is
intrinsically biodegradable and is character-
ized as having a moderately high rate of
biodegradation in soils (Brodhagen et al.,
2015; Sivan, 2011). These findings under-
score that the environment and cropping
system must be considered when selecting
a BDM, as mulch performance will vary
based on these factors. Additionally, while
complete biodegradation is an important
feature of BDMs, paper-based mulches may
be too quick to degrade and consequently
may not provide adequate soil surface cov-
erage for strawberry, depending on the pro-
duction environment and growers’ goals.
Conclusion
The plastic BDMs studied in this experi-
ment performed comparably to standard PE
mulch and may provide a suitable alternative
to PE mulch in plasticulture systems of day-
neutral strawberry in the PNW. Bio360 was
most similar to PE mulch in terms of PVC,
weed suppression, and yield, indicating this
material is compatible with commercial straw-
berry production systems and could replace
PE mulch. Re-formulations of the Exp. Pro-
totype could make it more compatible with
the requirements of day-neutral strawberry
production and additional testing of reformu-
lated and new plastic BDMs (e.g., BASF
Ecovio) is recommended. The question of
in-soil biodegradation of these materials
should also be pursued in future research, as
there has been limited evaluation of in-soil
biodegradation of plastic BDMs across dif-
ferent production systems and soil environ-
ments. It is important to note that while PE
and paper BDMs are allowed in organic
production, use of plastic BDMs is presently
not permitted (Miles et al., 2017). Plastic
BDMs may be a tool that enhances the
sustainability of conventional strawberry
production as a replacement for PE, but are
also chemically and physically different from
PE. Soil fumigation and concurrent BDM
application is not allowed and producers
should speak with mulch manufacturers if
they have questions regarding how certain
on-farm practices may impact plastic BDM
performance. Overall, plastic BDMs are
a promising tool for day-neutral strawberry
growers and continued research on plastic
BDMs as an alternative to PE could assist
growers in reducing plastic waste generation
while maintaining the profitability of their
farm enterprises.
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