May 2020 - 13

MSUE VEGETABLE NOTES
Cover crops between plastic mulch get mixed results
By Alyssa Tarrant
and Zachary Hayden
Michigan State University
Weed management between plastic
mulch beds influences in-season and
future weed pressure, and directly
impacts soil management. Most growers
use cultivation or herbicides to control
weeds between beds, but with up to 5075%
of a field covered with impervious
plastic, erosion and chemical runoff can
be substantial in plasticulture systems
(Figure 1).
Growing a cover crop as a " living mulch "
between vegetables grown on plastic has
the potential to suppress weeds while also
reducing erosion, scavenging nutrients,
building soil organic matter, and improving
harvest conditions on ground that
would otherwise be bare. Despite grower
interest in these advantages, uncertainties
surrounding impacts on vegetable
productivity and management remain.
Between 2017 and 2018, we conducted
two experiments and several on-farm
demonstrations in Michigan to investigate
tradeoffs in growing living mulches relative
to more commonly used between-bed
weed management practices (including
cultivation and straw mulch) in organic
plasticulture vegetable production. Here
we share some of what we learned to help
you decide if (and how) living mulches
might work for you.
Details of the research
In one two-year experiment, we
evaluated the system-level impacts of
different between-bed management
strategies on bell pepper and yellow
summer squash produced on plastic mulch
(Figure 2). We compared three betweenbed
living mulch treatments (cereal rye,
Italian ryegras and a rye-Dutch white
clover mixture) to mowed ambient weeds,
straw mulch and cultivated bare ground.
Figure 1. Cereal rye living mulch, left, reducing early season erosion when planted
between plastic mulch beds on sloping ground at Forgotten Harvest Farms in
Fenton, Michigan. Photos: Michigan State University
Results and implications
Living mulches can suppress weeds, but
don't count on it. Across all of our trials,
weed suppression by living mulches was
generally modest. Rye did particularly
poorly due to a combination of heat stress
and leaf rust that reduced growth in
both years. Summer annual grasses were
the most promising weed suppressors,
including Italian ryegrass and teff. In
fact, teff reduced weed biomass to levels
comparable to the cultivated control in one
summer. However, in most cases, there
was more weed than cover crop biomass in
our living mulches, allowing for weed seed
production late in the season that could
increase future weed pressure. While other
approaches to living mulch management
may improve weed suppression, don't
expect miraculous weed control from a
living mulch, particularly if weed pressure
in your fields is already high.
Anything growing between plastic
mulch beds has the potential to compete
with the cash crop. We expected the
spatial and physical barrier provided by
plastic mulch to reduce the potential for
competition between a living mulch and
the cash crop. While this is likely true
to some extent, all between-bed living
mulches and weeds significantly reduced
water and N levels within plastic mulch
beds (Figure 3).
Living mulches often reduce vegetable
Figure 2. Between-bed management
strategies including cultivation, straw
mulch, and living mulch were evaluated in
Michigan for impacts on summer squash
and bell pepper production.
A second two-year experiment
screened nine different cover crop species
between plastic to evaluate growth, weed
suppression and potential to compete with
a cash crop for in-bed water and nutrients.
The species included Italian ryegrass, teff,
sudangrass, barley, cereal rye, winter wheat
and three clover species grown in mixture
with rye (Dutch and New Zealand white
clover and yellow blossom sweet clover).
In all experiments, living mulches were
sown after bed formation and were mowed
three-to-four times during the summer.
yields on plastic, but some crops may be
more tolerant than others. In one year,
weeds and living mulches between plastic
mulch beds reduced bell pepper yields by up
to 60%. Similarly, our review of the literature
revealed that vegetable yield reductions
are common when living mulch is grown
between plastic mulch beds. However,
summer squash yields were unaffected by
between-bed management in either year
of our study, and pepper yields were less
affected in a wet year. Careful cash crop
selection and adjustments to fertility and
irrigation may help mitigate competition.
Soil health benefits are likely, but not all
are measurable in two years. Total living
mulch and weed biomass produced an
average of 2.5 tons of shoot biomass per
acre each summer on half the field that
would normally be left bare. This resulted
in significantly lower N leaching potential
in our studies. We would also expect
reductions in erosion and increases in
soil organic matter over time relative to
cultivating between beds. That said, we did
not detect changes in soil organic matter or
microbial biomass over two years, which
may not be surprising; these changes are
slow to accumulate.
Living mulch strategies should be
matched to the needs of your unique
production system. Growers interested
in planting living mulch between plastic
mulch beds should proceed with caution,
acknowledging that living mulch systems
entail new labor and management
requirements in addition to potential
risks to cash crop productivity. However,
services not measured here, including
Figure 3: Weed (common lambsquarters)
roots seeking water and nutrients under
an adjacent plastic mulch covered bed.
biological pest control, increased farm
biodiversity, reduced erosion or improved
harvesting conditions may offset potential
drawbacks. In addition, alternative
strategies, like delayed planting of living
mulches or different living mulch species
(like buckwheat or mustards) that can
be terminated at first mowing may offer
additional advantages.
You can find more information about
this project online in our final report
to the North Central SARE program
(https://projects.sare.org/project-reports/
gnc17-251), or email us at
tarran19@msu.edu or haydenza@msu.edu
with any questions. VGN
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VGN | MAY 2020 | 13
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May 2020

Table of Contents for the Digital Edition of May 2020
