Photos Courtesy of Dr. Julie Grossman, University of Minnesota
High tunnels, also called hoop houses or poly tunnels, are
an important and relatively inexpensive tool for horticultural
production to extend seasonal crops in cold climates.
However, their use creates challenges related to soil health
and decreased crop yields.
There is no question that using high tunnels results in
increased marketable crop yields and extended harvests,
but there are concerns that continued use of these high
tunnels will jeopardize the long-term sustainability due to
the loss of natural resources in the soil.
The greater number of available growing days in high
tunnels intensifies soil use due to longer and more frequent
planting cycles and increased nutrient demands. These
practices, coupled with high temperatures and irrigation
under protected conditions, leaves high tunnel soils
vulnerable to depletion of organic materials and increased
soil salinity. Soil health depletion is a major concern for
growers, especially those who are certified as organic.
The objective of a two-year study by researchers at the
University of Minnesota was to explore the effect of winter
cover crops on soil nutrients, soil health, and bell pepper
crop yield in high tunnels. Cover crops are often used to
manage soils issues, such as decreased organic matter,
degraded soil structure, increased salinity, and high
The study was conducted from August 2015 to September
2017 at three sites in Minnesota. The cover crops used in
this study consisted of red clover monoculture, winter
pea/rye bicultural, hairy vetch/tillage radish/rye, and a bareground, weeded control. Researchers sought to identify
productive winter cover crop mixtures in cold-climate high
tunnel environments, to quantify the effects of overwintered
cover crops on soil health, and to assess the impacts of
overwintered cover crops on cash crop productivity-in this
case, bell peppers.
During the study period, cover crops were seeded at two
different times-either in late August/early September
between standing rows of peppers, or in mid-September
after the pepper plants had been removed. The cover crops
were watered overhead as needed, and no additional
fertilizer was added to any of the study plots. In May, the
cover crops were mowed down, left to dry on the soil
surface, and then tilled into the soil. New peppers were
transplanted into the soil 5-10 days later.
Sowing cover crops later in the fall after pepper plants were
removed generally increased total biomass from 30% to 70%
that of the early planted plots, except at the warmest site.
The legumes (winter peas) were winter hardy to zones 4a and
4b in the high tunnels, extending past their open-field range
of zone 5a, and researchers will continue efforts to increase
the number of surviving legumes in order to increase the
nitrogen in the tunnel soil samples.
The researchers concluded that overwintered cover crops in
high tunnels may be a possible organic alternative for crop
producers to reduce the dependence upon compost and
manure to meet nitrogen requirements and replenish
organic soil matter. Further research will be needed to
determine the potential trade-offs between cash crop and
cover crop productivity.
According to Dr. Julie Grossman, Associate Professor at the
University of Minnesota, “Now that some high tunnels have
been on the landscape for almost 30 years, more farmers are
coming to us with questions about how they can improve soil
health in these unique environments. This project is the first
step to understand the role that cover crops may play in
improving long-term sustainability and productivity of high
You can learn more about this exciting research by reading
the full report in HortScienceDOI: https://doi.org/10.21273/HORTSCI15987-21
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