Options for Including Cover Crops in High Tunnel Rotations in the Northern United States

Introduction

High tunnels are a popular tool used to extend the growing season, particularly for organic vegetable, flower, and fruit growers (Pool and Stone, 2014, 2015). Their popularity is no surprise given increased yield and quality for many kinds of produce, in addition to a price premium for fresh, local products outside of the typical growing season. Construction expenses for high tunnels can be prohibitive for some growers, but beginning in 2009, the USDA-Natural Resources Conservation Service (USDA-NRCS) has funded a cost-share initiative for high-tunnel construction through the Environmental Quality Incentive Program (EQIP) that has resulted in over 10,000 new high tunnels since the start of the program (USDA-NRCS, 2018).

A growing number of farmers have begun to notice certain production problems unique to high tunnels; namely, an increase in some soilborne diseases, foliar diseases associated with greenhouse production, and an overall decrease in soil health (Fig. 1). The combination of these issues can decrease plant productivity over time. The soil health issues in particular include a loss of organic matter (which leads to degraded soil structure) and an increase in soil salinity from irrigation under dry conditions (eXtension Foundation, 2013; Magdoff and VanEs, 2009). Another challenging aspect is the increased growth rate of weeds due to increased heat in high tunnels (Blomgren et al., 2007). Intensive cropping cycles also require frequent fertilizer inputs, which for organic growers consist of compost, manures, or other animal byproducts. These organic inputs often provide phosphorus that exceeds plant nutritional needs (eXtension Foundation, 2016).

Many of these problems could be addressed by including cover crops in high-tunnel crop rotations. Cover crops are known to increase organic matter, disrupt disease cycles, and, in the case of legumes, add nitrogen through symbiosis with rhizobia bacteria without increasing salinity or phosphorus to the same degree as manure, compost, or fertilizers (Treadwell, 2009; Zieminski, 2018; Monfort et al., 2007; Zhou and Everts, 2007). Research exploring cover crops in high-tunnel crop rotations is relatively new, but initial findings are encouraging. The primary challenge for farmers who want to grow cover crops in their high tunnels is how to balance the time required to grow a cash crop and a cover crop. In this article, we will discuss management basics, benefits and challenges, and seasonal options for including cover crops in high-tunnel rotations in the northern United States, drawing on the combined farming and research experience of the authors in the Northeast, Midwest, and Pacific Northwest.

Figure 1. High-tunnel soil showing poor soil quality with compaction and surface cracking. Photo credit: Elizabeth Perkus, University of Minnesota.

General Management Considerations

The basic operations of growing a cover crop in the high tunnel are similar to growing a cover crop in the open field: planting, growth, and termination—though some specifics are slightly different. Growers can plant larger areas by broadcasting and raking seeds in to ensure good seed-soil contact, or between rows using a walk-behind direct seeder for more uniform stands (Fig. 2). Following planting, irrigation must be supplied, as the plastic cover on the high tunnel excludes rain. Irrigating cover crops is best done with an overhead source because drip tape needs to be removed before termination. However, if high temperatures make overhead irrigation impractical or the tunnel is constructed with water-sensitive materials, such as untreated wood, the task of removing drip irrigation before termination may be worthwhile.

Figure 2. Walk-behind six-row direct seeder. Photo credit: Emily Swanson, University of Minnesota.

Termination is a more intensive task and can be difficult in a high tunnel, especially with high biomass cover crops which may be very large by the termination date (Fig. 3). As with any cover crop, it is important to monitor each species and terminate all the cover crops before the initiation of seed set. This will provide the greatest nitrogen contribution, as nitrogen will still be stored in vegetative tissue instead of sequestered in seeds. This will also prevent cover crops from self-seeding and becoming weeds in the tunnel later. The most common method for cover crop termination in high tunnels is mowing, then tilling to incorporate biomass after a few days of drying the plant material on the soil surface. The drying period after mowing makes the cover crops easier to till into the soil. If field-scale mowing and tillage equipment is too large for the high tunnel, it is important to find a balance of machinery that will both fit into the tunnel and completely terminate the cover crops. Walk-behind flail mowers and small riding mowers work well, but smaller equipment, like a regular push lawn mower, will only work on low-biomass cover crops. Another option is to use a weed whacker to terminate taller-growing cover crops, then use the cut biomass as a surface mulch or chop the material more finely with a push lawn mower prior to incorporation. Many cover crops require tillage for complete termination, especially if they are mowed before flowering.

Figure 3. Overwintered cover crops in high tunnel. Photo credit: Elizabeth Perkus, University of Minnesota.

Once the cover crops have been fully incorporated, it is best to wait about 5–14 days to plant the following cash crop. During this time the fresh biomass in the soil will begin to break down, increasing soil moisture and the community of decomposers. In the first days of decomposition, nutrients may be less available for crops. Also in this environment, newly planted seedlings or germinating seeds can be subject to rot or predation (Dufour et al., 2013).

Summary: Cover Crops in High Tunnels

Seasonal Windows

Fall-planted Winter-Kill

Fall-planted cover crops that are intended to winter-kill may be a great option after an early or main-season summer crop. These winter-kill species are chosen to put on a quick flush of biomass in the fall and then die back in late fall or winter with the arrival of freezing temperatures. In climates without prolonged freezing temperatures, a winter-kill rotation may not be possible. Winter-killed cover crops require no mowing equipment after establishment, so they work well for growers who operate with less mechanization. They are also ideal to precede an early spring crop since the winter-killed residue will provide some amount of soil cover but not require active spring termination, leaving the soil ready for early spring planting. Growers who choose to incorporate remaining residue in the spring will need to use some tillage, which could be manual or mechanized. To maximize the soil health and early-spring planting benefits of a winter-killed cover crop, it is important to choose a species with low spring residue that will not require extensive tillage nor tie up nutrients in decomposing plant material (Dupont et al., 2018).

Winter-killed cover crops can provide several benefits including fall weed control, organic matter contributions, and soil compaction mitigation (Johnson, 2016). Species selection for winter-kill cover crops is region-specific, since timing and severity of cold temperatures will vary widely by growing zone. The high tunnel buffers cold temperatures, allowing some species to survive in the tunnel where they would not survive open field conditions. In these situations, winter-kill could be achieved by opening sidewalls to lower temperature, and minimizing irrigation inputs to force senescence. Depending on timing, drought may induce cover crops to set seed quickly, so this technique should only be used before flowering.

Species selection for winter-kill cover crops will depend first on grower goals, which are typically organic matter input, weed control, pest control, or nitrogen fixation. Tillage radish (Raphanus sativus) is an excellent winter-kill cover crop choice based on its ability to reduce soil compaction, scavenge fall soil nutrients, and leave very little spring residue. However, tillage radish has been observed to survive winters in high tunnels in zones 4b and warmer, so growers in many regions will need to monitor this species closely. Buckwheat (Fagopyrum esculentum) is also a quick-growing, frost-sensitive crop that is effective at weed control and improving soil aggregation. Oats (Avena sativaa) are a common winter-kill grass species in cold climates, and when paired with a frost-sensitive legume like cowpea (Vigna unguiculata), they can form a mix that provides nitrogen fixation, organic matter, and weed control.

Fall-planted Overwintered

Overwintered cover crops fit well into rotations with a winter fallow period. They are seeded in the fall, survive the winter, grow rapidly in the spring, and are terminated and incorporated in spring just before the cash crop is planted. Ideally, the cover crop should be flowering at termination. Overwintered cover crops fit best prior to a warm-season cash crop planted in the mid to late spring, such as tomatoes or peppers. Overwintered cover crops can improve soil quality and cash-crop productivity, and can increase soil microbial activity (Rudisill et al., 2015). A study in Arkansas found that high-tunnel tomato yield increased in plots following an overwintering legume cover crop compared to control plots with no cover crop (Freeman, 2016).

Timing of fall sowing and species selection for overwintered cover crops depends on the summer use of the tunnel. If tunnels are used to extend the summer growing season, growers may wish to plant cover crops fairly late in the fall. The increased daytime temperature and buffered nighttime lows in a tunnel allow later planting of fall cover crops than would be feasible in the open field. However, in cold climates, it is ideal to plant the cover crops as early as possible in the fall so they are able to establish some robust growth before temperatures become limiting. One way to have both season extension and longer cover crop growth is to interseed an overwintering cover crop into the existing summer crop (Fig. 4). The summer crop can be removed when its production is finished and the cover crop can continue to grow throughout the winter.

Figure 4. Vetch/rye/radish mix seeded between yellow bell peppers. Photo credit: Elizabeth Perkus, University of Minnesota.

Overwintering cover crops in cold climates require special attention to moisture and temperature management. Maintaining adequate soil moisture can be a challenge during winter in high tunnels since most tunnels are not equipped with a non-freezing water source for winter irrigation. For this reason, regions with cold, dry winter weather may see rapid desiccation of unfrozen soil. To address this, cover crops should be thoroughly watered right before the water supply is shut off for winter. At this point the sides of the tunnel should be rolled down and secured, and the doors closed. These steps can help prevent water loss from evaporation and wind, though this may not be necessary in regions with warmer, more humid winters.

Dramatic winter temperature fluctuations in the tunnel create a unique temperature environment for plants. Sunny days can raise the temperature in the tunnel much higher than outside temperatures. However, due to the minimal insulation of high tunnel plastic, nighttime lows in the tunnel will be similar to outside. Unlike plants in the open field in northern regions, high tunnel cover crops do not have an insulating blanket of snow. Using row cover within the high tunnel is a management technique to combat both moisture loss and temperature extremes (Fig. 5). This insulating layer holds heat in the soil, helps retain soil moisture, and mitigates air temperature fluctuations.

Figure 5. Row cover supported by low tunnel hoops over cover crops in the high tunnel. Photo credit, Charlotte Thurston, University of Minnesota.

Since overwintering cover crops in the high tunnel will experience low temperatures similar to crops in the open field, varieties should be selected for winter hardiness. Legumes such as hairy vetch (Vicia villosa) and red clover (Trifolium pratens) and grasses like winter rye (Secale cereale) are suitable for overwintering in high tunnels across the United States. Subterranean (Trifolium subterraneum) and Crimson clover (Trifolium incarnatum) will more reliably overwinter in USDA Hardiness Zones 6 and above (Young-Mathews, 2013.). In the spring, overwintering cover crops will grow quickly in response to higher temperatures and increasing light. If row cover is used, it can be removed as soon as night temperatures are reliably above freezing. Due to this accelerated spring growth, cover crops may be ready to terminate by March–April, depending on the region. Some cover crop species may harbor cash-crop pathogens, such as the Erysiphe genus of powdery mildew that can infect both clovers and cucurbits, so growers should consider this when choosing cover crop species (Mahr, 2016).

Spring-planted

Spring-planted cover crops could fit well into a high-tunnel rotation for growers who have a late fall crop and plan to start their summer crops in late spring. Cold-tolerant species, such as hairy vetch, red clover, field pea (Pisum sativum), winter rye, and oats, can be planted as soon as the soil can be worked and will withstand early spring frost without supplemental heat. A spring cover crop will not grow to full size or maturity before termination time. This younger plant material has more nitrogen and less carbon, so it will break down quickly in the soil. Th

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