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).

Read more at eXtension (Elizabeth Perkus, Anne Pfeiffer, Charlotte Thurston, Fucui Li, and Julie Grossman)