Cover Story

Cover Crop Options that Provide Benefits for Tennessee Nurseries

By Dr. Anthony Witcher, Associate Professor and Grayson DeLay, Graduate Masters Student – Tennessee State University, Otis L. Floyd Nursery Research Center

Tennessee has over 19,000 acres in production of nursery crops with more than half of that total area planted with field-grown trees and shrubs that will be harvested as barerooted liners or dug as balled and burlapped (B&B) stock. Fertile and well-drained soils are critical for producing quality nursery stock. Yet in these systems, continuous cycles of nursery production with successive crop harvests leads to removal of topsoil and reduced soil quality. In many scenarios, once a new crop has been transplanted to the field, the soil surface can remain bare for several months until grasses and broadleaf weeds are able to establish in the nonproduction middle areas between rows. In this period, bare soil is susceptible to structural breakdown and soil erosion that are made worse during heavy rain events and windy conditions (Figure 1). Soil degradation and erosion also occurs in nursery middles subjected to frequent tillage. The action of tilling will produce a shallow hardpan that limits water infiltration into the soil, making it more difficult for water to reach the crop root zone. Heavy equipment travelling through nursery middles for tillage and mowing also can degrade soil structure by compacting the soil leading to increased rainwater runoff and reduced water infiltration. Harvesting B&B crops removes large amounts of fertile topsoil from production areas, and it is cost- and labor-prohibitive to replace by importing new soil or organic material to the fields. Instead, producers can take advantage of implementing best management practices to their operation that will reduce soil degradation and improve soil properties, thereby maximizing the success of future crops. Cover crops are a versatile tool that can be used to reduce erosion and improve soil quality and cover crop options also can help restrict or suppress various crop pests.

FIGURE 1. Soil erosion resulting from heavy rainfall in a nursery field.

Types of Cover Crops

Cover crops are plant species that are grown during the fallow crop period or that can be intercropped between the primary crop rows to “cover” the soil while also providing numerous benefits. Cover crops can enhance soil properties by reducing soil erosion, facilitate increased rain infiltration into the soil, add organic matter and nutrients, and break up compacted soil. Cover crops can also complement existing pest management strategies by outcompeting weeds, providing habitat for beneficial insects, and reducing soil pathogen populations. There are two main groups of cover crops: grasses [sudex (Sorghum bicolor x S. bicolor var. Sudanese), winter wheat (Triticum aestivum), etc.] and broadleaf plants (clovers, mustards, etc.). Broadleaf cover crops are also subdivided into legume and non-legume crops. Legumes are able to supply additional nitrogen through the symbiotic relationship that these crops have with nitrogen-fixing bacteria (Rhizobium spp.). Cover crops may also be referred to as cool season [crimson clover (Trifolium incarnatum), winter wheat, etc.] or warm season [cowpea (Vigna unguiculata), sudex, etc.] crops, which provides an indication of the time of year that the cover crop will be actively establishing and growing. In Tennessee, cool season cover crops will typically be planted in late August through October. Warm season cover crops are planted in mid- to late spring. Although cover crops can provide numerous benefits, care should be taken to avoid selecting species that may reseed and become weedy [i.e., annual ryegrass (Lolium multiflorum)] or that grow with a vining/spreading habit [i.e., hairy vetch (Vicia villosa)] that can quickly overtake small crops. Cover crops can be utilized throughout the nursery crop cycle, yet cover crop species should be selected based on the intended primary goal or benefit they provide, such as erosion control, correcting compacted soil, or increasing organic matter.

Cool Season Cover Crops

Cool season cover crops are a great option for use in Tennessee nurseries during the crop cycle. Cool season cover crop species will establish during the fall/winter, mature in late spring, do not interfere with nursery tasks, and can improve field access for harvesting crops during the winter months (Figure 2). Planting a mix of two or more cover crop species is recommended to maximize establishment and gain advantages of each individual cover crop species. The USDA National Resource Conservation Service local county offices are an excellent resource for providing details on selecting cover crop species for mixes and for recommended planting rates. Once the desired cover crop mix has been selected, growers can contact a cover crop seed vendor which can mix the cover crop seeds based on the recommended planting rates.

FIGURE 2. Cool season cover crop mix (triticale and crimson clover) in April at a commercial nursery in Tennessee.

Crimson clover has been widely used in Tennessee nurseries due to its ease of establishment, low growth habit, and nitrogen fixation capability. Crimson clover is an annual plant that can reseed naturally but should be replanted each year to achieve optimum benefits. To achieve maximum nitrogen fixation, all legume cover crop seed must be pre-inoculated or inoculant can be purchased separately and added to the seed mix. Cereal grasses such as cereal rye (Secale cereale), winter wheat, and triticale (×Triticosecale; a hybrid of rye and wheat) are choices that grow well in Tennessee, survive throughout the winter, and naturally senesce in May.

FIGURE 3. A cool season mix including tillage radish that was planted in February at a Tennessee nursery.

FIGURE 3. A cool season mix including tillage radish that was planted in February at a Tennessee nursery.

TABLE 1. Cool season cover crop mixes for Tennessee nurseries.

FIGURE 4. All-purpose overseeder used for planting cover crops in between nursery production rows.

Using Cover Crops in Nurseries

Cover crops can be grown on fallow ground, into which no nursery crops have been planted, as a preliminary step intended to increase soil organic matter prior to transplanting a future nursery crop. To start, plow and fill in root ball holes from the previous crop. A cover crop of sudex, which is a hybrid of sudangrass and sorghum that increases organic matter and suppresses weeds, can be sown May to June at 50 lb/acre. Sudex can grow up to 8 ft tall but should be mowed once it reaches 3 to 4 ft tall to maximize root and shoot biomass (Figure 5). In September, sudex should be turned under using a chisel plow. At this time, a cool season cover crop mix should be planted to stabilize the soil and suppress weeds during the winter. After taking these steps, the field is ready for transplanting a nursery crop the following spring. After planting the nursery crop, a cool season cover crop mix can next be planted into the row middles in late summer/early fall each year to help replenish soil organic matter during the crop production cycle. Alternatively, a cool season cover crop mix can be planted in late winter (February to early March) to help stabilize soil and suppress weeds in spring. With this latter choice, growers will generally receive less biomass production from cover crops started in late winter. There are several ways to manage the cover crop residue produced by fall and spring-planted cover crops. The cover crop can be left to senesce naturally, allowing the standing stubble to slowly decompose and helping to suppress spring-germinating weeds. A roller-crimper can be used to physically push the cover crop to the ground forming a residual mulch which provides short-term weed control and increases rainwater infiltration, yet this must be done prior to plant maturation (during cover crop flowering but before seed set). The cover crop can also be mowed to chop the residue which will facilitate decomposition into the soil.

FIGURE 5. Sudex cover crop growing in a fallow nursery field.

Planting cover crops does require additional expenses for seed, equipment, and labor. However, several cost-share programs are available through the Tennessee Department of Agriculture Agricultural Enhancement Program (TAEP) and the USDA National Resource Conservation Service (NRCS) that can help offset the cost of seeders and cover crop seed. Equipment (all-purpose overseeder and roller crimper) can also be rented through the Warren County Soil and Water Conservation District (931-668-4383). Recent research (see Additional Reading section), including work at Tennessee State University, has identified several benefits to using cool season cover crops in nurseries. Other benefits have included deterrence against flatheaded borer (Chrysobothris spp.) beetle attacks on young trees, suppression of soil-borne pathogens, and suppression of weed establishment. Our team’s ongoing research continues to develop best management practices for using cover crops in nursery production systems.

For more information on cover crops or weed control practices in nursery crop production, contact Dr. Anthony Witcher ( 931-815-5147, awitcher@tnstate.edu. )

Additional Resources

Managing Cover Crops Profitably. Sustainable Agriculture Research and Education (SARE). https://www.sare.org/resources/managing-covercrops-profitably-3rd-edition

Southern Cover Crops Council. https://southerncovercrops.org

Cover Crops. Appropriate Technology Transfer for Rural Areas Sustainable Agriculture Program. https://attra.ncat.org/topics/cover-crops

Flatheaded Borer Management in Nurseries with Winter Cover Crops. https://www.tnstate.edu/extension/documents/ Flatheaded%20Borer%20Management%20 with%20Cover%20Crops%201.pdf

Cover Crop Usage for the Sustainable Management of Soilborne Diseases in Woody Ornamental Nursery Production System. Canadian Journal of Plant Pathology 44:432-452. DOI: 10.1080/07060661.2021.2020336.