Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
There is grower interest in using cover crops between the middles of plastic mulched vegetables. Cover crops can provide weed control, serve as a living mulch, reduce soil splash and soil borne diseases, reduce soil on harvested fruits, and improve drainage between rows.
Research has been conducted in the region on potential cover crops for row middles with mixed success.
The most common row middle cover is overwintered rye that serves as a windbreak for early warm season vegetables when planted between every bed. The rye is fall planted and then beds for the plastic are tilled in the spring and plastic laid with the remaining rye between the beds. After elongation, the rye windbreak then is killed by a non-selective herbicide prior to seed set. This killed rye can be left to breakdown over time or can be rolled (this is done by running a tractor down the rows). This rye residue helps to keep fruits of vining crops off the soil, producing a cleaner product with lower susceptibility to soil born diseases such as Phytophthora.
Another common cover crop for row middles being used in the region is annual ryegrass. Plastic beds are laid, and ryegrass is spun over top of beds to cover row middles. Rainfall or overhead irrigation is needed to germinate the ryegrass seed. This system has been successfully used in plasticulture strawberry production as ryegrass is a cool season annual that grows well in the fall and spring. It provides erosion control, particularly where irrigation is used for frost protection.
A novel system was developed by a Clemson University scientist in South Carolina for organic production. A vigorous low growing variety of Ladino (white) clover was planted in the fall and beds were tilled in and plastic laid in the spring. Another option is to lay plastic in the fall and overseed. Ladino clover produces rhizomes and will fill in up to the shoulders of the plastic but will not go over the plastic bed. When done right, the clover will provide near total weed control. It can also be mowed.
Some organic growers will also lay plastic beds in the fall and then plant grass/legume combinations such as rye-clover, ryegrass-clover, or perennial grass clover mixes (fescue-white clover for example). These will be maintained by mowing in the spring.
Some research has also been conducted in the region on using spring planted cover crops. In these systems the whole field is tilled, plastic mulch laid, and then cover crops are seeded between the beds using a modified drill. Seeds may also be broadcast and cultivated in. Many different crops have been tested but grasses are most preferred. Options include spring oats, annual ryegrass, sorghum-sudangrass, millets, teff, and rye or wheat. Spring oats can be planted as early as March and will elongate, annual ryegrass can also be early planted but is lower growing. Sorghum-sudangrass, millets, and teff are warm season grasses best planted in May and may reach as high as 5 feet. Rye and wheat when planted later than mid-April will not elongate (they have not been vernalized). Each grass has different management strategies and may be herbicide killed with shielded sprayers or selective grass herbicides, rolled green, allowed to grow naturally, or may be mowed. Options also depend on the crop being grown and available equipment. Mustards and radishes have also been tried in these systems.
Some challenges with spring planted cover crops include getting an adequate stand, weed control on plastic edges, herbicide selection for residual control and weed breaks through the cover.
Spring cover crops drilled between watermelon beds.