By Lynn Byczynski
Winter is a good time to be working on soil improvement, and you don't have to go out in cold weather to do it. By planting a cover crop in summer or fall and letting it overwinter, you can improve soil organic matter and soil fertility, suppress cool-season weeds, prevent soil erosion, and create a better seedbed for spring planting.
The best winter cover crops vary by region, depending on the crops' winter hardiness, but from a management perspective there are basically two types: crops that are killed by cold but have enough biomass to protect the soil and those that remain alive through winter and resume growth in spring.
Oats are an example of the first type. Sown in summer, they will put on a lot of growth before being killed by heavy frost. The killed mulch and root mass will hold the soil in place until the following spring. Other plants that may be grown for winterkilled mulch include field peas, oilseed radishes, and rapeseed. The disadvantage to this type of cover crop is that you have to clear the land and plant the cover crop early enough to get significant amounts of biomass to hold the soil over the winter. That could mean cover-cropping only on ground that grew spring vegetables, or it could require undersowing the cover crop in a summer crop such as corn. The big advantage of a winterkilled cover crop is that the mulch is easy to till under in spring and the land can be planted right away.
Cover crops that live through winter, or that go dormant and renew growth in late winter, can usually be planted after summer vegetable crops. They will grow in fall and establish root systems that protect the soil over winter. Some examples of crops that will survive winter (depending on winter low temperatures) include winter rye, winter wheat, hairy vetch, Austrian winter peas, and crimson clover. Winter rye and hairy vetch are recommended for the northern United States. In regions where these crops survive winter, they will grow vigorously in early spring. They will need to be mowed close to the ground to stop growth and then incorporated into the soil. Because decomposition of the cover crop debris will tie up nitrogen, it's a good idea to wait two or three weeks before planting.
Many growers use a mixture of cover crops from the two categories. Johnny's Fall Green Manure Mix is a blend of winter rye, field peas, ryegrass, crimson clover, and hairy vetch. The peas, clover, and ryegrass are winterkilled. The rye and hairy vetch regrow in spring.
• Winter Cover Crop Chart, by Pam Dawling in Growing for Market, September 2009
• Overview of Cover Crops and Green Manures, by ATTRA, the National Sustainable Agriculture Information Service
• Cover Crops and Living Mulches, Sustainable Practices for Vegetable Production in the South
• Improve Your Soil with Cover Crops, by Cornell University
Lynn Byczynski is the editor of Growing for Market and the publisher of The Hoophouse Handbook.