Using Winter Kill Cover Crops as a Part of Your Vegetable Cropping System

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Cover crops that will put on significant growth in the fall and then die during the winter can be very useful tools for vegetable cropping systems and the University of Delaware, University of Maryland, and other universities in the region have been conducting research on a number of these winter killed crops for use with vegetables.

Winter killed cover crops that are late summer and fall planted include spring oats, several mustard species, and forage and oilseed radish. Earlier planted summer annuals (millets; sorghums, sudangrasses, and hybrids; annual legumes such as sun hemp or forage soybeans; buckwheat and many others) can also be used as winter killed species. Timing of planting will vary according to the species being used and winter killed species selection will depend on when fields will be available for seeding. Spring oats, mustards, and radishes can be planted from late August through September. Once into October, they do not put on adequate fall growth. Summer annuals should be planted in late July or during August for use in a winter killed system to obtain sufficient growth.

The winter of 2011-2012 was extremely mild and gave us a good look at issues that occur when crops that normally winter kill do not. In our plots at the Georgetown, DE research farm last winter, forage radish, oilseed radish, spring oats, and edible greens type mustard (Tendergreen) did not winter kill completely. All the biofumigant mustards (Pacific Gold, Idagold, Caliente, and Kodiak) winter killed completely (as did summer annuals).

The following are several options for using winter killed species with vegetables:

1) Compaction mitigation for spring planted vegetables. Where there are compacted fields, the use of forage radishes has worked very well as a winter killed cover crop by “biodrilling”. The extremely large taproot penetrates deep into the soil, and after winterkilling, will leave a large hole where future crop roots can grow. Oilseed radish also provides considerable “biodrilling”. Winter killed radishes works well with spring planted crops such as peas, early sweet corn, and early snap beans.

2) Early planted vegetables. A wide range of early planted vegetables may benefit from winter killed cover crops. For example, peas no-till planted or planted using limited vertical tillage after a winter killed cover crop of forage radish, oilseed radish, or winter killed mustard have performed better than those planted after conventional tillage. Early sweet corn also has potential in these systems as do a wide range of spring vegetables. Winter killed radishes and mustards also have the advantage of outcompeting winter annual weeds leaving relatively weed free fields and also in recycling nutrients from the soil so that they are available in the spring for early crops (decomposition has already occurred).

3) Mixed systems with windbreaks for plasticulture. By planting planned plasticulture bed areas with winter killed cover crops and areas in-between with cereal rye you can gain the benefits of these soil improving cover crops and eliminate the need make tillage strips early in the spring. The winter killed areas can be tilled just prior to laying plastic.

4) Bio-strip till. By drilling one row of forage or oilseed radish and other adjacent rows with rye or other small grains, you can create a biodrilled strip that winter kills and that can be no-till planted into the spring without the need for strip-till implements. This opens up dozens of options for strip tilling (seed or transplanted) spring vegetables.

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