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Cover Crops for Vegetable Growers

Late-summer legumes for overwintering

Medium red clover and hairy vetch

Late summer is one of the best opportunities for vegetable growers to establish a legume cover crop to supply the following year’s nitrogen needs. Planting in late summer allows time for the legume to establish in the fall. Enough fall growth is needed for the plants to develop a strong root system and enough shoot growth to provide valuable winter cover. Most of the nitrogen is fixed during growth in May.

There are two legume cover crops that are reliable and economical: medium red clover and hairy vetch. Red clover is normally spring seeded, but the easy availability, low seeding rate, and high nitrogen fixation make it an attractive choice in the late summer as well.

Legume winter cover crops provide a great deal of nitrogen. If well-managed, they can provide as much nitrogen for the next crop as fertilizer containing 100 to 150 lb/ac of nitrogen. Whether you are committed to raising all your nitrogen on the farm or concerned about paying today’s high prices for nitrogen fertilizer, the nitrogen contribution from a late summer cover crop is an attractive option.

Choosing the legume
Hairy vetch is the best-known winter legume for vegetable rotations in the mid-Atlantic region. It is at its northern limit in much of New York, but does very well there. Depending on your needs, medium red clover can be more versatile and economical than hairy vetch. The decision of which legume to use is based on subtle criteria:

When will the summer legume be planted? If timing demands that the legume be overseeded into a vegetable crop at the time of the last cultivation, the choice must be red clover. It has the ability to establish in shade after being broadcast on the soil surface. Vetch seed needs to be drilled for reliable fall growth, and therefore best follows harvest and incorporation of the vegetable crop.

What is the current condition of your soil? If the soil is dry, clover will have a better chance of establishing because it needs less moisture than the large-seeded hairy vetch. Clover is also preferred if soil compaction is a problem. Neither cover crop does well on compacted soil, but hairy vetch is more sensitive to the resulting waterlogging.

What other crops do you plant on the field? If you raise small grain on the field, clover must be the choice. Hairy vetch has hard seed that will germinate in future small grain fields and produce vetch seed that contaminates the grain.

Nurse crops
Both of these legumes benefit from a nurse crop. Hairy vetch requires a nurse crop to reliably overwinter, and nurse crops are valuable for medium red clover as well. The nurse crop helps keep down weeds during the legumes' slow establishment, reduces winter-kill, and provides physical support to reduce matting under the snow and during spring growth.

For both legumes, a small-grain nurse crop should be sown at a low rate (approx 40 lb/ac). Wheat overwinters to provide support in the spring, especially for hairy vetch. It is likely the best choice for most situations. Oats can be used for early spring planting, as they die during the winter, allowing faster breakdown for earlier vegetable seeding. Rye is the classic nurse crop with vetch, but can be too vigorous for New York vegetable production.

Seed sources and sowing
Clover seed costs $25-30/ac (15 lb/ac at $1.75 - $2.00/lb). Hairy vetch seed costs $60 - $110/ac (40 lb/ac at $1.50 to $2.75/lb). Inoculum to insure nitrogen fixation costs $1 - $2/ac. The grain seed adds $5 - $10/ac. You can often buy clover and small grain seed of adequate quality for cover cropping at a significantly lower cost from local farmers.

Sow hairy vetch at 40 lb/ac. It can be sown at rates as low as 25 lb/ac, but the additional weed suppression and nitrogen fixation from the higher rate make it attractive for vegetable growers. Seed the small grain at 40 lb/ac. If the grain seeding rate is too low, winter-kill will be excessive. If the grain seeding rate is too high, competition will reduce vetch growth and nitrogen fixation. The vetch and grain seed can be mixed together in the drill.

Sow medium red clover at 15 lb/ac. You can broadcast the seed onto prepared ground or sow it with a grass seeder. If overseeded into a standing crop by broadcasting, it is worth using 20-25 lb/ac. Several varieties of red clover are available, but they don’t work nearly as well as medium red clover for this application.

Sow red clover in mid-August through early September. Later seeding results in inadequate growth. Even within this window, earlier sowing improves winter survival, nitrogen fixation, and weed control. The main cause of poor establishment of either crop is lack of moisture. New York usually receives adequate rain in late August and September for these crops to establish.

Spring management
In the spring, both crops are incorporated at early bloom (typically late May). Plan a rotation where vegetables are sown or transplanted in mid-June. The timing of incorporation maximizes nitrogen fixation and minimizes both regrowth and volunteer seed. Bloom time is determined mostly by spring temperature, so the date will vary. Both crops fix most of the nitrogen during the month of May, so premature incorporation costs a lot of nitrogen. In fact, if the field needs to go to vegetables in May, it is not worthwhile to do a fall-seeded legume because so little nitrogen will be fixed. In this situation, a more effective way to manage nitrogen is to have a vigorous overwintering cover crop that scavenges leftover nitrogen.

This article is intended for publication in Cornell Cooperative Extension newsletters, and similar outlets, that reach growers by early August.