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Winter cover crops

Sowing a cover crop into or after the final vegetable crop of the year has several advantages. Benefits for the soil include prevention of erosion, reduced leaching of nutrients, and improvement of soil structure by the dense root systems of the cover crop and the green foliage that will be incorporated into the soil in the spring. As explained in Tilth & weeding, maintaining good soil properties by using cover crops makes weeding easier. Cover crops also have a direct competitive effect on weeds, particularly species like common chickweed and shepherd's-purse that thrive in cool weather.

Various crop species in a garden are harvested on different dates, and this will determine to a large extent the type of cover crop that you will want to use. In New York and New England only two commonly used cover crop species survive the winter well. These are grain rye and hairy vetch. Annual rye grass (a finer-leaved and smaller seeded grass than grain rye) can also over winter, but the seed is so commonly contaminated with perennial rye grass that the species cannot be recommended. For information on cover crops suitable for other parts of the U.S.A., see Resources.

Hairy vetch is a legume that can convert nitrogen gas in the air into fertilizer nitrogen (nitrogen fixation). It grows in a lush, tangled mat that by May may be three to four feet thick. It must be sowed by the last week of Aug. to have a good chance of surviving the winter, but sowings in early Sep. may be successful if the autumn is long and mild. If it is sowed earlier than late July, the plants may flower and then die during the winter. Birds and mice eat the seeds, and the germinating seeds are attacked by many species including slugs and earthworms. Consequently, hairy vetch seed should be incorporated into the soil by light hoeing. It is a good cover crop to follow early crops of potatoes, sweet corn or cole crops, and can be undersown into any crop that is not casting dense shade. Hairy vetch seed is relatively expensive ($1.50 to several dollars per pound) and usually has to be ordered from a seed company. The seed is cheaper if purchased in bulk, and if kept cool and dry it will stay viable for at least a decade. Large scale farmers typically sow hairy vetch at 0.05 to 0.2 lb/100 ft2, but in home gardens, rates of 0.2 to 0.5 lb/100 ft2 are financially practical, and these higher rates quickly smother out most annual weeds.

Rye does not "fix" nitrogen, but it is an effective scavenger of any nutrients that were not used by the crop and produces huge amounts of green matter. By late May it will be five to seven feet tall on fertile ground, so cutting it back in late April or early May is wise. If you do not keep it cut back, the long, coarse flowering stems will be hard to incorporate into the soil. Rye can be planted from late Aug. to mid Sep. in New York State. Consequently, it can be planted after frost killed crops like squash, tomatoes and peppers. Although rye will sometimes establish if sown onto the surface of loose soil, it establishes best if it is hoed lightly into the ground. This also has the salutary effect of removing weeds after the crop has been harvested, a time when most gardeners ignore that part of the garden. Rye can also be interseeded into fall cole crops and late varieties of sweet corn at the final hoeing. The seed is usually available locally at farm and garden stores. Large scale farmers sow rye at 0.25 to 0.40 lb/100 ft2, but since the seed is usually cheap ($0.10 to $1.00/lb), using 1-2 lb/100 ft2 is often financially feasible and is more effective for smothering weeds. Always check cover crop seed for contamination with weed seeds before you sow it, but be especially cautious of inexpensive, locally produced rye seed.

Winter wheat can be used as a substitute for grain rye. It is less competitive than rye but it produces less growth in the spring, and the flowering stalks form later in the season, so it is easier to incorporate. Untreated seeds are often difficult to obtain locally and expensive to order from seed companies. Planting and management practices generally follow those for rye.

Both rye and hairy vetch should be cut before incorporating the plants in the spring. This can be done with a string trimmer, a grass whip or a scyth. If the cover crop has grown tall, cut the top first and then cut closer to the ground. Rye stems longer than 12 to 18 inches will make rototilling the ground completely impractical! Hairy vetch is much easier to incorporate. Most experienced growers incorporate their cover crops about two weeks before they plan to plant. This gives the material time to rot and release nutrients so that the young crop plants are not starved by competition with decomposers. Hairy vetch, however, rots much more quickly than rye. A lag between incorporation and seedbed preparation also gives many weed seeds an opportunity to germinate and be killed before the crop is planted (Tillage & germination).

Red, white, alsike and sweet clovers will overwinter in New York and New England. Crimson clover will overwinter in Maryland and Delaware. Clovers usually must be established before about July 1 to be successful. Consequently, they are usually established by interseeding into a summer crop. They do well in sweet corn, but tomatoes and squash are often too competitive to allow the clover to survive. Since the seeds are small, they should be sown onto the surface of loose soil after the crop is well established and competitive. Summer sowings of clover should be watered to ensure establishment. Since the young plants are small and slow growing, they are ineffective for smothering out weeds until well into the fall. Since hoeing kills the young clover, hand pulling of weeds will probably be unavoidable unless your garden is exceptionally weed free.

Oats usually winter kill in New York and New England, but if planted by early Sep. and sown thickly they will provide substantial soil cover before death. An advantage of oats as a cover crop is that they are already partly decayed by early spring and so make a good cover to precede early-planted crops. Sowing rates for home gardens range from 0.1 to 1.0 lb/100 ft2.

For more information on cover crops, see Resources.