Cover crops are grown to protect and/or enrich the soil rather than for short term economic gain. When turned into the soil, a cover crop is called a green manure, so the terms are reasonably interchangeable. Cover crops are an important part of a crop rotation plan to maintain soil health and reduce insect, weed, and disease pressure.

Cover crops protect the soil from wind and water erosion, and they can help alleviate compaction. With the exception of legumes, which fix nitrogen from the atmosphere, cover crops don't actually create nutrients, but they can conserve nutrients which may otherwise be lost through leaching. Some deep rooted crops can obtain nutrients from below the root zone of most vegetables. When these cover crops are turned under, the nutrients will be released to the upper zone of soil. Fast-growing cover crops are well-suited to suppressing weeds, by "smothering" them and starving them for light. Use high seeding rates if cover crops are grown for weed suppression.

Depending on their growing requirements, cover crops can be sown after vegetable harvest, between a spring and fall crop, or by overseeding into a standing vegetable crop after a final cultivation. They can also be used as a long-term fallow when a field is taken out of vegetable production for a season or more.

In selecting a green manure crop, consider the following: seed cost, winter hardiness (if applicable), ability to fix nitrogen, suppress weeds, and suitability to soil conditions, tillage equipment and the crop to follow. Here is a list of some common cover crops in New England and a description of their uses.

Nonlegumes

These are selected when N contribution to the soil is not a priority. They tend to grow more rapidly and thus are better at short-term weed suppression than legumes. Late-season grasses are useful for recovering leftover nitrogen after crops have been harvested.

Winter Rye is a common winter cover crop, sown after cash crops are harvested in the fall. It is very hardy, adapted to a wide range of conditions, and seed is inexpensive. The latest-sown cover crop, it produces a lot of biomass in the spring. This adds organic matter to the soil but may be difficult to incorporate prior to crop planting. Sow 60 to 120 lb/acre if drilled, 90 to 160 lb/acre if broadcast, from late summer to mid-October in most areas. Incorporate rye in early spring before it gets too rank for your equipment to handle.

Some growers leave narrow strips of rye untilled as windbreaks between blocks of crops.

Oats are used as a winter cover crop to protect the soil without requiring intensive management in the spring, because they are frost-killed. Shallow incorporation of residues may still be necessary before crop planting. Enough growth is needed before first frost to adequately protect the soil, so plant from mid-August to mid-Sept in most areas. Sow 80 to 110 lb/acre if drilled, 110 to 140 if broadcast. Oat residues left on the soil surface may chemically suppress weed growth, and act as a physical barrier. Oats are also a good cover crop to plant any time during the spring or fall to get a quick cover.

Annual Ryegrass is a low-growing cover crop that produces an extensive root system that is good at capturing leftover nitrogen. It is well suited to undersowing, after last cultivation of a cash crop, in order to establish a winter cover prior to harvest. Annual ryegrass is less expensive than perennial ryegrass, and is more likely to winterkill; however, it may overwinter in milder areas, and perennial ryegrass may winterkill in harsher zones. These crops form a dense sod that reduces erosion. Sow from mid-summer to September at 5 to 10 lb/acre if drilled, 15 to 30 lb/acre if broadcast.

Sudangrass and Sorghum-sudangrass (Sudex) are fast-growing, warm season crops that require good fertility and moisture to perform well. Under such conditions, their tall, rank growth provides excellent weed suppression. Such heavy growth can be difficult to cut and incorporate if left unmanaged. Mowing several times during the season makes it easier to turn in residues, and it promotes tillering and root growth, which reduces soil compaction. Sow once soil has fully warmed, in early summer at 35 lb/acre if drilled, 40 to 50 lb/acre if broadcast.

Buckwheat is a fast-growing summer annual that can be used to protect the soil and suppress weeds for a month or two between spring and fall cash crops. It grows fairly well on acid and low phosphorus soils. It decomposes rapidly, so is easy to incorporate. Mow or incorporate at flowering, prior to setting seed so it does not become a weed in subsequent crops. Sow from early to mid-summer at 50 to 70 lb/acre if drilled, 60 to 100 lb/acre if broadcast.

Japanese Millet is an annual grass that grows about 4 ft tall and can provide good weed suppression. It is about the stature of buckwheat but can keep the ground covered from early summer through fall without mowing if sown heavily. Sow at a rate of 20 to 25 lb/acre if drilled, 30 to 40 lb if broadcast. It performs poorly on sandy soils without supplemental fertilization.

Brassica cover crops such as Oriental mustard have been associated with disease suppression in a subsequent cash crop. When the green residues of the Brassica cover crop are incorporated, some pathogens may be suppressed, such as Verticillium in potato; Pythium, Fusarium and Rhizoctonia root rots in beans or peas; Phthium in lettuce; pink root in onion and Fusarium in carrot. This effect is associated with gluconsinolate compounds, which produce the sulfur smell associated with some brassicas.  Mustards, radish and rapeseed covers are broadcast at 8 to 20 lb/acre or drilled at 5 to 13 lb/acre.  Be aware that these crops can host flea beetles, crub root and other pests of Brassica vegetable crops.

Interseeding, or under-sowing, a cover crop into a standing cash crop is a way get a jump on the winter cover crop season and can help protect soil between rows from erosion and compaction.  When interseeding covers, sowing should be delayed enough to minimize competition with the vegetable crop, but early enough so the cover crop can establish well and then withstand the harvest traffic. Typically, a good time to sow is at last cultivation, before the crop canopy closes. Less competitive crops such as carrots, onions and the like are poorly suited to intercropping while vigorous vegetables, like winter squash and sweet corn, can often tolerate early-summer interseeding with a cover crop such as ryegrass and/or red clover. Late summer is a better time for interseeding crops like peppers, staked tomatoes, fall crucifers and the like. Traditional winter covers like rye, oats and/or hairy vetch can be used at that time. A good seedbed and timely rainfall or irrigation helps with establishment. Interseeding is not advisable if no irrigation is available or if there are disease problems in the crop that call for the field to be tilled after harvest.

Legumes

Sown when "free" nitrogen is desired for a subsequent cash crop with a high nitrogen demand. Legumes generally require good drainage and fertility. Most grow slowly at first so they do not compete much with weeds until well established. Drill seed for best stands. Mix seed with proper inoculant to ensure that root nodules are formed. Can be sown with a nurse crop such as oats to provide early ground cover and weed suppression during establishment. When legumes are mowed, tarnished plant bugs may be driven into adjacent vegetable crops increasing the likelihood of damage.

Red Clover is a short-lived perennial that is somewhat tolerant of acid or poorly drained soils. Mammoth red clover produces more biomass for plow-down than medium red clover, but does not regrow as well after mowing. Mammoth will often establish better than medium in dry or acid soils. Sow in early spring or late summer. Can be undersown in midsummer into corn, winter squash before it vines, and other crops if soil moisture is plentiful. Sow at 8 to 10 lb/acre if drilled, 10 to 12 lb/acre if broadcast.

White Clover is a low-growing perennial, tolerant of shade and slightly acid soil. Ladino types are taller than the Dutch or wild types. White clover is a poor competitor with weeds unless mowed. Suitable for use in walkways or alleys. Expensive seed. Sow in spring or fall at 3 to 9 lb/acre drilled, 5 to 14 lb/acre broadcast.

Sweetclover is a biennial (except for the annual type Hubam) that is deep-rooted and adapted to a wide range of soils. It is a good soil-improving crop with a strong taproot that opens up subsoil. Yellow sweetclover is earlier maturing and somewhat less productive than white sweetclover. Sow in early spring or summer at 6 to 10 lb/acre drilled, 10 to 20 lb/acre broadcast. Heavy growth is produced in spring after overwintering. Incorporate in late spring or mid-summer at flowering. May deplete soil of moisture, which can be a problem for subsequent crops in dry years.

Hairy Vetch has become increasingly popular as a cover crop. It can fix tremendous amounts of nitrogen. Generally this cover crop is seeded in the fall after August 15 or before mid September in most areas. It should be allowed to grow at least until mid May before plowdown. It is advisable to seed winter rye (30 to 40 lbs/acre) or oats (40 to 50 lbs/acre) with the vetch when seeded in the fall to take up unused nitrogen and to ensure a good ground cover for erosion control. Sow vetch at 15 to 20 lb/acre if drilled, 25 to 40 lb/acre broadcast. The oats will not overwinter, leaving the vetch alone the following spring, which is easier to turn under and prepare for direct seeding; with rye there is more residue to turn under which can be OK if transplanting. Hairy vetch can also be seeded in early spring or summer and allowed to grow until the following spring. Treat seed with a pea-type inoculant.

Alfalfa requires deep, well-drained soil with a pH near neutral for good growth. It is a long-lived perennial that is probably not worth the expense of establishment in a short-term rotation; it makes more sense if also used for production of livestock feed for 2 or 3 years. Alfalfa fixes large amounts of N that can meet most or all of the needs of a subsequent vegetable crop if allowed to regrow after cutting before it is turned in. Seed in early spring at 6 to 10 lb/acre if combined with a grass nurse crop, otherwise seed at 10 to 15 lb/acre; drill if possible.

Mixtures

Legumes and grasses are often mixed as cover crops to hedge against failure of one and to get some of the benefits of both. The grass will usually establish quickly, holding soil in place and "nursing' the legume along. By taking available soil N, the grass promotes N-fixation by the legume. Fertilization with N or the absence of mowing favors growth of grass over legume. Some common mixtures, in addition to vetch and rye described above, are red clover and oats (combine or mow oat heads, leaving established clover); ryegrass and white clover for mowed alleys. Timothy is often used as a nurse crop for alfalfa. It is advisable to trial unfamiliar cover crops or mixtures on a small scale to determine if they're suited to your climate and management resources before growing them widely. Note: N fixed in root nodules moves to the leaves and stems of legumes. If hay is harvested from the field prior to plowing, very little N will be contributed to the subsequent crop unless regrowth is allowed to occur.

For more information, see the book Managing Cover Crops Profitably, available on-line at: http://www.sare.org/publications/covercrops/covercrops.pdf or call (301) 374-9696 to order.