Cover Crops and Green Manures

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 may also be called a green manure. Cover cropping is an important part of a vegetable crop rotation plan in order to maintain soil health and manage insect, weed, and disease pressure.

Cover crops offer many benefits, but not all at once, nor from one species. Identifying management priorities helps one select the best cover crop to use in a given field. Do you need to protect the soil from intense erosion, alleviate compaction, suppress weeds, build organic matter, and add N or mop-up available nutrients after the growing season? Fast-growing, thick covers are best for erosion control and weed suppression; high biomass covers add the most organic matter; legumes provide N; cold-hardy covers can mop-up nutrients that remain in the soil at the end of the growing season.

Cover crops can be alternated with vegetables in many ways, including: as a winter cover sown in early fall; as a summer cover sown in late May or June; as a spring cover sown as soon as the ground can be worked; as a strip crop in-between rows, beds or blocks of vegetables; or as a long-term fallow in a field taken out of vegetable production for a season or more.

To get a strong cover crop stand requires good soil to seed contact, uniform seed distribution and seeding depth, and adequate soil moisture and fertility. A weak or inconsistent cover crop stand will not provide many benefits to soil health and may allow for high levels of weed growth within the cover. Recommended seeding rates for a cover crop vary depending on the equipment used and soil conditions. Drilling requires less seed than broadcasting as it enhances soil to seed contact, and less seed is recommended for a well-prepared seedbed with optimal moisture and nutrient levels than for sub-optimal conditions.

Fall-seeded cover crops. These include hardy small grains sown primarily for winter soil protection and nitrogen scavenging, and a few legume cover crops. Small grain options include rye, oats, wheat, spelt, and triticale. Rye is the most cold-tolerant and puts on growth even late into the fall during mild days. It develops a root system that holds soil in place over the winter and in early spring. Oats are not as cold tolerant as rye and not at all winter hardy so they create a winter-killed ground cover that is easily incorporated before planting vegetables the following spring. Wheat, spelt and triticale grow more slowly than rye and are easier to incorporate in the spring. Triticale can be sown earlier to produce more fall growth; spelt grows well on low N soil. Hairy vetch is the most winter-hardy annual legume cover crop. It may be planted alone or in combination with small grains. The later these winter cover crops are planted the smaller the plants will be over the winter, so it is advisable to double or even triple the recommended seeding rate when sowing late in the fall. 

Spring-seeded cover crops. These are used to provide early-season soil cover, add organic matter and provide some weed suppression after a winter-killed cover crop or on land left bare over winter. Field peas are the only legume option to use as a source of N, though they are only a modest N-fixer. They can be mixed with oats. Yellow mustard can be used as a good source of organic matter, with potential for soil-borne disease suppression. It can also break up hardpans and suppresses weeds, as can annual ryegrass. These crops are sown as soon as the ground can be worked in early spring.

Early summer-seeded cover crops. These fast-growing crops are used primarily to suppress weeds and add organic matter. Commonly choices are Sudangrass (or sorghum-Sudangrass) and buckwheat. Both grow rapidly if there is sufficient warmth, moisture and fertility. Sudangrass is preferable for adding to soil organic matter as it produces a lot of biomass when grown for the entire summer; it also has a deep root system that helps reduce compaction and it can reduce root-knot nematode pressure. If a cover crop is needed for less time, and/or if weed suppression is the main goal, then buckwheat is preferable as it covers the ground earlier than Sudangrass, especially in early June, and needs only 35 to 40 days to produce most of its biomass whereas Sudangrass needs 60 to 70 days.

Late summer-seeded cover crops. These are sown after an early-harvested vegetable crop, a month or so two before frequent frosts; mid-August to mid-September in most locations. Winter covers such as rye or oats are an option; when sown early they will produce more fall growth. But when sufficient growing time remains in the season several other covers can be used including annual ryegrass, forage radish, hairy vetch, and mustard.

Overview of non-legume cover crops

Annual ryegrass, also called Italian ryegrass, is a turf grass with a dense, shallow root system. Its extensive root system tolerates compacted soil and makes it effective at mopping up available N remaining in the soil after a vegetable crop. It competes well with late summer annual weeds as well as winter annuals that start in the fall, such as chickweed. Ryegrass will tolerate a wide range of soils but performs best on moderately- to well-drained soils with high fertility. 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. Perennial ryegrass may winterkill in harsher zones. Sow from mid-summer to early fall at 5 to 10 lb/A if drilled, 15 to 30 lb/A if broadcast.

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

Cereal rye is commonly sown after cash crops are harvested in the fall. It is very hardy, an efficient N scavenger, adapted to a wide range of conditions, and seed is inexpensive. The latest-sown cover crop, it produces a lot of biomass if allowed to grow into late spring. This adds organic matter to the soil but may be difficult to incorporate prior to crop planting. In late spring rye must be carefully managed to prevent excessive growth, and allowed time to break down so it will not interfere with establishment of a subsequent cash crop.  Sow at 60 to 120 lb/A if drilled, 90 to 160 lb/A if broadcast, from late summer to mid-October in most areas. Incorporate in spring before it gets too rank for equipment to handle. Some growers leave narrow strips of rye untilled as windbreaks between blocks of crops in the spring.

Forage radish is a fall-seeded Brassica that is not winter hardy. This crop forms thick, white taproots that can grow 8 to 14 inches. Radishes are excellent at breaking up shallow layers of compacted soils; the end of the taproot can penetrate deeper layers of compaction. The roots die over the winter and leave channels so that the soil dries and warms up faster in the spring. Forage radishes also suppress fall weeds. Plant into a smooth seedbed. Sow 4 to 10 weeks before fall frost at 10 lb/A if drilled in good soil conditions or 13 lb/A if broadcast or drilled into sub-optimal condition. Using higher rates leads to overcrowding and weaker growth. Drilling gives a much better stand; broadcasting should be reserved for when the soil is too wet to drill. After seeding, roll the ground to improve seed-to-soil contact but do not break up soil aggregates. Plant at 1/4" to 1/2" deep. Forage radish can be planted with 40 lb/A of wheat for spring cover and weed suppression.

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

Mustard can be used as a fall-planted cover crop that winter-kills. It adds organic matter, breaks up a hardpan, and suppresses weeds in the following crop. Soil-borne diseases may be suppressed by glucosinolates in mustard and other Brassica family crops, but results may vary from year to year and in different locations. Further, different species and varieties contain different amounts of bioactive chemicals. Prepare a firm, weed-free seedbed with adequate levels of available N to assure a good stand. Sow mid-July through August at 5 to 12 lb/A if drilled and 10 to 15 lb/A if broadcast. Roll the ground to improve seed-to-soil contact but do not break up soil aggregates. In the spring, yellow mustard can also be frost-seeded or sown as soon as the ground can be worked. Do not let mustards go to seed. Mustards attract flea beetles and diamond-back moths, but the risk is lowest in the fall. They can also host Brassica crop diseases such as clubroot.

Oats are often 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/A if drilled, 110 to 140 lb/A if broadcast. Oat residues left on the soil surface may chemically suppress weed growth, and also 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.

Sudangrass and sorghum-Sudangrass are fast-growing, warm season crops that require good fertility and moisture to perform well (sorghum-Sudangrass is often referred to generically as Sudex, although that is a trademarked name). Under such conditions, their tall, rank growth provides excellent weed suppression. The heavy growth can be difficult to cut and incorporate if left unmanaged. Sudangrass growth is easier to manage because the stems are narrower, and it can be sown a little earlier than sorghum-Sudangrass. These crops provide abundant root biomass, which is useful for increasing soil organic matter. Mowing when 2' to 3' tall encourages root growth. Mowing several times during the season makes it easier to turn in residues later, and it promotes tillering and root growth. These crops may suppress root knot nematodes. Sow once soil has warmed to 60o F, in early summer at 35 lb/A if drilled, 40 to 50 lb/A if broadcast. Provide adequate moisture and apply N fertilizer if grown on low fertility soils.

Teff is a warm-season grass useful for suppressing weeds if sown at a high plant population. It has a fine plant structure that doesn't leave soil clumpy for the next crop. Although buckwheat and Sudangrass are more common choices for early-summer sowing, Teff tolerates dry conditions better. It also requires less maintenance: buckwheat must be controlled when it matures and Sudangrass should be mowed several times. Teff needs minimal mowing and generally does not produce seed, so volunteers are not an issue. Sow in June-July into a very firm seedbed so that the tiny seeds stay near the surface.  The crop needs 40 to 60 lb/A N, so follow a high-N vegetable or manure application. Sow 5 to 8 lb/A raw seed or 8 to 10 lb/A coated seed, or if soil moisture is uneven. Use a Brillion seeder or broadcast followed by roller to press seed into soil. Plant 1/8" to 1/4" deep. Needs frequent light rain or irrigation for rapid uniform emergence.

Overview of legume cover crops

Legume cover crops are often used when "free" nitrogen is desired for a subsequent cash crop with high N demand. Legumes generally require good drainage and adequate P fertility (other than N). Most grow slowly at first so they do not compete well with weeds until established. Drill seed for best stands. Treat legume seed with the proper inoculant to ensure that root nodules are formed. Legume cover crops can be sown with a nurse crop such as oats to provide early ground cover and weed suppression during establishment. When legume cover crops with flower buds are mowed, tarnished plant bugs may be driven into adjacent vegetable crops.

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 forage 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/A if combined with a grass nurse crop, otherwise seed at 10 to 15 lb/A; drill if possible.

Hairy vetch is a winter-hardy, annual legume that is an effective N-fixer. It is useful in vegetable crop rotations as a tool for growing N without taking land out of cash crop production. Once established, it is good at weed suppression and soil conditioning. Generally this cover crop is seeded in late summer from mid-August to mid-September in most of New England. To gain the most N benefit, it should be allowed to grow until early flowering, about mid-May, before being incorporated the following spring. Sow vetch at 15 to 20 lb/A if drilled, 25 to 40 lb/A broadcast. Use vetch/pea type inoculant (not crown vetch type.) Since it is slow to establish, sow vetch with a nurse crop such as rye at 30 to 40 lb/A or oats at 40 to 50 lb/A. The grass takes up unused soil N and ensures a good winter ground cover for erosion control. 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 biomass produced and more soil residue after incorporation which is better suited to transplanting. Hairy vetch can also be seeded in early spring or summer and allowed to grow until the following spring.

Red clover is a short-lived perennial that is somewhat tolerant of acidic or poorly drained soils. It is useful for adding N and organic matter to soils on land that is taken out of production for a season or two. Mammoth red clover produces more biomass for plow-down than medium red clover, but does not regrow as well after mowing. Mammoth red clover will often establish better than medium in dry or acid soils. Seed in early spring or late summer or undersow in early summer into corn, winter squash before it vines, and other crops if soil moisture is plentiful. Sow at 8 to 10 lb/A if drilled, 10 to 12 lb/A if broadcast. Can be mixed with Sudangrass, sown at half the recommended rate, seeded in early summer.

Sweet Clover is a deep-rooted biennial (except for some annual types) that is adapted to a wide range of soils. It is a good soil-improving crop with a strong taproot that penetrates into subsoil, reducing compaction. Yellow sweet clover is earlier maturing and somewhat less productive than white sweet clover. Sow in early spring or summer at 6 to 10 lb/A if drilled, 10 to 20 lb/A if broadcast. Heavy growth is produced in the spring after overwintering. Incorporate in late spring or mid-summer at full flowering.

Soybean or cowpea. These warm-season legumes have potential as cover crops sown in early summer to provide some weed suppression and add N to the soil. They are frost and drought sensitive. Though typically grown for their seeds these crops will primarily produce foliage if long-season varieties are used in the Northeast. Forage cultivars may produce more biomass than horticultural varieties. Drill at 30 to 40 lb/A if drilling, 60 to 100 lb/A if broadcasting; use high rates in sub-optimal conditions, or to improve weed suppression. Avoid damaging seed when handling. Plant 1 to 2 inches deep, firm up the soil and provide adequate moisture to get good germination. Good soil seed contact and well-drained soils are needed to establish strong stands. Use cowpea/peanut, or soybean type inoculant. Can be grown in mixture with Japanese millet or Sudangrass; the latter is taller and may suppress legume growth via shading so used reduced rates.

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. It is suited for use in walkways or alleys. Once established, it provides long-term cover, either alone or with a low-growing turfgrass. It can be used in high traffic areas to minimize soil compaction and improve soil health. White clover tolerates wet conditions. Sow in early spring, frost-seed in March, or seed in early fall, along with a turfgrass, at 3 to 9 lb/A if drilled, 5 to 14 lb/A if broadcast.

Cover crop mixtures. Cover crop mixtures are used to hedge against failure of one component and to get some of the benefits of both. A grass will usually establish quickly, holding soil in place and "nursing' the legume along. By taking up 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.

Interseeding, or under-sowing a cover crop into a standing cash crop is a way get a jump on the fall/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.

For more information:

Managing Cover Crops Profitably: www.sare.org/publications/covercrops/covercrops.pdf

Cover Crops for Vegetable Growers: http://www.hort.cornell.edu/bjorkman/lab/covercrops/