Cover Crops and Green Manures

Cover crops are grown to protect and/or enrich the soil, rather than for short-term economic gain. When incorporated into the soil for fertility, a cover crop may be called a green manure. Cover cropping is an important component of vegetable crop rotations, as cover crops can maintain soil health and manage insect, weed, and disease pressure.

Cover crops can provide a range of benefits, depending on the species. Identifying goals and management priorities is key in selecting the best cover crop to use in a given field. Various cover crop types and species can protect the soil from intense erosion, alleviate compaction, suppress weeds, build SOM, add nitrogen, or scavenge excess soil nutrients until the following season. Fast-growing, thick cover crops are best for erosion control and weed suppression; high biomass cover crops add the most organic matter; legumes provide N; cold-hardy cover crops can take up nutrients that remain in the soil at the end of the growing season.

Cover crops can be added to a vegetable rotation at several points in the year.  They can be grown in the winter when sown in early fall, in the summer when sown in late May or June, as a spring cover sown as soon as the ground can be worked, as an intercrop 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.

When growing cover crops from fall until spring, it is important to consider the cold hardiness and biomass production of potential species. Winter annual species, such as winter rye, dependably overwinter and provide large amounts of biomass by spring. Such species are suitable for subsequent warm-season cash crops, and work well for no-till and zone-till systems. On the other hand, high-residue winter-killed cover crops, like sudangrass, provide substantial ground cover over the winter (if seeded early in fall) while still allowing early spring planting.  Low-residue winter-killed cover crops like forage radish, oat, and field pea allow for planting of early season small seeded crops and are suitable for operations with limited tillage equipment.

Growing a robust cover crop stand requires good soil-to-seed contact, uniform seed distribution and seeding depth, and adequate soil moisture and fertility. A weak or spotty stand will not provide full benefits to soil health and can 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 generally 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. When broadcasting seed, germination rates can be improved through shallow incorporation, via tilling or disking, or rolling or culti-packing.

Having the appropriate termination equipment and labor is imperative to successful cover crop management. Letting cover crop biomass grow beyond what equipment can handle will make termination and incorporation efforts very difficult. It is also important not to let cover crops go to seed, which can happen when they are left to mature in the field. This can lead to long-term weed problems with some species.  Selecting appropriate, manageable species is key for avoiding these problems.

Fall-seeded cover crops. These include hardy small grains sown primarily for winter soil protection and nitrogen scavenging, and a few legume species. Small grain options include rye, barley, oats, wheat, spelt, and triticale. Rye is the most cold-tolerant and puts on growth even late into the fall when days are mild. It develops a root system that holds soil in place over the winter and in early spring. Oats and barley are not winter-hardy, and create a winterkilled ground cover that is easily incorporated before planting vegetables the following spring. Wheat, spelt and triticale grow more slowly than rye or barley and are easier to incorporate in the spring. Triticale can be sown earlier to produce more fall growth; spelt grows well on low N soils. Hairy vetch is the most winter-hardy annual legume cover crop. It may be planted alone or in combination with small grains, which will boost its biomass production and nitrogen delivery. Later plantings of these winter cover crops will result in smaller plants 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. Legumes can be mixed with oats or barley, which serve as nurse crops to outcompete weeds as legumes get established. Yellow mustard can be used as a good source of organic matter, with potential for soil-borne disease suppression. It can also suppress 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. Common 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 SOM, as it can produce tremendous amounts of biomass when grown for the entire summer. It also has a deep root system that reduces 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 35-40 days to produce most of its biomass whereas sudangrass needs 60-70 days.

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

Non-legume cover crops

Annual ryegrass, also called Italian ryegrass, is a turf grass with a dense, shallow root system. This root system tolerates compacted soil, making it effective at scavenging excess available soil N. It competes well with late summer annual weeds, as well as winter annuals that germinate in the fall, such as chickweed. This grass 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 the 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 winter-kill; however, it may overwinter in milder areas. Sow from mid-summer to early fall at 10-20 lb/A if drilled, or 20-30 lb/A if broadcast.

Buckwheat is a very fast-growing summer annual used to protect the soil, add organic matter, and suppress weeds for a month or two between vegetable crops. It grows well on nutrient-poor soils, but requires good tilth and drainage. It decomposes rapidly, making it easy to incorporate. Timely termination is important (within 7-10 days of flowering), so it does not become a weed in subsequent crops. Sow from early to mid-summer at 50-60 lb/A if drilled, or 90 lb/A if broadcast.

Cereal rye, also known as winter rye, is commonly sown after cash crops are harvested in the fall. It is inexpensive, very hardy, an efficient N scavenger, and adapted to a wide range of conditions. The latest-sown cover crop, it produces ample biomass if allowed to grow into late spring. This adds organic matter to the soil, but can be difficult to incorporate prior to crop planting. Late spring growth must be carefully monitored to prevent full maturation and to allow time for the residue to break down. Otherwise, the carbon-rich biomass may tie up soil N, interfering with subsequent crop N requirements. Sow at 60-120 lb/A if drilled, or 90-160 lb/A if broadcast, from late summer to mid-October in most areas. Incorporate in spring before it gets too large for equipment to handle. Some growers leave narrow strips of rye untilled as windbreaks between blocks of crops in the spring.

Forage radish, oilseed radish, and tillage radish are late summer-seeded brassicas that are not winter-hardy. These crops form thick, white taproots that can grow 8-14 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, leaving channels that allow soil to dry and warm up faster in the spring. Radishes also suppress fall weeds. However, some vegetable growers with several Brassica cash crops in their rotation avoid this cover crop, to minimize Brassica-specific pests and diseases. Plant into a smooth seedbed. Sow 4-10 weeks before fall frost at 5-10 lb/A if drilled in good soil conditions or 10-13 lb/A if broadcast or drilled into sub-optimal conditions. Sowing higher rates leads to overcrowding and weaker growth. Drilling produces 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. Forage radish can be planted with 40 lb/A of wheat for spring cover and weed suppression. Higher seeding rates will increase leaf growth for weed suppression, while lower seeding rates will produce deeper tap roots for alleviating compaction.

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, providing ground coverage from early summer through fall without mowing if sown heavily. Sow at 20-25 lb/A if drilled, or 30-40 lb/A if broadcast. This species performs poorly on sandy soils without supplemental fertilization.

Mustard can be used as a fall-planted winterkilled cover crop. It adds organic matter, and suppresses weeds in the following crop. Soil-borne diseases are suppressed by glucosinolates in mustard and other Brassica family crops, but results may vary from year to year and across locations. Different species and varieties contain varying amounts of bioactive chemicals. To increase the benefits of biofumigation with mustards, the cover crop should be flail mowed at peak bloom and then incorporated immediately before a rain event. Plan to either roll the soil and/or cover the area with a tarp to trap in the gases from the glucosinolates. When planting, prepare a firm, weed-free seedbed with adequate levels of available N to ensure a good stand. Sow any time the soil temperature is above 40ºF and the field is available for 5-7 weeks at 5-12 lb/A if drilled or 10-15 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; they can easily become weed problems. Mustards attract flea beetles and diamond-back moths, and can host Brassica pathogens such as clubroot.

Oats are often used as a winter cover crop that protects soil without requiring intensive management in the spring, since they are frost-killed. Shallow incorporation of residues may still be necessary before crop planting. Enough growth is needed before first frost to provide adequate ground coverage, so plant from mid-August to mid-Sept in most areas. Sow 80-110 lb/A if drilled, or 110-140 lb/A if broadcast. Oat residues left on the soil surface can suppress weeds through allelopathy (chemical suppression), or via a physical barrier to emergence. Oats are a good cover crop species to plant any time during the spring or fall for quick coverage.

Sudangrass and Sorghum-Sudangrass (or sudex) are fast-growing, warm-season species that require good fertility and moisture to perform well. Under such conditions, their tall, prolific 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 SOM. Mowing when 2-3 ft. tall encourages root growth. Mowing several times during the season makes it easier to turn in residues later, and promotes tillering and root growth. These crops may suppress root knot nematodes. Sow once soil has warmed to 60ºF, in early summer at 35 lb/A if drilled, or 40-50 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 density. 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 cover crop species, teff tolerates dry conditions better. It also requires less maintenance compared to buckwheat, which must be controlled when it matures to prevent seed set, and sudangrass, which 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 40-60 lb/A N. Sow 5-8 lb/A raw seed, or 8-10 lb/A coated seed or if soil moisture is uneven. Use a Brillion seeder or broadcast followed by roller or culti-packer to press seed into the soil. Needs frequent light rain or irrigation for rapid uniform emergence.

Legume cover crops

Legume cover crops are often used when "free" nitrogen is desired for a subsequent cash crop with high nitrogen demand. Legumes generally require good drainage and adequate phosphorus fertility (other than nitrogen). An abundance of available soil N will not inhibit growth, but reduce biological nitrogen fixation. Most legume species grow slowly at first, so they do not compete well with weeds until established. Drill seed for best stands. Treat legume seed with the appropriate inoculant to ensure optimum nitrogen fixation, unless the field has a known recent planting of the same species. Legume cover crops can be sown with a nurse crop such as winter rye 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 2-3 years of forage production. Alfalfa fixes large amounts of nitrogen that can meet most or all of the needs of a subsequent vegetable crop if multiple cuts are made before it is turned in. Seed in early spring at 6-10 lb/A if combined with a grass nurse crop, or otherwise seed at 10-15 lb/A; drill if possible.

Hairy vetch is a winter-hardy annual legume that is an effective nitrogen fixer. It is useful in vegetable crop rotations as a tool for providing nitrogen without taking land out of cash crop production. Once established, it is good at weed suppression and soil conditioning. In most of New England, this cover crop is seeded in late summer, from mid-August to mid-September, and over-wintered. To gain the most nitrogen benefit, it should be allowed to grow until early flowering, about mid-May, before being incorporated. Sow vetch at 15-20 lb/A if drilled, or 25-40 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 30-40 lb/A, or oats at 40-50 lb/A. The grass takes up unused soil N and ensures a good winter ground cover for erosion control, while also providing the vining vetch species a natural trellis to produce more biomass. Oats will not overwinter, leaving the vetch alone the following spring and making for easier management ahead of direct seeded crops. When planted with rye, more overall biomass is produced, which is more suitable for transplanting into rather than direct-seeding after termination. 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 and poorly drained soils. It is useful for adding nitrogen and organic matter to soils on land that is taken out of production for a season or two. Mammoth red clover produces more biomass than medium red clover, but does not regrow as well after mowing. Mammoth red clover will often establish better than medium red clover 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 8-10 lb/A if drilled, or 10-12 lb/A if broadcast. It 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 cover crop with a strong taproot that penetrates subsoils, reducing compaction. Yellow sweet clover is earlier maturing and somewhat less productive than white sweet clover. Sow in early spring or summer at 6-10 lb/A if drilled, or 10-20 lb/A if broadcast. Heavy growth is produced in the spring after overwintering. Incorporate in late spring or mid-summer at full flowering.

Soybean and Cowpea are warm-season legumes that have potential as cover crops sown in early summer to provide some weed suppression and add high amounts of nitrogen to the soil. They are sensitive to frost and drought. 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, which will optimize nitrogen delivery. Drill at 30-40 lb/A, or 60-100 lb/A if broadcasting; use high rates in sub-optimal conditions, or to improve weed suppression. Avoid damaging seed when handling. Plant into a firm seedbed and provide adequate moisture for good germination. Good soil-seed contact and well-drained soils are needed to establish strong stands. Use cowpea/peanut, or soybean type inoculant. These can be grown in mixture with Japanese millet or sudangrass; the latter is taller and may shade out legumes, so reducing seeding rates is recommended.

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 3-9 lb/A if drilled, 5-14 lb/A if broadcast.

Cover crop mixtures are used to diversify benefits as well as provide resilience should one species or another fail. A grass will usually establish quickly, holding soil in place and ‘nurse' the legume along. By taking up available soil nitrogen, the grass promotes biological nitrogen fixation by the legume species. Fertilization with nitrogen, or the absence of mowing, favors growth of grass over legume. Planting multiple cover crop species can increase the number of benefits provided, but can also decrease the magnitude of each benefit. For example, several grasses and brassicas in a mix will result in less nitrogen fixed by legumes. Quick-growing, competitive grass and brassica species seeding rates should be reduced in mixes, while less competitive legumes should be kept close to monoculture seeding rates. 

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 cover crops, 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 the cultivation, before the crop canopy closes. Less competitive crops such as carrots, onions, etc., are poorly suited to intercropping. Vigorous vegetables, like winter squash and sweet corn, can better 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, etc. Traditional winter cover crops 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 when no irrigation is available, or if there are disease problems in the crop that necessitate post-harvest tillage. It should also be noted that interseeding cover crops can lead to increased rodent damage to crops like winter squash.

For more information:

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

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