10.3: Types of Cover Crops

Many plant species can be used as cover crops. Legumes and grasses (including cereals) are the most extensively used, but there is increasing interest in brassicas (such as rapeseed, mustard and oilseed radish, which is also known as forage radish) and continued interest in summer cover crops, including buckwheat, millets and summer legumes such as cowpeas and sunn hemp. Some of the most important cover crops are discussed below.

Legumes

Leguminous plants are often very good cover crops. Summer annual legumes, usually grown only during the summer, include soybeans, cowpeas and sunn hemp. Winter annual legumes that are normally planted in the fall and counted on to overwinter include winter field peas (such as Austrian), crimson clover, hairy vetch, Balansa clover and subterranean clover. Crimson clover reliably overwinters in hardiness zone 6 and farther south, and sometimes in zone 5. Winter peas have a similar region of adaptation as crimson clover, although it might be usable a little farther north if planted early enough. Berseem clover will overwinter only in zones 8 and above. Hairy vetch is able to withstand fairly severe winter weather. Balansa clover is still being evaluated in colder regions but has in some cases overwintered in zone 5. Sweet yellow clover is an example of a biennial legume, while perennial legumes include red clover, white clover and alfalfa. Crops usually used as winter annuals can sometimes be grown as summer annuals in cold, short-season regions. Also, summer annuals that are easily damaged by frost, such as cowpeas, can be grown as a winter annual in the deep southern United States.

Figure 10.2. Root systems of five legume cover crops at early stages of growth (two months in a greenhouse). From left: alfalfa (winter perennial), yellow-blossom sweet clover (winter biennial), hairy vetch (winter annual), sunn hemp and cowpea (summer/tropical annual). Photos by Joseph Amsili.

One of the main reasons for selecting legumes as cover crops is their ability to fix nitrogen from the atmosphere and add it to the soil. But legumes need to be grown later in the spring—typically until a few weeks after cereals elongate—to reach the early flowering stage and achieve near-maximum nitrogen fixation. Legumes that produce a substantial amount of growth, such as hairy vetch, crimson clover, red clover and Austrian winter peas may supply over 100 pounds of nitrogen per acre to the next crop if allowed to grow to the flowering stage or longer. Other legumes may supply considerably less available nitrogen. Legumes also provide other benefits, including attracting beneficial insects, helping control erosion and adding organic matter to soils.

Inoculation. If you grow a legume as a cover crop, don’t forget to inoculate seeds with the correct nitrogen-fixing bacteria. Different types of rhizobial bacteria are specific to certain crops. There are different strains for alfalfa, clovers, soybeans, beans, peas, vetch and cowpeas. Unless you’ve recently grown a legume from the same general group you are currently planting, inoculate the seeds with the appropriate commercial rhizobial inoculant before planting. The addition of water to the seed-inoculant mix, just enough to moisten the seeds, helps the bacteria stick to the seeds. Plant right away, so the bacteria don’t dry out. Inoculants are readily available only if they are commonly used in your region. It’s best to check with your seed supplier a few months before you need the inoculant, so it can be specially ordered if necessary. Keep in mind that the “garden inoculant” sold in many garden stores may not contain the specific bacteria you need. Be sure to find the right one for the crop you are growing and keep it refrigerated until used.

Winter Annual Legumes

Crimson clover is considered one of the best cover crops for areas with mild climates, like the southeastern United States and the southern Plains, such as Oklahoma and parts of Texas. Where adapted, it grows in the fall and winter, and matures more rapidly than most other legumes. It also contributes a relatively large amount of nitrogen to the following crop. Because it is not very winter hardy, crimson clover is not usually a good choice in hardiness zones 4 or colder, and it can be marginal in zone 5 (snow cover and/or early planting can help with winter survival). Crimson clover survival can also suffer from poorly drained soil conditions. In northern regions, crimson clover can be grown as a summer annual, but that prevents an economic crop from growing during that field season. Varieties like Chief, Dixie and Kentucky Select are somewhat winter hardy if established early enough before winter. Crimson clover does not grow well on high-pH (calcareous) or poorly drained soils.

Field peas are grown in colder climates as a summer annual and as a winter annual over large sections of the South and California. They have taken the place of fallow in some dryland, small-grain production systems. Austrian winter peas (bred for winter hardiness) and Canadian field peas (bred for good spring growth) tend to establish quickly and grow rapidly in cool moist climates, producing a significant amount of residue: 2 1/2 tons or more of dry matter. They fix plentiful amounts of nitrogen, from 100–150 or more pounds per acre. Austrian winter peas will perform best as a winter cover crop if seeded in early fall.

Hairy vetch is winter hardy enough to grow well in areas that experience hard freezing, and it can be planted later than most other legumes. Where adapted, hairy vetch produces a large amount of vegetation and has an impressive root system (Figure 10.2). It fixes a significant amount of nitrogen, thereby contributing 100 pounds of nitrogen per acre or more to the next crop. Hairy vetch residues decompose rapidly and release nitrogen more quickly than most other cover crops. This can be an advantage when a rapidly growing, high-nitrogen-demand crop follows hairy vetch. Hairy vetch will do better on sandy soils than many other green manures, but it needs good soil potassium levels to be most productive. Where wheat is part of the rotation, hairy vetch should be avoided, as hairy vetch may volunteer in the wheat, and the seed sizes are similar enough to make it hard to separate the vetch seed from the wheat seed during harvest.

Subterranean clover is a warm-climate winter annual that, in many situations, can complete its life cycle before a summer crop is planted. When used this way, it doesn’t need to be suppressed or killed and does not compete with the summer crop. If left undisturbed, it will naturally reseed itself from the pods that mature belowground. Because it grows low to the ground and does not tolerate much shading, it is not a good choice to interplant with summer annual row crops. Balansa clover is a new winter annual clover getting some use. The exact extent of its winter hardiness is still a question, and it is currently recommended for growing in zone 5 and farther south. It produces excellent spring growth, but because Balansa clover is a relatively new cover crop species, some small-scale testing for various uses may be appropriate for your location, including evaluation of how it does in mixes.

Summer Annual Legumes

Berseem clover is grown as a summer annual in colder climates. It establishes easily and rapidly and develops a dense cover, which makes it a good choice for weed suppression. It’s also drought-tolerant and regrows rapidly when mowed or grazed. Berseem has the advantage of being unlikely to cause bloat in grazing livestock. It can be grown in mild climates during the winter. Some newer varieties have done very well in California, with Multicut outyielding Bigbee. Frosty is another new berseem clover introduction that is supposed to have improved cold tolerance and is able to be cut multiple times in a season.

Cowpeas are native to Central Africa and do well in hot climates. The cowpea is, however, killed by even a mild frost. It is deep rooted and is able to do well under droughty conditions. It usually does better on low-fertility soils than crimson clover. Cowpeas can perform well in mixes with summer grass cover crops such as pearl millet or sorghum-sudan. The most common variety of cowpeas for cover crop use is the iron clay type.

Sunn hemp is a warm-season tropical legume that grows vigorously as a summer legume for much of the United States; it is also a popular inter-seasonal cover crop in the tropics. Sunn hemp can grow from several feet to as much as 7 feet tall and is used frequently in mixes with other summer cover crops. It greatly reduces soybean cyst nematode populations and is a good nitrogen fixer. Sunn hemp has been noted as a summer cover that deer like to browse, which can be a positive or negative depending on the goals for cover crop use. Soybeans, usually grown as an economic crop for their oil- and protein-rich seeds, can serve as a summer cover crop if a farmer has leftover seed and if allowed to grow only until flowering. They require a fertile soil for best growth. As with cowpeas, soybeans are killed by frost. If grown to maturity and harvested for seed, they do not add much in the way of lasting residues or nitrogen.

Soybeans, usually grown as an economic crop for their oil- and protein-rich seeds, can serve as a summer cover crop if a farmer has leftover seed and if allowed to grow only until flowering. They require a fertile soil for best growth. As with cowpeas, soybeans are killed by frost. If grown to maturity and harvested for seed, they do not add much in the way of lasting residues or nitrogen.

Figure 10.3. Velvet beans grown on hillsides in Central America, as growing vines (left) and mulched under a corn crop (right). Photos by Ray Bryant.

Figure 10.3. Velvet beans grown on hillsides in Central America, as growing vines (left) and mulched under a corn crop (right). Photos by Ray Bryant.

Figure 10.3. Velvet beans grown on hillsides in Central America, as growing vines (left) and mulched under a corn crop (right). Photos by Ray Bryant.

Velvet beans (mucuna) are widely adopted in tropical climates. It is an annual climbing vine that grows aggressively to several feet high and suppresses weeds well (Figure 10.3). In a velvet bean–corn sequence, the cover crop provides a thick mulch layer and reseeds itself after the corn crop. The beans themselves are sometimes used for a coffee substitute and can also be eaten after long boiling. A study in West Africa showed that velvet beans can provide nitrogen benefits for two successive corn crops.

Lablab beans (also called hyacinth beans) are another tropical legume being evaluated as a cover crop in the Southeastern U.S. Once established, they grow quickly in hot weather and can produce vines several feet long. Given their viney, climbing growth habit, they might be best paired with an upright grass cover like pearl millet or sorghum-sudan. As with other warm season legumes, they are killed by a light frost.

Similar tropical cover crops include Canavalia and Tephrosia, which can also be used as mulches after maturing. Pigeon peas are yet another tropical legume that may have some potential as a cover crop.

Biennial and Perennial Legumes

Red clover is vigorous, shade tolerant, winter hardy, and can be established relatively easily. It is commonly interseeded in early spring with small grains. Because it starts growing slowly, there is minimal competition between it and the small grain. Red clover also successfully interseeds with corn in the Northeast if the herbicides used do not have significant residual activity.

Figure 10.4. Root systems of four grass cover crops at early stages of growth (two months in a greenhouse). From left: annual ryegrass, barley, triticale (winter biennials) and sorghum-sudangrass (summer annuals). Photos by Joseph Amsili.

Sweet clover (yellow blossom) is a reasonably winter hardy, biennial, vigorous growing crop with an ability to get its roots into compacted subsoils. It is able to withstand high temperatures and droughty conditions better than many other cover crops. Sweet clover requires a soil pH near neutrality and a high calcium level; it does poorly in wet, clayey soils. As long as the pH is high, sweet clover is able to grow well in low-fertility soils. While it is sometimes grown for only a year, a good use for this legume is to allow it to flower and complete its life cycle in the second year, when it produces a large amount of biomass. Like red clover, a typical way that sweet yellow clover has been used is to overseed it into winter wheat in March, then let it grow after wheat harvest until the following spring. When used as a green manure crop, it is incorporated into the soil before full bloom, especially when followed by early spring corn planting.

Alfalfa is not used as a cover crop, but growing it for multiple years as part of a rotation provides some of the same benefits. It improves soil aggregation and water infiltration, and breaks up compacted layers through its deep taproot (Figure 10.2). It also adds significant amounts of carbon and nitrogen to the soil. Following a three-year stand of alfalfa, there should be sufficient nitrogen for most crops for the next year, along with more N than might otherwise be present in the subsequent year or two. It is best grown on well-drained soils that are near neutral in pH and high in fertility. Alfalfa is commonly interseeded with small grains, such as oats, wheat and barley, and it grows after the grain is harvested.

White clover does not produce as much growth as many of the other legumes and is also less tolerant of droughty situations. (New Zealand types of white clover are more drought tolerant than the more commonly used ladino and Dutch white clovers.) However, because it does not grow very tall and is able to tolerate shading better than many other legumes, it may be useful in orchard-floor covers or as a living mulch. White clover has been evaluated for early summer interseeding into corn, but its survival in corn is often not as good as more shade-tolerant species such as annual ryegrass. White clover is also a common component of intensively managed pastures.

Grasses

Commonly used grass cover crops include the annual cereals (rye, wheat, triticale, barley oats), annual or perennial forage grasses such as ryegrass, and warm-season grasses such as sorghum-sudangrass. Grasses, with their fibrous root systems, are very useful for scavenging nutrients, especially nitrogen, left over from a previous crop. They tend to have extensive root systems (Figure 10.4), and some establish rapidly and can greatly reduce erosion. In addition, they can produce large amounts of residue and a large amount of roots. Both the residue and the roots can help add organic matter to the soil. The aboveground residue also can help suppress weed germination and growth.

Figure 10.5. Left: Cereal rye grows in late fall and early spring, and is an effective catch crop and soil conditioner in cool regions. It is widely used in Maryland to reduce nutrient loading into the Chesapeake Bay. Right: Buckwheat establishes quickly in hot and dry conditions, and is an excellent short-duration summer cover crop that improves soil and suppresses weeds. Photo by Thomas Bjorkman.

Figure 10.5. Left: Cereal rye grows in late fall and early spring, and is an effective catch crop and soil conditioner in cool regions. It is widely used in Maryland to reduce nutrient loading into the Chesapeake Bay. Right: Buckwheat establishes quickly in hot and dry conditions, and is an excellent short-duration summer cover crop that improves soil and suppresses weeds. Photo by Thomas Bjorkman.

Figure 10.5. Left: Cereal rye grows in late fall and early spring, and is an effective catch crop and soil conditioner in cool regions. It is widely used in Maryland to reduce nutrient loading into the Chesapeake Bay. Right: Buckwheat establishes quickly in hot and dry conditions, and is an excellent short-duration summer cover crop that improves soil and suppresses weeds. Photo by Thomas Bjorkman.

A problem common to all the grasses is that if you grow the crop to maturity for the maximum amount of residue, you reduce the amount of available nitrogen for the next crop. This is because of the high C:N ratio (low percentage of nitrogen) in grasses near maturity, which ties up nitrogen when decomposing after termination, especially when plowed under. This problem can be avoided by killing the grass early or by adding extra nitrogen in the form of fertilizer or manure. Another way to help with this problem is to supply extra nitrogen by seeding a legume-grass mix.

Cereal rye, also called winter rye, is very winter hardy and easy to establish. Its ability to germinate quickly, together with its winter hardiness, means that it can be planted later in the fall than most other species, even in cold climates. Decomposing residue of cereal rye has shown to have an allelopathic effect, which means that it can chemically suppress germination of small broadleaf weed seeds. It grows quickly in the fall and also grows readily in the spring (Figure 10.5). It is often the cover crop of choice as a catch crop and also works well with a roll-crimp mulch system, in which the cover crop is terminated by rolling and crimping while the cash crop (for example, soybeans) is no-till planted or transplanted into the resulting mulch (see Figure 16.10).

Triticale, a cross between wheat and rye, is almost as winter hardy as cereal rye. It is also easy to establish and has good production of spring vegetation and roots (Figure 10.4), though is somewhat shorter than cereal rye. It can be used for fall or spring grazing. If triticale does go to seed, it is easier to control than many other cover crops that might be grown singly or in a cover crop mix.

Oats are another popular cover crop. Many farmers like to use spring oats for fall cover crop planting because they will not overwinter and thus don’t need spring termination. Summer or fall seedings, usually planted about a month before the last seeding date for cereal rye, will winterkill under most cold-climate conditions. This provides a naturally killed mulch the following spring and may help with weed suppression. As a mixture with one of the clovers, oats provide some quick cover in the fall. Oat stems help trap snow and conserve moisture, even after the plants have been killed by frost. There are oat types that can overwinter in mild climates, such as winter oats or black oats. Black oats, which are a different species of oats compared to spring oats, are popular in no-till systems in South America, where crops such as soybeans are planted into the oat mulch. In the Midwest, black oats often get more fall growth than spring oats, but the seed of black oats can be harder to find and more expensive (note that they are also black-seeded winter oats, which are not true black oats).

Annual ryegrass (not related to cereal rye) grows well in the fall if established early enough. It develops an extensive root system (Figure 10.4) and therefore provides very effective erosion control while adding significant quantities of organic matter. The roots may grow 3–4 feet deep even when aboveground growth is 6 inches or less. Annual ryegrass may winterkill in cold climates. Some caution is needed with annual ryegrass: because it requires a careful approach to termination, it may become a problem weed in some situations.

Sudangrass and sorghum-sudan hybrids are fast growing summer annuals that produce a lot of growth in a short time (Figure 10.4). Because of their vigorous nature, they are good at suppressing weeds. If they are interseeded with a low-growing crop, such as strawberries or many vegetables, you may need to delay seeding so the main crop will not be severely shaded. They have been reported to suppress plant-parasitic nematodes and possibly other organisms, as they produce highly toxic substances during decomposition in soil. Sudangrass is especially helpful for loosening compacted soil. It can also be used as a livestock forage and so can do double duty in a cropping system with one or more grazings and still provide many benefits of a cover crop. If grazing is not an option, periodic mowing helps to control excessive sudangrass stem growth and residue management issues. Mowing also stimulates root development, leaving more belowground residue. Dwarf and brown midrib (low lignin) varieties of sorghum-sudangrass are available and might be considered for cover cropping.

Millets are another group of summer annual grasses used as cover crops. There are actually several different plant species that are called millets, from different regions of the world. The two most commonly used as cover crops in the United States are pearl millet (from Africa) and foxtail millet (from Asia). Forage types of pearl millet can be tall, vigorous crops similar to sorghum-sudan and are drought tolerant. Foxtail millet is also drought tolerant and a fast maturing cover crop, sometimes used in mixes or after vegetable crops.

Other Crops

Buckwheat is a summer annual that is easily killed by frost (Figure 10.5, right). It will grow better than many other cover crops in low-fertility soils but is less tolerant of compacted soils. It also grows rapidly and completes its life cycle quickly, taking around six weeks from planting into a warm soil until the early flowering stage. Buckwheat can grow more than 2 feet tall in the month following planting. If planted in early summer, it may get 3–4 feet tall at maturity but will stay shorter with late summer planting. It competes well with weeds because it grows so fast and, therefore, is sometimes used to suppress weeds following an early spring vegetable crop. It has also been reported to suppress important root pathogens, including Thielaviopsis and Rhizoctonia species. It is possible to grow more than one crop of buckwheat per year in warmer regions. Its seeds are not “hard” and do not persist for multiple years in the soil, but it can reseed itself and become a volunteer weed. Mow, roll, or till it before seeds develop to prevent reseeding. On the other hand, self-seeding can be taken advantage of, and if using tillage, work with a shallow pass with harrows.

Buckwheat grown for grain … occupies the land only during three months of the year, and which consequently figures in the first rank among catch crops, which accommodates itself to all soils, requires little manure, has scarcely any exhausting effect upon the land, keeps the ground perfectly clean by the rapidity of its growth, and which, notwithstanding, yields on an average fifty-fold, and may easily be raised to double that quantity.”

—Léonce de Lavergne (1855)

Brassicas used as cover crops include mustard, rapeseed, oilseed radish, forage turnips and other species. They are increasingly used as winter or rotational cover crops in vegetable and specialty crop production, such as potatoes and tree fruits.

Rapeseed (canola is a type of rapeseed) grows well under the moist and cool conditions of late fall, when other kinds of plants are going dormant for winter. Rapeseed is killed by harsh winter conditions but is grown as a winter crop in the middle and southern sections of the United States. Both winter annual and spring-types of rapeseed and canola are available in the market.

Oilseed (forage) radish has gained a lot of interest because of its fast growth in late summer and fall, which allows significant uptake of nutrients. It develops a large taproot, 1–2 inches in diameter and a foot or more deep, that can break through compacted layers, allowing deeper rooting by the next crop (Figure 10.6). Oilseed radish will winterkill and decompose by spring, but it leaves the soil in friable condition with remnant root holes that improve rainfall infiltration and storage. It also eases root penetration and development by the following crop. All of the brassicas get much better growth as fall cover crops if planted in late summer or early fall. For winter-hardy crops, such as canola, early fall planting is critical to ensure winter survival.

Rapeseed and other brassica crops may function as biofumigants, suppressing soil pests, especially root pathogens and plant-parasitic nematodes. Row crop farmers are increasingly interested in these properties. Don’t expect brassicas to eliminate your pest problems, however. They are a good tool and an excellent rotation crop, but pest management results are inconsistent. More research is needed to further clarify the variables affecting the release and toxicity of the chemical compounds involved. Because members of this family do not develop mycorrhizal fungi associations, they will not promote mycorrhizae in the following crop.

Cover Crops In Perennial Systems

In perennial systems like orchards and vineyards, groundcover management (floor management) can help improve soil health and crop quality. In this case, the cover crop should be a perennial with special characteristics. It should not overly compete with the main crop, and it should be persistent with minimal maintenance and provide good erosion and weed control. Also, it should be able to tolerate the conditions of the orchard floor, such as shade, traffic and drought. Basically, it functions more as a living mulch and therefore should not be too aggressive or spread laterally. A good species for this purpose is Dutch white clover, which also provides modest amounts of nitrogen. Perennial grasses like certain fescues can be attractive as a ground cover if they have a low-growing habit with dense, fine roots and require minimal mowing. Combinations of legumes and grasses may also be attractive. Sometimes, cover crops are used to deliberately compete with grapevines to reduce excessive vegetative growth, but in this situation they are kept away from the immediate vicinity of the vines.