Four Outstanding Cover Crops
The following is an expanded version of my article “Best Summer Cover Crops,” published in Mother Earth News (Aug/Sep 2014). It opens with a brief review of cover cropping (explored in greater depth in “The Joys of Cover Cropping”) which emphasizes the special challenges and particular benefits of cover cropping in the summer. (Added to the site October 1, 2016.)
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No gardening practice brings so many benefits as cover cropping, so the wise gardener tries as hard to cover-crop in every season as to grow an abundance of fresh vegetables for the table and for storage.
We too often think of gardening as reducing fertility in the soil. Though plants do of course remove mineral nutrients from the soil to grow leaves, tubers, and fruits, they also add much to the soil: Their roots release exudates containing carbohydrates and proteins that feed organisms—bacteria, fungi, and more—which in turn help make minerals in the soil more available, increase water uptake, and protect from nematodes and disease. Especially deep-rooted plants actually increase mineral nutrients by drawing them from the deep subsoil and making them available to more shallow-rooted plants. When cover crops are tilled into the top few inches of soil, decomposition releases their stored nutrients for use by following crops. Decomposing roots open up channels through which even heavy rains are absorbed (rather than running off erosively), oxygen moves into the soil to support soil life, and earthworms and other organisms migrate to achieve their good work. Repeated seasons of such organic matter deposition result in tremendous increases in soil carbon or humus, key to soil fertility, friable texture, and water retention. So in many ways, the more plants growing on a piece of ground, the better. Cover crops are the key to getting lots more plants in play.
Ecological and sustainability benefits go far beyond improving garden soil. Tilling in a lot of nutrient-dense fresh plant matter or manures can release a surge of nutrients the soil community cannot take up before they leach to groundwater. An immediately following cover crop scavenges excess nutrients, “banking” them for later crops. Cover crops boost biological diversity in and around the garden—of spiders and ground beetles under their shelter, of pollinators and beneficial insects feeding on their flowers, and even of bird and amphibian populations. This increase in ecological complexity is the key to prevention of disease and insect damage. Some cover crops do double duty as forage for poultry and livestock, making the entire farm or homestead a more closed circle, less dependent on purchased inputs from far away.
Too often, however, gardeners practice cover cropping only in the “off season”—after food crops are harvested in the fall, for soil protection over winter; or early in the gardening year, for an opportunistic round of soil-buliding preceding such warm weather crops as corn and sweet potatoes—and too many gardeners assume that cover cropping is not on option in summertime. If you are one of those gardeners, you’re passing up opportunities with big payoffs.
Certainly there are obstacles to cover cropping in summer. Remember Eliot Coleman’s advice to give the same care to starting your cover crop as you do to planting harvest crops—that is especially true for summer covers.
Hot, dry soil is inimical to seed germination, so starting the cover crop is more difficult than when the ground is cooler and more moist. Though some cover crops may establish in those favorable conditions when simply broadcast, it is unlikely that any summer cover will succeed without a little tender loving care. If you do broadcast seeds, work them into the soil and tamp with a rake to cover and ensure good soil contact. A better practice is to drill the seed: Open closely spaced furrows with the corner of a hoe, dribble in the seed, cover, and tamp. If the sowing doesn’t get a nice rain after you seed, water immediately, and as often as needed until germination to keep the top quarter inch of soil a bit damp. A light mulch is a great boost to germination, both cooling the soil and retaining moisture.
Take advantage of the shade offered by an existing crop such as spring broccoli and seed a following cover crop beneath it a couple of weeks before removal of the harvest crop. And remember that most cover crops are adapted to very close planting. The closer you plant, the sooner a tightly shading canopy will form.
Drought is more likely in summer, so choose cover crop species that not only thrive in heat but are tolerant of drought.
The greatest difficulty with summer cover cropping? We are already pushing the garden for maximum production—where do we find any unused space?
If you realize first of all just how important regular cover cropping is in every way, try hard to dedicate at least one bed in the rotation to growing a cover crop the entire growing season, maybe even an entire year. You will be amazed how much an uninterrupted year under cover will do for your soil, and, as the seasons roll, each of your beds will receive those benefits in turn.
Fortunately, there are a couple of other ways to sneak in a lot more cover cropping than you’ve ever managed in the past. Even in the most tightly planted garden, there is frequently a gap between the harvest of a spring crop and the planting of a later one. Even a gap of a few weeks gives the opportunity to avoid the bane of any garden—bare soil—provided the chosen cover crop grows really fast. There are as well many possibilities for interplanting cover crops with harvest crops. Low-growing covers planted at the base of taller crops, for example, provide all the benefits of cover cropping in the same space we might otherwise have used solely for such main crops as corn, trellised tomatoes, or pole beans.
It’s important not to lose summer as a major cover cropping season because many of the benefits are multiplied during that part of the year. Larger yields of biomass are typical, especially if we cut the cover crop during its vegetative stage—perhaps more than once—encouraging rapid regrowth.
Summertime is weed time! Fast-growing cover crops suppress weeds. They are especially useful for filling that “blank spot” between early and later crops which otherwise is a field day for weeds.
Cover crops grown in summer make greater contributions to biodiversity, enhancing populations of birds, amphibians, and spiders and insects, especially pollinators, when they are feeding and reproducing—and helping establish ecological balances which limit diseases and surges of crop damaging insects.
Though overwinter covers can provide valuable green fodder as well, summer covers provide a wider range of home-produced feeds for poultry and livestock: cut-and-come-again greens, dried cover crops as hay, grains and seeds.
See “The Joys of Cover Cropping” for more on killing cover crops to feed the soil and use as mulches. But note that we can simply let nature do that work if we grow any of the covers below—all are intolerant of frost, and die down into a protective mulch as freezing temperatures begin.
Though there are other good covers for summer, let’s focus on four that best rise to the challenge of hot, dry conditions and offer a broad range of benefits: two legumes, cowpea and sunn hemp; and two non-legumes, sorghum-sudangrass (grass) and buckwheat (broadleaf).
This broadleaved annual’s great virtues are extremely rapid growth and profuse flowering. Its greatest limitation—extreme sensitivity to frost—can actually be turned to advantage.
Buckwheat, Fagopyrum esculentum, is an excellent choice for a protective cover in a hurry. Seeds germinate in three to five days and form a tight canopy with three-inch leaves within two weeks. It outstrips most weeds, and its tight shade suppresses their subsequent establishment. Even in patches of tough established weeds buckwheat’s prowess at suppression offers an ecologically responsible alternative to toxic herbicides: A program of tillage and back-to-back successions of buckwheat has proven effective at suppressing perennial weeds such as Canada thistle, sowthistle, creeping jenny, leafy spurge, Russian knapweed, and perennial peppergrass.
Adapted to a wide range of soil types, including those of low fertility—so long as they are not highly alkaline, compacted or excessively dry or wet—buckwheat is often used as a first crop in rejuvenating exhausted soils.
Plant buckwheat after all danger of frost is past—make additional plantings any time, up to thirty-five days or more before fall frosts. Either drill the seeds or broadcast and work in with a rake. (Vigor tends to be greater from drilled plantings.) It can be grown in mixes with sorghum-sudangrass, cowpeas, and sunn hemp (as shown in the photo at the top of this page).
Though its yield of biomass is modest—a couple of tons per acre—buckwheat achieves that yield in an impressive six to eight weeks. Encourage greater yield by cutting before it reaches twenty-five percent bloom—regrowth will be rapid and a second such cutting may be possible.
Mild acids from buckwheat roots release mineral nutrients from the soil—buckwheat is an especially capable scavenger of phosphorus—and rapid breakdown of plant residues make the minerals available to following crops. Decomposition of the dense, fibrous root system, concentrated in the top ten inches, makes buckwheat an excellent soil conditioner to precede fall crops. Do note, though, that rapid breakdown together with this loosening of the soil may increase chance of runoff erosion absent an immediately following crop or cover.
Bare ground should be anathema to us gardeners at any time of year—certainly we don’t want our soil exposed to baking sun in the height of summer. Buckwheat may be the most useful of all covers to fill a gap between the harvest of a spring crop and the planting of a fall crop. Though not particularly drought tolerant, often wilting during the hottest days, buckwheat usually recovers during the cool of night.
With its hollow, fragile stems, buckwheat is the easiest of all cover crops to kill—you can literally “sweep” it off the bed with a garden rake. The planting melts away in a couple of days if chickens are assigned the task. If the planting has matured seeds, all the better—chickens love buckwheat seeds.
Though buckwheat is killed by the slightest touch of frost, that very vulnerability makes it a useful “nurse” for late-planted crops. Alfalfa for example should ideally be planted forty-five days before freezing weather to establish well before winter, but germination can be inhibited by the late summer heat. Mixing in a moderate amount of buckwheat seeds when sowing alfalfa yields a quick cover to shade and cool the soil and conserve moisture. The alfalfa grows slowly in the tight shade, but the first killing frost mows down the buckwheat—freeing the sun-loving, cold-hardy alfalfa for a surge of growth before winter dormancy.
Buckwheat flowers early (thirty days from seed to bloom in my garden) and profusely, and its nectar and pollen feed many pollinators including honeybees and many classes of beneficial insects such as hover flies, predatory wasps, minute pirate bugs, insidious flower bugs, tachinid flies, and lady beetles. Even at points in my garden rotation when I don’t need buckwheat as a cover crop, I make sure to have a scattering of flowering buckwheat in the garden to support these essential populations.
Hybrid crosses of forage-type sorghums and sudangrass (Sorghum bicolor x S. bicolor var. sudanese) yield dramatic improvements to soil texture and increases in organic matter.
Plant sorghum-sudangrass hybrids after the soil is thoroughly warmed, about two weeks after the established date for planting sweet corn in your area. Make additional plantings during the season until six weeks before first fall frosts—they grow fast and thrive in summer heat. These hybrids will tolerate a wide range of pH and soils, including those of low fertility, though do best in soils of good fertility and near-neutral pH. I plant in rows spaced eight inches apart, with seeds drilled about an inch deep and spaced at an inch and a half. Planted this tightly, they largely suppress weed competition. Once established, sorghum-sudangrass is highly resistant to drought.
Strains of sorghum-sudangrass grow five to twelve feet tall and produce a lot of biomass (about 4,000 to 5,000 pounds per acre, though yields several times greater can be achieved in good conditions and with multiple cuttings). Roots are aggressive and become more so with cutting: Cutting back to six inches after the stalks reach three to four feet stimulates deeper penetration and mass (five to eight times greater) of roots. More open texture and increase in soil carbon from decomposing roots can be major factors in soil improvement. Cutting back boosts top growth as well, in the form of multiple new, thicker stalks per plant. Cut stalks, with their high cellulose content, make persistent mulches.
Allelopathic compounds exuded from the roots suppress damaging nematodes and weeds such as velvetleaf, large crabgrass, barnyardgrass, green foxtail, smooth pigweed, common ragweed, redroot pigweed, and purslane. (Be aware that persistent traces of these allelochemicals can affect some following crops, for example lettuce seedlings.)
Sorghum-sudangrass can be grown in mixes with buckwheat, sunn hemp, or cowpeas (or all of them, as in the photo at the top of this page). If growing with cowpeas, choose a viny type which will run up the stalks; if with sunn, match with strains that grow to about the same height.
If you plan to use sorghum-sudangrass for forage, be aware that young plants (less than 24 inches) and those stressed by drought or killed by frost can cause prussic acid poisoning. If these limitations are observed, sorghum-sudangrass makes good livestock forage. Ducks and geese love the leaves—goats eat the stalks like candy.
The most broadly useful of nitrogen-fixing cover crops that can be started in summer’s heat.
Cowpeas, Vigna unguiculata, are legumes related to beans (Phaseolus) and peas (Pisum) but in a different genus, Vigna. They thrive in summer heat, grow fast, and—with taproots that probe almost eight feet into the deep subsoil for water—are highly tolerant of drought. Cowpeas can set 100 to 150 pounds of nitrogen per acre, possibly more; and with typical biomass production of 3000 to 4000 pounds per acre contribute significantly to replenishment of soil organic matter. They do well in a wide range of soil types, from highly acidic to neutral (though not highly alkaline), including soils of low fertility.
Don’t rush cowpeas, which require thoroughly warmed soil (65°F minimum, 70° even better—a week or two after recommended date for planting sweet corn). Successive plantings can be made anytime during the summer, up to nine or ten weeks before the first killing frosts.
Don’t forget to inoculate the seeds, using an inoculant specific to cowpeas (different from rhizobial strains for garden peas and beans), to ensure maximum fixation of nitrogen. It can help to soak the seed a few hours before drilling an inch deep into a moist seedbed, spaced at two to three inches in the row, with rows six or seven inches apart for fast ground cover (fifteen inches or more for more viny varieties). Planted this closely, the dense canopy shades out weeds and conserves soil moisture.
Cowpeas help feed beneficial insect populations such as honeybees, lady beetles, and various predatory wasps, not only with their flowers but via “extrafloral nectaries”—nectar-secreting glands near leaf nodes.
Its moderate shade tolerance makes cowpea a good candidate for interplanting with other crops. Plant seeds between corn plants when they are four to six weeks old for a companion plant that suppresses weeds, fixes nitrogen, and remains as cover after the corn harvest. Mixing cowpeas with buckwheat and sorghum-sudangrass improves their effectiveness for weed control and biomass production.
If you find a variety of cowpea you like, saving your own seed couldn’t be easier. Even if you grow several varieties, you can still save seeds so long as your varieties don’t flower at the same time or you allow at least 50 feet between them.
The green plants make good fodder—an efficient way to kill the crop is to graze it off with cattle or pigs—which can be dried as hay. Matured seeds are not only excellent feed for poultry and livestock, but a delicious table legume as well. (Unlike some other seed-bearing legumes, cowpeas leave a net gain of nitrogen in the soil even if the seed is harvested.) Since a number of varieties set seeds as early as two months, they could serve as a nitrogen-fixing cover between an early spring harvest and the planting of heavy-feeding fall brassicas. They are an outstanding candidate to precede fall planted garlic and shallots: The cowpeas fix nitrogen for the heavy-feeding alliums and die down into a protective mulch with the killing frosts that precede planting of the overwinter alliums in October or November.
A tall-growing legume from the tropics can help fill your summer cover cropping needs.
Crotalaria juncea originated in India, where it has been grown since prehistoric times for the tough fibers in its stems. Called “sunn” in its native land, it is commonly referred to here as “sunn hemp”—because its fibers, like those of industrial hemp (Cannabis sativa), have been important for sacking, canvas, and cordage, rather than any botanical relation to Cannabis hemp, which is not a legume.
Sunn germinates and grows rapidly—to as much as nine feet tall. It has a deep tap root and vigorous side roots and a stem up to half an inch or more in diameter. When inoculated with a proper rhizobial inoculant (same as for cowpea) it can fix 120 pounds of nitrogen and more than 5000 pounds of dry matter per acre in nine to twelve weeks (twice that rate or more with multiple cuttings over a longer season in ideal conditions). Its fast growth makes it useful for filling mid-summer niches between spring and fall crops, especially when the latter would benefit from a nitrogen boost.
Crotalaria is adapted to a wide range of soils—including marginal, unimproved soils of low fertility—so long as they are not waterlogged. It will do best with a near-neutral pH (6.0 to 7.0), but will tolerate a range from 5.0 to 7.5. It is not tolerant of frost, so wait until the soil is thoroughly warmed in the spring—at least until it is time to plant sweet corn and preferably a week or two later. After that, plant any time in the growing season, up to nine weeks before killing frost. Drill the inoculated seed an inch deep, spacing an inch and a half in the row and with rows at six inches apart. Though tall, the plants not only tolerate such close spacing but will better suppress weeds and produce greater biomass.
There is more nitrogen in the younger plants, so up to 60 days they can be cut for composting or for nutrient mulches that break down rapidly. After 60 days the stems thicken and become fibrous, with a much higher carbon content (cellulose and lignin)—plants cut at this stage are good for long-lasting mulches and increased soil carbon. Once the plants are several feet to head high, I like to cut them back to a foot tall, to encourage deeper, more aggressive root growth and the regrowth of multiple stems in place of the original single one—for heavier biomass production and new stems and leaves with high nitrogen content.
Last year I grew sunn as an undersown cover crop with black oilseed sunflowers, cutting back the sunn as often as needed to keep it from competing for sunlight with the sunflowers. Seedlings in one of my plantings were hit hard by rabbits (deer will find them tasty as well if they can get to them)—be sure to protect plantings from marauders.
It is important to note that Crotalaria juncea can be used as a valuable fodder, but only if the variety grown is Tropic Sun, bred jointly by the University of Hawaii and the USDA. Other strains of Crotalaria contain alkaloids toxic to livestock. Tropic Sun is the only strain that has been proven safe for consumption by poultry and livestock such as cattle and goats. Tropic Sun is drought tolerant—though more productive if it receives the equivalent of an inch of rain a week—and its roots are resistant to and suppressive of root damaging nematodes.
Sunn flowers at the end of the growing season—obviously pea-family flowers that are sunflower yellow, quite pretty—but will not mature seeds in a non-tropical climate. Most seed used in this country is imported from Hawaii.