Sow the four best summer cover crops for fast-growing, weed-suppressing soil builders in any patch possible, even during your prime gardening season.
No gardening practice yields as many benefits as cover cropping, and growing cover crops in every season is as important as producing vegetables for your table. We too often think gardening reduces soil fertility, but in fact, the more you keep live plants growing, the richer your soil will become. Roots exude substances that feed beneficial soil organisms, including the amazing mycorrhizae (see Mycorrhizal Fungi). Deep-rooted plants draw minerals from subsoil, which makes the minerals available to shallow-rooted crops. Dead plants, including invisible roots, decompose and release nutrients for use by subsequent crops. Plants also prevent soil erosion, and decomposing roots open channels for oxygen and rain, and provide pathways through which earthworms and other important organisms can migrate. Repeated seasons of organic matter deposition will increase soil carbon, or “humus,” which is crucial to soil fertility, friable texture and water retention.
The ecological benefits of cover cropping go beyond soil improvement: Cover crops scavenge nutrient surges in soil that result from tilling in fresh plant matter or manure. They also prevent runoff pollution and “bank” excess nutrients for later crops. These beneficial covers boost biological diversity in and around gardens, a key to naturally preventing plant diseases and insect damage. Plus, many cover crops perform double-duty as forage for poultry and livestock, making a homestead less dependent on purchased inputs.
Too often, gardeners practice cover cropping only in the off-season — for instance, to protect soil in winter — and assume it’s not a summer option. But it is, and planting summer cover crops provides big payoffs.
Unique Challenges of Summer Cover Crops
Drought and heat. Drought is more likely in summer, so gardeners must choose cover crop species that not only thrive in heat but are also drought-tolerant. Hot, dry soil is also inimical to seed germination, so no summer cover is likely to succeed without a little loving care.
If you broadcast seeds, first work them into the soil, then tamp the soil down with a rake. Even better, drill the seed by hand: Open closely spaced furrows with the corner of a hoe, dribble in seed, cover and tamp. Water immediately and as often as needed to keep the top quarter-inch of soil damp until germination. After plants become established, apply a light mulch to cool the soil and conserve moisture. You can also take advantage of shade offered by an existing crop, such as spring-planted broccoli, by seeding a cover crop into the same bed a couple of weeks before you remove the main crop. Most cover crops are adapted to close planting — the closer you plant, the sooner a tight, shading canopy will form.
Limited space. The greatest difficulty with summer cover cropping is revealed by the first question every gardener asks: “We’re already pushing the garden for maximum production — where do we find any unused space?” First, try hard to dedicate at least one bed in your rotation to growing a cover crop through an entire summer season, or maybe even a whole year. You’ll be amazed by how much an uninterrupted year under cover will do for your soil — and your future crops.
A couple of ways to sneak in more cover cropping: Gaps between early and late crops are opportunities to avoid the bane of any garden — bare soil — provided your chosen cover crop grows quickly (keep reading for suggestions). You can also interplant low-growing covers at the base of tall crops — such as trellised tomatoes or pole beans — to achieve all the benefits of cover cropping and avoid any wasted bed space.
Unique Benefits of Summer Cover Crops
- Abundant biomass. In summer, large yields of biomass are typical, especially if you cut the cover crop during its vegetative stage to encourage rapid regrowth. You can work this surplus biomass into the soil or use it as livestock feed, compost fodder or mulch. Although overwintered covers provide those same benefits, summer cover crops yield a wider range of home-produced feeds for poultry and livestock, including cut-and-come-again greens, grains and seeds, and dried cover crops used as hay.
- Weed suppression. Summertime is prime weed time, and fast-growing summer cover crops suppress weeds. They’re especially useful for filling that blank spot between early and later crops — space that would otherwise offer a field day for weeds.
- Biodiversity boost. Summer covers make greater contributions to biodiversity because insects, birds and amphibians feed and reproduce during the growing season. Biodiversity creates ecological balance that can help mitigate plant diseases and damaging insects.
Let’s consider four summer cover crops that best rise to the challenge of warm summer conditions and offer a broad range of benefits, especially in mixed plantings. Nature will do the work of killing these cover crops at the end of the season: All four are intolerant of frost and will die down into a protective mulch as freezing temperatures set in.
Buckwheat
This broadleaf annual’s greatest virtues are extremely rapid growth and profuse flowering. Its greatest limitation, extreme sensitivity to frost, can actually be turned into an advantage.
Buckwheat can form a tight canopy within two weeks, outstripping and shading out weeds. Its weed-suppressing prowess offers a responsible alternative to toxic herbicides. Tillage plus back-to-back successions of buckwheat have proved effective at suppressing even tough perennial weeds.
For preventing soil’s exposure to baking sun, buckwheat may be the best of all covers to fill a gap between early and later crops. Plant buckwheat after all danger of frost has passed and make additional plantings anytime, up to 35 days before frost. Buckwheat flowers early (30 days from seed to bloom in my northern Virginia garden) and profusely, encouraging honeybees and other beneficial insects. Increase biomass yield by cutting the crop just before it reaches 25 percent bloom. Regrowth is rapid and a second such cutting may be possible. Plants make good fodder for poultry or rabbits, and chickens love buckwheat seeds: Just toss cut stems with seedheads to your flock.
Buckwheat’s vulnerability to frost makes it a useful “nurse” for fall-planted, cold-tolerant crops, such as alfalfa and winter greens, which are often difficult to germinate in late-summer heat. The quick cover of some buckwheat sown with a winter crop will shade and cool the soil. The cold-hardy crop will grow in buckwheat’s shade until a killing frost mows down the buckwheat, freeing the other plants for a surge of growth before winter dormancy.
Sorghum-Sudangrass
Hybrid crosses of forage-type sorghum and sudangrass yield dramatic improvements to soil texture and increases in organic matter. Strains of sorghum-sudangrass grow 5 to 12 feet tall and produce an impressive amount of biomass. Cut back to 6 inches when the crop reaches 4 feet high to stimulate regrowth and encourage deeper, more aggressive root growth for opening compacted soil. The cut stalks make long-lasting mulches.
Plant sorghum-sudangrass about two weeks after the date for planting sweet corn in your area and anytime thereafter until six weeks before frost; it thrives in summer heat. After it’s established, sorghum-sudangrass is highly drought-resistant.
If planted tightly — in rows spaced 8 inches apart and seeds at 1.5 inches apart, planted 1 inch deep — sorghum-sudangrass will beat out weed competition. Allelopathic compounds exuded from this crop’s roots will suppress damaging nematodes and inhibit many sprouting weeds and crop seeds. However, this means gardeners should wait six to eight weeks after killing sorghum-sudangrass before sowing another crop in the same spot.
Sorghum-sudangrass makes good livestock forage, though you must not feed your animals young plants (those less than 24 inches high) or those stressed by drought or killed by frost, which may cause prussic acid poisoning. Ducks and geese love the leaves, and goats eat the stalks like candy.
Cowpeas
Cowpeas thrive in heat, grow fast and — with taproots reaching almost 8 feet deep — are highly drought-tolerant. They set about 130 to 200 pounds of nitrogen per acre and typically contribute a couple of tons of biomass to replenish soil organic matter. They do well in a wide range of soils, except highly alkaline soil.
Plant cowpeas in thoroughly warmed soil a week or two after your recommended date for planting sweet corn. Make successive plantings up to nine weeks before a killing frost. Treat the seeds with a rhizobial inoculant specific to cowpeas to ensure maximum nitrogen fixation. Tight plantings shade out weeds and conserve moisture, so plant seeds 1 inch deep in rows 6 inches apart (up to 15 inches apart for viny varieties) with seeds 2 inches apart in each row.
Beneficial insects feed on cowpeas’ flowers and “extrafloral nectaries” (nectar-secreting glands near leaf nodes). The green plants make good fodder, which can also be dried for hay. Mature seeds provide feed for poultry and livestock and are a delicious table legume, too. Because a number of varieties set seeds at as early as two months, cowpeas are outstanding candidates to follow spring crops and set nitrogen for heavy-feeding, fall-planted alliums.
Sunn Hemp
Sunn hemp is a tropical legume that quickly grows to 9 feet tall. In dense plantings, it can set more than 120 pounds of nitrogen and 5,000 pounds of biomass per acre — or twice that if you cut the crop back, stimulating branching and additional root penetration. It can fill midsummer niches between spring and fall crops, especially if the latter would benefit from a nitrogen boost. Sunn hemp is adapted to a wide range of soils (so long as they aren’t waterlogged), but isn’t frost-tolerant.
Plant sunn hemp a week or two after you’d plant sweet corn in your area, and succession sow up to nine weeks before a killing frost. Plant inoculated seed (use the same inoculant as for cowpeas) 1 inch deep, spacing 1.5 inches in the row and with rows 6 inches apart. Planting densely will crowd out weeds.
Younger sunn hemp plants contain more nitrogen, so cut them for composting or for a nutrient-rich garden mulch that will break down rapidly. After 60 days, the stems thicken and become fibrous and higher in cellulose; cuttings at this stage make long-lasting mulches that increase soil carbon.
Sunn hemp can be a valuable fodder, but only if you grow the variety known as ‘Tropic Sun.’ Other strains contain toxic alkaloids. ‘Tropic Sun’ is drought-tolerant and resistant to and suppressive of root-damaging nematodes.
Cover Cropping Resources
Molokai Seed Company in Hawaii is the only source for nontoxic Certified Organic ‘Tropic Sun’ sunn hemp seed.
The 244-page compendium Managing Cover Crops Profitably (third edition) is indispensable for gardeners as well as farmers. Published by the Sustainable Agriculture Research & Education Program (SARE), you can order it as a book for $19 or download it as a free PDF from SARE.
Harvey Ussery, a veteran of raising homegrown produce and poultry, homesteads and grows cover crops year-round in northern Virginia.
