Cover crops have many benefits for your garden soil, and for the food crops you’ll later plant in their place. (See “5 Benefits of Cover Crops,” below) To reap these benefits, you’ll need to cultivate strong stands of growth that can suppress weeds and add lots of organic material, or “biomass.” Late summer and fall are the easiest times to start cover crops, because the crops won’t need to compete with any other plants in your garden except weeds. And cover crops planted late in the season have a long period in which to grow — throughout winter, for those that aren’t frost-sensitive.
Here are some recommendations for winter cover crops, based on my decades of gardening experience at Twin Oaks Community in central Virginia. At Twin Oaks, we practice intensive vegetable gardening, both outdoors and within hoop houses, to feed about 100 people year-round. The fertile, productive soil that cover cropping brings us helps feed our residents on a daily basis.
Crop Choice and Timing
Timing is critical, because you’ll be planting these cover crops late in the traditional growing season. Work back from your area’s first frost date to choose from the options below. For a frame of reference, I’ve included the applicable dates for Twin Oaks (Zone 7, with an average first frost date of October 15). “Frost-killed” cover crops will be killed by frost. “Winterkilled” means the plants will be killed by midwinter temperatures. “Winter-hardy” means the plants will live through winter.
60 to 80 days before first frost (July 26 through Aug. 17 at Twin Oaks). Frost-killed: buckwheat; sunn hemp; soybeans; Southern peas; spring peas; lablab; Japanese millet; sorghum-sudangrass hybrids. Winterkilled: Austrian winter peas (Zone 6 or lower); oats. Winter-hardy: crimson and red clovers. Note: Don’t sow oats too early, or they may set seed in fall.
40 to 60 days before frost (Aug. 17 to Sept. 6 at Twin Oaks). Frost-killed: soybeans; spring peas. Winterkilled: Austrian winter peas (Zone 6 or lower); barley; oats. Winter-hardy: winter wheat; winter rye; Austrian winter peas (Zone 7 or higher); red clover; crimson clover; hairy vetch; fava beans. Note: Don’t sow rye in August in Zone 7 or higher, as it may set seed.
0 to 40 days before frost (Sept. 6 to Oct. 15 at Twin Oaks). Winter-hardy: winter rye, winter wheat, or winter barley with crimson clover; red clover; Austrian winter peas (Zone 7 and higher). Note: This is too late for crops that aren’t frost-hardy.
0 to 24 days past frost date (Oct. 15 to Nov. 8 at Twin Oaks). Winter-hardy: winter rye or wheat with Austrian winter peas. Note: This is too late for crops that aren’t frost-hardy.
Next, I’ll discuss the pros and cons of a couple of classes of winter cover crops: grasses and legumes.
All cereal grasses prevent erosion, suppress weeds, add organic matter, and scavenge excess nutrients. They improve soil structure and encourage helpful microorganisms.
Winter wheat (Triticum aestivum) is a winter survivor that’s cheaper and easier to manage in spring than rye, because it’s slower to go to seed and has less bulk. It’s also easier to kill before putting in a food crop, and less likely to become a weed than barley or rye. Winter wheat tolerates heavy soils better than barley or oats, but it has poor tolerance to flooding, and is slightly more susceptible than rye or oats to insects and disease.
Winter rye (Secale cereale), also known as “cereal rye,” also survives winter, and grows 5 to 7 feet tall, making massive amounts of biomass. You’ll need to allow 3 to 4 weeks after tilling in rye in spring before planting a food crop, because it takes that long for rye’s biochemical compounds (aka “allelopathic”) to stop inhibiting seed germination.
Winterkilled cover crops smother weeds and grow biomass that will provide mulch to cover and hold the soil in place until early spring. The dead material is more easily incorporated in spring than a green cover crop, or it can be raked aside for planting.
Oats (Avena sativa) tend to winterkill. Seedlings die at 17 degrees Fahrenheit, and large plants are seriously damaged by temperatures lower than about 20 degrees. They have some tolerance to flooding (better than wheat), but not much tolerance to heat or drought (more than rye). They’re easier to incorporate into the soil than rye or wheat. Oats aren’t as good as rye at breaking up compacted subsoil, but they have less allelopathic effect. They’re ideal for areas you plan to plant with early spring crops the following year.
Oats should be sown at least 40 days before first frost so they provide biomass before being killed by the cold. In our Zone 7 garden, this requires a plot that’s open for cover cropping in August. We sow oats after harvesting early sweet corn, spring broccoli, cabbage, and spring-planted potatoes. The next year, we till in the oats before planting peas, cabbage, broccoli, carrots, potatoes, spinach, and the first sowing of sweet corn.
Other frost-tender grains and grasses to try include sorghum-sudangrass hybrids (Sorghum bicolor x S. bicolor var. sudanese), buckwheat (Fagopyrum esculentum), barley (Hordeum vulgare), and millet (Echinochloa esculenta).
To fix the maximum amount of nitrogen in your soil, only cover crop with legumes if they have time to flower in spring before you need to till them in. A good legume stand can provide all the nitrogen the following food crop needs. If you include multiple legumes in your winter cover crop mix, note that one may struggle while another thrives.
Hairy vetch (Vicia villosa) is widely adapted, grows quickly in fall, and is hardy to minus 15 degrees. It’s drought-tolerant after it’s established, but it doesn’t tolerate flooding or shade. The plant may harbor pest nematodes. During spring, it reaches a height of 2 feet alone, or 6 feet if supported by tall cover crops. Hairy vetch can be invasive if it sets seed, and the seed is toxic to poultry.
Clovers (Trifolium spp.) winterkill at minus 10 degrees. They attract beneficial insects that eat aphids. Clovers are best planted with a grass crop for support. We plant crimson clover and wheat together after harvesting vegetable crops that finish by mid-October: sweet corn, June-planted potatoes, watermelons, tomatoes, and peppers. We turn the clover mix under in late April, and plant the plot with vegetables in May or June.
Austrian winter peas (Pisum sativum var. arvense), or “field peas,” tolerate a wide range of soil types and are good at emerging through crusted soil, suppressing weeds, and preventing erosion. They winterkill at 0 degrees. Sowing them in a mix with a grass will improve their cold-weather survival by reducing soil freezing and heaving. They make rapid growth in cool spring weather and will bloom two weeks before hairy vetch, meaning they can be tilled in earlier.
We seed Austrian winter peas after winter squash, melons, sweet potatoes, tomatoes, peppers, midseason sweet corn, June-planted potatoes, and other late-finishing food crops.
Other winterkilled legume cover crops to try include Southern peas (Vigna unguiculata), spring peas and Canadian field peas (P. sativum), fava beans (Vicia faba), soybeans (Glycine max), sunn hemp (Crotalaria juncea), ‘Lana’ woollypod vetch (Vicia villosa Roth ssp. varia), and purple vetch (Vicia americana Muhl. ex Willd.).
At Twin Oaks, we use an undersowing technique to pack even more cover crops into our plots. Because most of our food crops aren’t finished in time for us to sow cover crops in August, we undersow the food crop. We choose vigorous food crops, but only moderately vigorous cover crops. Timing is critical: We sow the cover crop late enough to minimize competition with the food crop, but early enough to get light and be strong enough to endure foot traffic during harvest. The best time is often one month after planting the food crop, before its leaf canopy has closed. With vining food crops, we sow the cover crop before the vines run. Rye can be undersown in sweet corn or fall brassicas in early September, and left as a winter cover crop.
We plant our sixth sowing of sweet corn in mid-July, and then undersow it with oats and soybeans in mid-August. These cover crops are both somewhat shade-tolerant. Both are winterkilled in our climate. We wait until early spring to mow down the corn, and follow with potatoes in mid-March.
“Green fallow” is a term for planting a long-term cover crop. We green fallow at Twin Oaks by undersowing our fall broccoli with a clover mix that will grow the entire following year. We transplant broccoli starts in mid-July, cultivate between the rows two weeks later, and then cultivate again two weeks after that, in mid-August. Then, we broadcast the clover mix, using a dense rate of 1 ounce crimson clover, 1 ounce Ladino white clover, and 2 ounces medium, multi-cut red clover per 100 square feet. Crimson clover grows quickly in fall, and the others take over the next year.
Another cover crop technique you can use in an organic food plot is no-till: Kill the cover crop by freezing, mowing, or rolling; use it as mulch; and then plant the food crops into the decomposing residue. This will keep the soil covered, reducing erosion and increasing resilience, so it can absorb and retain more water. No-till’s yields are higher under drought conditions than are tilled soil’s yields. Biomass is conserved, rather than burning up as quickly as it does when it’s incorporated. Earthworms and beneficial soil microbes increase because the cover crops’ root channels are undisturbed. No new weed seeds are brought to the surface. Sufficient legumes in your cover crop mix of choice can provide all the nitrogen the next crop needs — and nitrogen from vetch seed is half the cost of commercial fertilizers.
One con is that winter snow and ice can compact the soil, making transplanting into untilled soil harder work than planting into loose, tilled soil. This technique is unsuitable for small-seeded, closely-spaced vegetables.
When choosing no-till cover crops, consider whether you want a winterkilled cover crop or a winter survivor. Cold-hardy cover crops need time in spring to grow to optimal size before you mow them, so they’re not suitable for early spring food crops. Plus, untilled soil retains cooler temperatures into summer, delaying growth and harvest by 1 to 2 weeks, so no-till isn’t good for growing watermelons.
If you want to plant early spring food crops in a no-till plot, you’ll need to choose frost-tender, low-residue cover crops, such as forage radish, lablab, Southern peas, and small-seeded fava beans. They’ll die back over winter and leave almost bare soil for planting in spring. The soil will be colder than if it had been left bare all winter; try this with cabbage and broccoli. Note that fast-maturing spring food crops won’t prosper with no-till cover crops unless you add fertilizer, as they need nitrogen more quickly than can be had from no-till.
If you plan to plant mid-spring food crops, choose winterkilled cover crops, such as mustards and oilseed radish.
For late-spring food crops in a no-till plot, you can limit the loss of nitrogen from the cover crop by planting a mixture of winter-hardy grasses and legumes — but legumes are wasted money if you can’t let them grow to flowering. Rye will mow-kill at flowering, but not earlier. Hairy vetch activates plant genes that increase disease-tolerance and plant longevity, giving tomatoes an extra 2 to 3 weeks of production. Peppers; eggplants; pumpkins; or successions of tomatoes, cucumbers, and squash can do very well as late-spring transplants. We maintain one plot in 10 as no-till for Roma paste tomatoes, transplanted in early May. They’re a good no-till food crop because we don’t need them to ripen early.
Be sure to keep notes on how well your cover crops work for you, and reconfigure your crop rotation and planting schedules to get timely sowings.
5 Benefits of Cover Crops
1. Soil fertility. Organisms will decompose dead cover crops into nutrients that are taken up by the roots of food plants, and add to your soil’s humus content.
2. Soil texture. Mycorrhizal fungi grow in association with plant roots, producing a substance that glues microscopic clay and organic matter particles into aggregate clumps. This makes the soil more porous to oxygen and water.
3. Erosion prevention. A cover crop’s tight canopy protects the soil from the drying effects of wind and the impact of heavy rain, and its roots hold the soil together.
4. Moisture. Organic matter added by cover crops acts like a sponge in the soil,
absorbing rainwater and holding it for gradual release to plant roots.
5. Weed, disease, and insect protection. Cover crops suppress weeds, and their roots release nutrients that feed beneficial microbes in the soil. These microbes then suppress pathogens that cause root diseases. When you cut a cover crop and use it as mulch, you boost the population of beneficial insects.
— Harvey Ussery
Pam Dawling works in the vegetable gardens at Twin Oaks Community in central Virginia. She often presents at MOTHER EARTH NEWS FAIRS and sustainable agriculture conferences, and is the author of The Year-Round Hoophouse and Sustainable Market Farming.