All About Growing Cover Crops

Begin by identifying a possible niche for a cover crop, and think about which of these benefits are your priorities at that site:

• Prevent weeds growing and seeding
• Add organic matter to the soil and increase the biological activity
• Improve the soil and sub-soil structure, tilth, drainage and water-holding capacity
• Prevent erosion by keeping something growing (roots anchor the soil)
• Add nitrogen to feed the next crop (leguminous cover crops)
• Absorb any nitrogen and other nutrients left over from feeding the previous crop (non- leguminous cover crops)
• Encourage beneficial insects (flowering cover crops)

The Second Step

Look at the “cover crop window” you have:

• Winter cover crops are usually sown after harvesting a major summer vegetable crop. Sometimes they can be undersown in the vegetable crop, while it is growing. They will then continue to grow after the vegetable is finished. Winter-killed cover crops are another option for planting after your main crop. The dead mulch covers the soil and is easy to till in as soon as spring arrives. Winter cover crops include grasses, legumes and brassicas that grow in cool and cold seasons.

• Spring gaps are locations where you plan to grow a crop later in the season. If you have six weeks or more in spring, you can till in the winter growth and sow oats. Plan ahead and order oats along with winter cover crop seed. Quick-growing cool-season grasses and brassicas may be suitable.

• Summer gaps may occur between the end of one vegetable crop and the planting of the next, or if a cash crop fails. Cover crops will reduce weeds. You could sow a quick-growing cover like mustard, buckwheat or soy, to till in before the next cash crop. Or you could plant a warm-weather cover crop, then till strips to plant fall vegetables, and perhaps till the rest of the cover crop later. (Winter can offer time to reconfigure planting schedules to make more future windows for short-term summer cover crops.)

• Full-year cover crops (green fallow) can be used to rebuild fertility, using perennials or biennials such as clovers. When we set up our ten-year rotation in a more organized fashion than we had been using, we discovered to our surprise that we had a plot “spare,” which we now use to grow cover crops to replenish the soil and reduce annual weeds.

• Undersow a cover crop between rows of a food crop. Perhaps undersow a spring vegetable crop with buckwheat, white clover or sorghum-sudan, which will take over after the spring food crop is finished. Summer vining crops like winter squash, pumpkins or watermelon, if not on plastic mulch, also make good use of this technique, provided the cover crop between the rows is progressively mow-killed or tilled in, or is low-growing and will not compete with the crop.

The Third Step

Identify suitable cover crops for each situation. Ray Weil divides cover crops into six groups:

• Cool-season grasses
• Cool-season legumes
• Cool-season brassicas
• Warm-season grasses
• Warm-season legumes
• Warm-season broadleaved crops

How We Use Winter Cover Crops at Twin Oaks, in Central Virginia, Zone 7

We sow oats in August (or early September—later is too late to make enough growth) in the areas where we plan to plant the early spring crops next year. The oats will be killed by hard frosts, creating a mulch that will be very easy to till under, come spring. We do this before peas, cabbage, broccoli, March-planted potatoes, spinach and sometimes the first sweet corn. This necessitates a crop rotation that clears those patches before the end of the previous August. For us, that’s early sweet corn, spring broccoli and cabbage and spring-planted potatoes.

Most of our crops are not finished before then, so in order to have more areas in winter-killed cover crops, we undersow some crops. We have good success undersowing our last sowing of sweet corn four weeks after seeding, with oats and soy. The next year, this area is disked in early spring for potatoes. It’s at least theoretically possible to transplant into the winter-killed oat mulch in the spring without tilling, although weeds may be a problem and the soil will be colder than bare soil—this may work for cabbage and broccoli.

In addition, we have the option of tilling in all or part of our green fallow plot, which has been in a clover mix, and sowing oats there in August. If the clover is growing well and the weeds are not too bad, we leave the clover to overwinter, and disk it in February. But if the weeds are gaining the upper hand in August, sowing oats (perhaps mixed with soy) is a better bet. If the weeds are bad in July, we disk in the clovers and sow sorghum-sudan hybrid mixed with soy. While this deals effectively with the weeds, it is a poor crop rotation, as the next year’s crop there is early sweet corn, which is related to sorghum-sudan.

During August and September, we sow other warm-weather cover crops that will be winter-killed, into areas that will be planted early the next year. Examples include sorghum-sudan hybrids, buckwheat, soy, cowpeas, Miami peas and millet.

The spring broccoli and cabbage finish in early July. We follow them with a round of buckwheat summer cover crop. Then, on September 7–14, we sow winter rye, Austrian winter peas and hairy vetch for the following year’s paste tomatoes and peppers. The next year, we do not till in this cover crop but mow it very close to the ground and transplant into the dying mulch. The vetch supplies all the nitrogen the tomatoes need, and the dead mulch keeps weeds away for weeks. The peas are thought to help limit Septoria leaf spot, a troublesome disease of tomatoes in this region. (We do eventually roll hay between the rows, in July, to top up the mulch). We have also used this no-till cover crop technique for watermelons, planted out in soil blocks, but watermelons like warm soil, so plastic mulch is a safer bet.

We plant crimson clover with oats or winter rye where we plan to have the late crops, which will not be planted till late May or June next year (chiefly the later sweet corn plantings, June-planted potatoes, winter squash and watermelons). This gives the leguminous cover crops time to flower before we need to disk them in. Once again, the legume supplies all the nitrogen for the following crop. Oats will winter-kill and be easier to incorporate, but rye will make more biomass, most of it in early spring. In practice we usually go for the rye, as we are too late for oats. In recent years, we have moved towards bringing our watermelon, sweet potatoes and winter squash to a timely end, in order to get good winter cover crops established. We used to hold onto those crops until the last minute to maximize crop yields, but realized we would do better with a perspective on productivity that went beyond one season.

The introduction of biodegradable black plastic mulch revolutionized our watermelon crop! The melons really do ripen three to four weeks earlier than with organic mulch. So now we harvest until we have enough melons to see us through to early October, then disk in the plot. We store our watermelons outdoors in the shade of trees or the eaves of a building, where they store quite well for several weeks. When the cover-crop sowing happens before mid-October, we use rye and crimson clover. If we’re later, we use rye and winter peas.

We plant winter rye and Austrian winter peas (which can be sown later than crimson clover) during late October or even early November after the late-finishing crops (winter squash, sweet potatoes, tomatoes and peppers, middle sweet corn). We will not disk these areas until mid-April the next year. Examples in our rotation include the watermelons to middle corn mentioned above; the winter squash to sweet potatoes and late sweet corn; the tomatoes and peppers to watermelons; and the June potatoes to winter squash. We go through the winter squash field for the last time just before Halloween and pull out the large semi-mature squash and give these away for lantern carving. Then we disk that patch and sow rye and winter peas. We aim to harvest the sweet potatoes in the week we expect the first frost (mid- October), then disk that plot and sow the cover crops.

We use winter rye alone after early November, as it is too cold for anything else to make enough growth. Rye needs three to four weeks after tilling in, in spring, to break down and to disarm the allelopathic compounds that prevent small seeds from germinating. Rye is our Last Chance cover crop, and we reckon that we can sow until mid- November, although it’s only worth sowing in November if it will have time to make growth in spring. For example, we harvest our fall carrots (sown in early August) in November, and if we are too late for rye, we simply spread the tops over the area to protect the soil and let weeds grow. Rye can be sown up to a month after the first frost and also in the early spring. If we are sowing in early spring, though, we prefer to sow oats, as they break down quicker.

How to Simplify Cover Crop Options

Bart Hall once published a very useful table in Growing for Market, which works back from your farm’s first frost date, to see what options you have. If you are more than 120 days before frost, you would most likely plant another food crop. If you are 80 days before your frost date, you could sow buckwheat, oats, Japanese millet, sorghum-sudan or possibly another vegetable crop. If only 60 days remain before frost, sow oats, Austrian winter peas, crimson clover or red clover; or soybeans, buckwheat, Japanese millet, sorghum-sudan, winter barley, Miami peas to winter-kill. When about 40 days from frost, use oats, Austrian winter peas, crimson clover, hairy vetch, red clover, fava beans or winter barley; or soy beans or Miami peas to winter-kill. When only 20 days remain before frost, sow winter rye with or without hairy vetch, winter wheat, winter barley, Austrian winter peas, red clover or crimson clover. It is too late to usefully sow crops that are not frost-hardy. Up to 10 days past the frost date, you can sow winter rye, winter wheat or perhaps red clover or crimson clover. After that, options become fewer, but it is still possible to sow winter rye up to a month past your average frost date.

How We Use Summer Cover Crops at Twin Oaks

Unlike our winter cover crops, we usually only plant our summer cover crops in small areas each time, so we broadcast and till in with our walk-behind BCS tiller. It is also possible to use an Earthway-type seeder for small areas of cover crops; good information about this, including which plates to use for which crops, can be found in the Virginia Association for Biological Farming Infosheet Seeders: Using Manually Operated Seeders for Precision Cover Crop Planting, by Mark Schonbeck and Ron Morse.

We sow oats in February or March, when we have winter weeds and will not be planting a food crop for eight weeks. Once we get to March 31 here, it is too late in the year for oats (they will quickly head up after making very little growth) and too soon to rely on frost-tender cover crops. We sometimes sow winter rye, which “languishes” here once it gets hot. I did this one year when our spring potatoes got flooded in April — we actually transplanted potatoes from the flood margins to the drier end of the patch and sowed rye in the lower end, once the floods had subsided. This kept the ground covered, and was easy to deal with in July at potato harvest time.

Once we get to late April (close to our average last frost), we are ready to sow frost-tender cover crops like buckwheat or soy, if mixed with a grain such as winter rye or wheat for insurance and some shielding from harsh weather. Warm-weather cover crops we use include sorghum-sudan hybrid, buckwheat, soy, cowpeas, Miami peas and the millets. After a bad experience battling oversize millet with small-scale equipment, I avoided growing them, but some are better behaved than the giants I had. German foxtail millet grows to 3’–4′ (1–1.3 m), Japanese millet to 3’–5′ (1–1.6 m), and pearl millet (my nemesis) gets taller, at 5’–10′ (1.6–3.2 m).

We like to include legumes in our cover crop mixes whenever we can, to add nitrogen to the soil. Soy has been a cheap legume for us, because Twin Oaks also has a business making and selling organic tofu. We can get the beans at wholesale price and be certain they are organic. Almost all non-organic soy grown in the US is genetically modified, so if you don’t want to add to the problems caused by GMOs, buy organic, or if you are not certified organic, Identity Preserved. See the Organic & Non-GMO Report’s Non-GMO Sourcebook for a searchable database of non-GMO suppliers.

Full-year Cover Crops

You can plant long-term cover crops in a section of your crop rotation to replenish the soil. They can also be planted as perennials in areas that are too challenging to use for production: edges, slopes, tight corners. This is farmscaping. These cover crops can attract beneficial insects, and the pollen and nectar can offer an alternative food source to beneficial insects when their insect prey is scarce. This can also be achieved by leaving an unmowed section when most of the crop is mowed, so that the beneficials don’t all die.

Undersowing (Inter-seeding)

The timing of undersowing is critical. Sow the cover crop late enough to minimize competition with the food crop, but early enough so that it can survive and be big enough to endure foot traffic. The leaf canopy of the food crop should not yet be closed. Often the best time is at the last cultivation (known in the UK as “lay-by”). With vining food crops, it’s important to sow the cover crop before the vines run. Choose vigorous food crops, but cover crops that are only moderately vigorous. Buckwheat, millets and cowpeas all have their fans. Ensure the seedbed is clean and the soil crumbs small enough. Use a high-seeding rate, whether broadcasting or drilling. And irrigate sufficiently—this is the second critical component of successful undersowing.

Kale can be undersown with rye and hairy vetch, rye alone or oats in mid-late August in New York, but in Virginia, winter rye sown in August will go to seed the same year. And perhaps more importantly, we expect kale to overwinter, and cover crops would compete too much.

Peas, eggplant and peppers can be undersown with oats. Living mulches cool the soil, which is not usually wanted for early plantings of warm-weather crops, but works well for cool-weather crops like peas. It is important to keep mowing the living mulch so that it doesn’t outcompete the food crop.

We tried undersowing winter squash and pumpkins with red clover, which works well in New York State, sown when the vines are just starting to run. The clover germinates OK in the low-light levels under the squash, and survives foot traffic. Financially, this is probably only worthwhile if the clover grows for a full year. We also tried undersowing corn with crimson clover, which also works in NY. The corn can be mowed after harvest, and the clover left to grow over the winter. In the South, corn and squash grow too fast compared to clover for this scheme to work. Our vines very quickly cover the whole field, once they start running. It is difficult in Virginia to get the clover to germinate in the heat and dryness of June and July. Soy is much easier to deal with, and cheaper.

Buckwheat can be undersown with winter squash or sweet potatoes, and mowed as soon as the vines start to run. One year we tried buckwheat between our squash rows to keep the weeds down, but failed to mow or till in the buckwheat, and had to wade in and pull it by hand—the crew hasn’t yet forgiven me! One trial of undersowing buckwheat in corn reported that, if sown the same day as the corn, the buckwheat outcompeted the crop; if sown at the eight-leaf stage of the corn, the buckwheat did not get enough light. Find the happy medium!

We tried a forage brassica (canola/rape), before a new strawberry planting, but it encouraged too many brassica pests, harlequin bugs being the worst. We decided brassica cover crops are not for us. In areas where they work, try daikon, forage radish, mustards or canola. Do this when the soil is 45°F–85°F (7°C–30°C). That’s up until early October for us. Aim to get six to eight leaves before the killing frost. The brassicas produce allelopathic compounds that inhibit weeds and biotoxins (glucosinolates) that kill pests.

Organic No-Till Cover Crops

Some cover crops can be killed by mowing, rather than having to till them in; food crops can then be planted in the dying residue. Roll-killing is another option, but usually requires special equipment. Mow-killing and roll-killing can be challenging to do successfully, so it’s best to experiment with a small area first.

Advantages of no-till include:

• Reduction of soil disturbance and the associated burn-up of organic matter;
• Prevention of new weed seeds being brought to the surface;
• Smaller number of tractor passes across the field, saving fuel and preventing soil compaction;
• Improvement of soil life diversity in the top layers of soil;
• Cooler soils in summer;
• And reduced need to bring in other mulches to control weeds.

Disadvantages include:

• Cooler soils in spring, meaning slower crop growth and later harvests;
• Slower rate of nitrogen release from the cover crop;
• Possibly more fungal diseases and slugs;
• Possibly problems with regrowth of the cover crop;
• And possibly weeds growing in the cover crop.

Summer vegetables, especially tomatoes, can do very well after a legume-grass mix no-till cover crop, needing no additional source of fertility. Fast-maturing spring vegetables will not do well with no-till cover crops as they need nitrogen more quickly than can be got from no-till. It is sensible, especially if you haven’t got much experience with no-till, to have a backup plan in case cover crop stands end up too thin, or the weeds are too plentiful for the no-till system to work.

Usually this means tilling in the poor cover crop and adding compost or some other source of fertility.

A related approach is reduced tillage: take an existing plot of cover crops and till out strips in summer and plant the next food crop. Sometimes the rest of the cover crop is tilled under later, as the food crop needs the space.

How to Clarify Your Options for Summer Gaps

If you have only 28 days until the patch is needed for a food crop, you can grow mustards or buckwheat; or weeds, if you’re careful not to let them seed! If you have at least 45 days, you can grow soy or Japanese millet. If you have 50–60 days, browntop millet is possible. In the right climate, sunn hemp can mature in 60 days. With 60–70 days, German/foxtail millet, pearl millet and some cowpeas will mature.

In high-moisture years, grow the most weed-suppressing crops, e.g., alfalfa. If fall moisture is low, sow peas, or wait until spring and sow a fast-maturing legume. Spring-planted peas can produce more nitrogen than fall-planted Austrian winter peas.

Cover Crop Mixes

A 2011 ATTRA webinar advocates mixes of cover crops. Using mixes can give the plot the advantages of each of the components, and also insure that regardless of the weather or rainfall, some cover will grow. Additionally, most mixes include some crops that attract beneficial insects and some legumes to add nitrogen. Mixes can generally be sown at a depth of one inch (2.5 cm), regardless of seed size.

Major ingredients for a summer mix could include soy, cowpeas and buckwheat. Lesser ingredients could include pearl millet, proso millet, radish, turnips, sunflowers and sunn hemp.

A spring mix could have oats and peas as the main ingredients, with hairy vetch, radish, turnips and red clover as minor ingredients.

Nitrogen Availability

How much nitrogen becomes available to the next crop depends on the C:N ratio in the biomass of the cover crop. C:N ratio is as important here as in compost-making. The soil microbes that digest the cover crop have a C:N ratio of 10:1. When a cover crop is incorporated into the soil, the microbes use the carbon and some of the nitrogen in the cover crop to build more microbes, tying it up until they die, meaning it is not immediately available to the next crop. If the cover crop has a C:N ratio of 50:1 (like sorghum-sudan), the microbes will need to find extra N to make use of all the C. They will use N from the soil, tying it up until they die. Hence any crop following immediately after a high C:N ratio cover crop will need a different source of nitrogen.

Legumes have a lower C:N ratio (from 30:1 down to 12:1), and when they are incorporated, soil N is unlikely to be tied up, so is available for the next crop. This process is explained in the North Carolina Extension Service Horticulture Information Leaflet 37 cited in the Resources section. This explains the advice to incorporate high-C cover crops a few weeks ahead of planting the following crop. It also allows time for any cover crop allelopathy to subside.

If cover crop residues are left on the surface rather than incorporated, the rate of decomposition is slowed. Some nitrogen is lost to the air (denitrification), but the increased organic matter can boost the diversity of microorganisms at the surface. Some of the carbon from cover crops is below the top eight inches (20 cm), where almost all soil data are collected. Remember the value of the roots.

This excerpt has been reprinted with permission from Sustainable Market Farming: Intensive Vegetable Production on a Few Acres, published by New Society Publishers, 2013. Buy this book from our store: Sustainable Market Farming, Intensive Vegetable Production on a Few Acres.