By Pam Dawling

Cover crops offer so many benefits that your first step in deciding what to plant is to identify which of these are priorities for each field:
•Prevent lots of weeds growing and seeding;
•Add organic matter to the soil, increasing the biological activity;
•Improve the soil and sub-soil structure, tilth, drainage and water-holding capacity;
•Prevent erosion by keeping something growing because roots anchor the soil;
•Add nitrogen to feed the next crop (Leguminous cover crops)
•Absorb any nitrogen leftover from feeding the previous crop (Non-legumes)
•Encourage beneficial insects (flowering cover crops)

The second step is to look at the “cover crop window” you have:
•Winter cover crops, the focus of this article, are usually sown after harvesting a major summer vegetable crop. Sometimes they can be under-sown 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.
•Spring gaps where you plan to grow a crop later in the season. If you have six weeks or more, you can till in the winter weeds and sow oats. Plan ahead and order seed now, along with winter cover crop seed.
•Summer gaps between the end of one vegetable crop and the planting of the next, or to cover soil if a cash crop fails, and prevent weeds. Either sow a quick-growing cover like buckwheat or soy, or perhaps under-sow a spring vegetable crop with buckwheat, white annual clover, or sorghum-sudan. Or even plant the cover crop and later till out strips to plant fall vegetables, and perhaps till in the rest of the cover crop later. No-till cover crops for organic vegetable growing is an area of current research. The winter can offer time to reconfigure planting schedules to make more windows for short term summer cover crops.
•Full year cover crops (green fallow), to rebuild fertility, using perennials, or biennials, often clovers. When we set up our 10-year rotation, 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 the annual weed-seed load.

Our winter cover crops
At Twin Oaks, in central Virginia (Zone 7), we sow oats in August (or early September if necessary), in the areas where we plan to plant the early spring crops next year. The oats will be killed by hard frosts, (20°F), and the killed mulch will be very easy to till in, come spring. We have done 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 August. Most of our crops are not finished before then, so we also do under-sowing. We have good success under-sowing oats and soy in our late sweet corn four weeks after seeding. This area is then used for spring potatoes the next year. 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. The areas that are cleared in August are where the early sweet corn was. 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 over winter, 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 also sometimes use other warm weather cover crops that will be winter-killed, if we will be planting early the next year. Examples include sorghum-sudan hybrid, buckwheat, soy, cowpeas, Miami peas, and the millets. (After a bad experience battling over-size millet, I’ve been avoiding growing them, but some are better behaved than the giants I had.)
We plant rye, Austrian winter peas and hairy vetch for the following year’s paste tomatoes and some of the peppers, sowing September 7-14. 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 choose to have the tomatoes and peppers follow the spring broccoli and cabbage, which finishes in early July. This gives us time for a round of buckwheat summer cover crop before the rye, vetch and peas go in. We have also used this no-till cover crop technique for watermelons, planted out in soil blocks.
We plant oats and crimson clover, or rye and crimson clover 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. 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 more 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 longer term perspective on productivity that went beyond one season. The introduction of biodegradable black plastic mulch revolutionized our watermelon crop! The melons really do ripen 3-4 weeks earlier than with organic mulch. Naturally, people want watermelon in July and August, not October. 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. They store quite well like this for several weeks, especially once the nights start to cool down. When the end happens before mid-October, we use rye and crimson clover. If we’re later we use rye and winter peas. The next year’s crops there are the middle three successions of sweet corn plantings, sown 6/6-7/7.
We plant winter rye and Austrian winter peas in late October or early November, after the late-finishing crops (winter squash, sweet potatoes, tomatoes and peppers, middle sweet corn) have been frosted. We plant in plots where we will not disk until mid-April 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. The winter squash get harvested once a week through September and October. We go through the 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 rye alone after early November. Rye needs 3-4 weeks in spring to break down and disarm its allelopathic compounds, which prevent small seeds from germinating. So we try to allow enough time to till in the rye ahead of the next crop. Rye is our Last Chance cover crop, and we reckon we can sow till mid-November, although it’s only worth sowing in November if it will have time to make growth in spring. Our fall carrots (sown at the end of July), are harvested in November, and if we are too late for rye, we simply spread the tops over the area to protect the soil, and then 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 usually sow oats, as they break down faster.

Simplify your options
Work 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, red clover; or soy beans, 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, winter barley; or soy beans or Miami peas to winter-kill. When only 20 days 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 which 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. Options become fewer, but it is still possible to sow winter rye up to a month past your average frost date. This information, for GFM readers who save their issues, comes from an April 1999 article by Bart Hall; that issue is no longer available.

Other possibilities
These are ideas that don’t work for us, but might work for you.
We tried under-sowing winter squash and pumpkins with red clover, which works well in New York State, done when the vines are just starting to run. But in our climate the vines grew so fast, they overwhelmed the clover. Our vines quickly cover the whole field, once they start running. One year we tried buckwheat between the rows to keep the weeds down, but failed to till in the buckwheat, and had to wade in and pull it by hand – the crew hasn’t forgiven me!
We also tried under-sowing corn with crimson clover, but it was difficult to get it to germinate in the heat and dryness of June and July, and it died. Soy is much easier to deal with, and cheaper.
We tried a forage brassica (canola/rape), before a new strawberry planting, but it encouraged too many brassica pests, the Harlequin bug being the worst. We decided brassica cover crops are not for us. In areas where they work, sow daikon, forage radish, mustards or canola at 4-14 lb. per acre drilled, 10-20 lb. per acre broadcast. Do this when the soil is 45-85°F. Aim to get 6-8 leaves before the killing frost. The brassicas produce allelopathic compounds that inhibit weeds, and biotoxins (glocosinolides) that kill pests.
Farther south than us, in zones 8b-10a, black oats can be sown in late September-October at 50-70 lb. per acre. Lupin can be used in September or early October at 70-120 lb. per acre. It does not tolerate waterlogged soil. Crotolaria (Sunnhemp) can be sown at least 9 weeks before the frost at 40-50 lb. per acre. These southern cover crops are relatively expensive.

Five for the future
•Sow crimson clover in September, and in spring strip-till rows and plant eggplant. Apparently the clover flowers may attract assassin bugs which will eat Colorado Potato Beetle.
•Sow Lana Woolypod Vetch or Berseem clover with oats as a winter cover before early spring vegetable crops. It will provide nitrogen and then be winter-killed. We use soy instead.
•Annual ryegrass, sown 9/10-10/10 @ 2oz/100 sq. ft., rapidly forms a soil-protecting mat, and then dies in winter. Be careful – it can become a weed if it seeds! We think the risk is not worth the possible benefits.
•Winter rye can be undersown in tasselling or silking corn or fall brassicas in late August, and left as a winter cover crop.
•“Frost Seeding”: Early in the morning after a hard frost, broadcast clover seed on prepared soil. The thawing will wet the seeds and pull them down into the soil.

Resources
Among the many guides to cover crops, my favorite is Managing Cover Crops Profitably, 3rd edition 2007, from the Sustainable Agriculture Network/SARE. It’s great value for $19, and the entire text is on their website.  http://www.sare.org/publications/covercrops/covercrops.pdf. I’ve also gleaned ideas for my region from the Virginia Co-op Extension Service publication. Cover Crops, http://pubs.ext.vt.edu/426/426-334/426-344table.html and the Kansas State University Extension publication Cover Crops for Vegetable Growers at  http://www.oznet.ksu.edu/library/hort2/mf2343.pdf.

Pam Dawling is the garden manager at Twin Oaks Community in Louisa, Virginia. She can be emailed at pam@twinoaks.org.