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Late Summer and Fall Intercropping of Cover Crops in Vegetable Crops, aka Undersowing

I wrote about Intercropping – Companion planting that really works! in April and Intercropping Vegetables in Late Spring and Early Summer, aka Relay Planting in June. Now we have reached summer, many of us will be thinking in terms of establishing winter cover crops, rather than planting lots more food crops.

The principles of intercropping can apply here too. Instead of tilling up the soil after the last food crop, in some cases you can sow your cover crop while the vegetable crop is still in the ground.


Advantages of undersowing cover crops:

• Increased organic matter for the soil, because the cover crop has longer to grow, compared to sowing after the food crop is finished.
• Increased choice of winter cover crop, including more opportunities to use oats, which readily winter-kills, making for an easier start next spring.
• A way of being able to have a winter cover crop after a very late vegetable crop. Much better than leaving bare soil! Better organic farming!
• More opportunities to include legumes for adding nitrogen to the soil.
• A way of capturing nutrients not used by the food crop and storing them over the winter.
• Reduced tillage, reduced fossil fuel use, a more sustainable farming method.
• Overhead irrigation does double duty: waters the vegetable and the cover crop.
• Reduced erosion: the vegetable crop holds the soil together while the cover crop gets established.
• Better weed control, therefore higher yields of your home grown vegetables.
• Better pest control, better-looking produce and more of it.
• More habitat for beneficial insects.
• An earlier sense of relief at getting the plots “put to bed” for the year.


Fall broccoli showing small clover cover crop.

Examples of undersown cover crops in late summer and fall vegetable crops

• We undersow our fall broccoli and cabbage with a mix of Crimson clover, white clover and medium red clover in August, to form a green fallow crop (all-year cover crop) for the following year.
• We undersow our last sweet corn planting with oats and soybeans, as the winter cover crop, which winter-kills, leaving a plot that is easily worked up next spring.
• Sweet corn can be undersown with clover rather than soy or oats. We tried clover but found it harder to germinate in hot weather, and harder to keep the tiny seed damp. Buckwheat can be undersown in corn as a short term summer cover, but according to Sue Ellen Johnson, (co-editor of Crop Rotation On Organic Farms: a planning manual, it grows rather straggly in the shade of the corn. Soy has the advantages of tolerating shade as well as foot traffic.
• An idea I’ve meant to try for years, but have so far not planned in time, is to sow crimson clover in late September (central Virginia) in the beds where you will grow eggplant next year. This is said to deter Colorado Potato Beetle and flea beetle.This is the reverse of sowing a cover crop into a standing vegetable crop, so maybe it’s in the wrong article!
• Winter squash or other vining crops can be undersown with cover crops just before the vines start to run, after cultivation between the crop rows. If you have left it a bit late, you can fold the vines over to one side, cultivate and sow, then fold the vines over to the other side and repeat. It’s important to return the vines to their right-side-up positions as, like other cucurbits, they don’t do well if inverted.  The timing isn't right for winter rye, oats, or other winter grains, nor do you want something that grows too tall by harvest. Low-growing red clover is a better choice. High seeding rates will help assure establishment. We tried this without success. I think it works better in cooler climates. Here in the mid-Atlantic, squash vines grow very fast, so the undersowing opportunity is short. And the soil is too hot for good clover germination (the same problem with the clover under corn idea mentioned above).
• After the squash and clover experience, we sowed buckwheat between our squash rows, intending to till it in strip by strip, getting ever closer to the squash. (This is not an example of undersowing a winter cover crop either, but it is related!) We failed to till in the buckwheat in time and had to wade in pulling it by hand. I wasn’t popular with the crew that week.
• Garlic and winter-killed oats – this idea comes from New York State, and it too, seems dependent on the right climate. The reason it doesn’t work for us is that we need to sow oats in August or the first half of September to get a good stand, and we plant garlic in early November. It’s too late to sow oats after garlic planting here, and too messy to plant garlic into standing oats. We prefer to roll out hay mulch over the garlic beds directly after planting.
• Peppers, tomatoes, cabbage, kale and rye, hairy vetch, or oats. Someone, somewhere has tried each of these combinations of a food crop and an undersown cover crop. Remember not to cultivate deeply near the roots of your food crop when sowing the cover crop.


Sweet corn undersown with soy beans.

Suitable and Unsuitable Situations for Intercropping – to help with designing your own system.

• Timing must work: the vegetable crop must be large enough to withstand cover crop competition, but young enough to cultivate between the rows (the crop canopy cannot be too closed). With sweet corn 28-30 days after sowing the corn is about right. This is generally the time to cultivate between the rows for the second and last time. Tilling between corn plants that are more than knee-high can damage too many roots. I confess I have at times pretended I was 6’6” rather than 5’ 5” in order to gain a bit on that “knee-high” measurement!
• Timing must work for the cover crop too: on our central Virginia farm, August is the time for sowing oats, but not for winter rye, which could head up before winter. Oats sown too late just don’t make enough growth to provide much weed control or enough winter-killed biomass. And the cover crop has to be able to compete enough with the food crop to get established.
• Timing must work for whatever comes next. With sweet corn, we will be walking between the rows to harvest. The cover crop needs to be large enough to withstand the traffic, as well as be of a type that can take some traffic. If your crops are on raised beds, traffic is not an issue.
• Vertical growing vegetables which have only a small “footprint,” like tomatoes, peppers and sweet corn, benefit from ground-covering cover crops such as Dutch white clover which comes up fast and produces a weed-suppressing soil cover which retains soil moisture, adds biomass and nitrogen and encourages soil micro-organisms.
• Vining vegetables benefit from fast-growing cover crops that establish before vining takes over the space. The rate of growth of the cover crop has to be faster than the rate of vining!
• Small and slow-growing vegetables such as onions or carrots will be unlikely to do well with undersown cover crops. The vegetable cannot out-compete the cover crop.
• If you have a dry climate and a shortage of irrigation water, it would be unwise to stress the vegetable crop by introducing a competing cover crop.
• If you suffer bad diseases of the vegetable crop, you may be wiser to till in the crop residues rather than leave them on the surface as undersowing a cover crop requires.

A Technically Tricky Situation

Broadcasting clover seed in broccoli does result in some seed lodging on the broccoli leaves – not much, and adjusting your broadcasting technique to a lower “throw” can help. With cabbages, it is more difficult, as the leaves cup and hold the seed. Another option is to drill the seed with an EarthWay seeder. Plate 5 is suitable for crimson clover, radish, sorghum-sudangrass and the millets; plate 14 works for Austrian winter peas; plate 22 for the grains, field peas, soybeans, cowpeas and buckwheat. See the Virginia Association for Biological farming Infosheet Using Manually-Operated Seeders for Precision Cover Crop Plantings on the Small Farm by Mark Schonbeck and Ron Morse.


Cover Crops and Green Manures

Managing Cover Crops Profitably

Soil Care Basics: Increasing Organic Matter and Mineral Availability

Crop Rotation On Organic Farms: a planning manual edited by Charles Mohler and Sue Ellen Johnson. Especially see Table 7.1 Cover crop interseeding systems. 

Photos by Nina Gentle (broccoli and clover); Kathryn Simmons (corn and soy; crimson clover)

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