Cover Crops for September: wheat and crimson clover

Crimson clover is a beautiful and useful cover crop.
Photo Kathryn Simmons

Note: the day after I posted this I found I was mistaken in believing barley to die at a warmer temperature than oats, so I’ve edited it.

Focus Cover Crops for September: Winter Wheat and Crimson Clover

In August I wrote about cover crops such as millets, southern peas, buckwheat which are frost-killed. If it’s still too early to sow your winter cover crops, sow summer cover crops. Before I get to the wheat and crimson clover, I’ll mention some other useful seasonal cover crops.

Winter-killed, not frost-killed, cover crops

There are also cover crops that are not frost-killed, but die later in the winter, at colder temperatures, such as oats and barley. Only sow oats or barley if you are sure you can get them turned under or killed by cold winter weather before they seed. They will not mow-kill, so if the weather doesn’t kill them, you will have to turn them under. Be careful buying feed-grade seeds (rather than Organic seed-grade), as they can contain weed seeds including GMO canola.

Late corn undersown with oats, now mowed high, and the sweet potato patch now sown in winter wheat and crimson clover.
Credit Ezra Freeman

If the area is clear of vegetable crops by 40-60 days before frost, sow oats to winter-kill. If possible add a legume (soy and spring peas are easy, and will be killed by the frost, so they won’t complicate food crops next year). For us with a first frost date of October 14-20, the cut-off date for oats is September 7, or September 15 if we really push it. Sowing too late means you don’t get enough growth in the fall, and the soil is not adequately protected from erosion or from weed growth.

We sow oats after growing early sweet corn, spring broccoli, spring-planted potatoes, cabbage, kale, or early season spinach, lettuce, beets, carrots. Spring oats die after three nights at 20°F (-7°C), or a single plummet to 6°F (-17°C), leaving the plot quick to prepare for early crops next year. Winter oats are hardier, but my goal with growing oats is for them to die in winter. After oats or other winter-killed cover crop, we like to plant our early spring food crops, peas, cabbage, broccoli, carrots, March-planted potatoes, spinach and the first sweet corn.

Don’t let your cover crop barley go to seed! Photo USDA

Fall-sown barley (Hordeum vulgare), grows even faster than oats, but not as quickly as winter rye, and it won’t die as early in the winter as oats. Barley dies at 17°F (-8°C). It usually will die in Zone 7 and colder regions. The dead barley residue protects the soil through the winter, and dries into what Barbara Pleasant calls “a plant-through mulch” in spring in cold zones.

See Planning Winter Cover Crops, a post that includes my Short Simple Guide to Winter Cover Crops and my slideshow Cover Crops for Vegetable Growers. Oats, barley, wheat and rye sown too early can head up and seed before you get to winter, making them less useful, and more of a weed problem.

Winter-hardy grass cover crops to sow in September

It is too late for us to usefully sow cover crops that are not frost-hardy, as they won’t make enough growth before getting killed.

Winter rye and winter wheat are two grass cover crops that can be sown in the mid-Atlantic in September. Wheat is easier to incorporate than rye and has less of an allelopathic effect on small seeds, the inhibition of germination that lasts three weeks after rye is turned under. It’s true wheat doesn’t produce as much biomass as rye, so there’s the tradeoff. We sow wheat if the area is ready for cover crops 20-40 days before frost. This allows us to make faster use of those plots in the spring, compared to plots sown to rye.

Winter wheat
Photo USDA

For us wheat is a good, trouble-free winter cover crop. Winter wheat prevents erosion, suppresses weeds, scavenges excess nutrients, adds organic matter, encourages helpful soil microorganisms, and the fine root system improves the tilth. It is less likely than barley or rye to become a weed; easier to kill than barley or rye; cheaper than rye; easier to manage in spring than rye (less bulk, slower to go to seed); tolerates poorly drained, heavier soils better than barley or oats. If you have leftover seed, wheat can be sown in spring – it will not head up, but “wimps out” when the weather gets hot.

The challenges of wheat are that it does not have good tolerance of flooding, and is a little more susceptible than rye or oats to insects and disease.

Secondary cover crops in September:  Include legumes where possible

With careful planning, you can grow next year’s fertilizer for your later spring-planted vegetables! Legumes grow nitrogen-fixing bacteria on their roots, which can feed the next food crop. You may need to buy a suitable inoculant if you are introducing a new legume species. You may decide to inoculate anyway, for insurance, even if that type of legume is already somewhere in your garden. Before sowing the legume seed, dampen it, sprinkle the inoculant over the seed at a “pepper on your dinner” rate, and stir it in. Then sow the seed. Be sure not to make the seed wetter than slightly damp, or you’ll need to spread it out to dry a bit before you sow.

Two other key parts of being successful are to sow the legume early enough to establish before winter halts growth, and to plan not to need that plot next year until flowering time for that legume. At flowering time, legumes have the maximum amount of the nitrogen nodules they will have. Don’t let the legume flowers set seed, or they may become a weed problem. Take notes on when various legumes flower. If you have a legume that doesn’t reach flowering, it’s not the end of the world, you just get less nitrogen for your money, and won’t be able to supply all the N needs of the following food crop.

September (40-60 days before frost) is a good time to sow clovers here, provided you can supply enough overhead irrigation. They will make some growth in our climate before winter, and then a lot more once spring arrives.

Crimson clover cover crop with bumblebees.
Photo Bridget Aleshire

Crimson clover is a good September choice, if you won’t need to prepare the area before it flowers (in central Virginia 4/16-5/2, most usually around 4/20).

For a cover crop to survive the winter, sow winter wheat with Austrian winter peas, crimson clover, hairy vetch, red clover, white clover or fava beans. Hairy vetch takes a few weeks longer than crimson clover to reach flowering. Which you choose will depend what you want to grow there next spring and when you need to plant it.

Clover for green fallow in early September

See August’s post for info on planting a Green fallow plot (Full year cover crops)

Time is running out on this for us, but you may still have enough warm weather where you are. A green fallow crop (all-year cover crop) will replenish the soil and reduce annual weeds for the following year. In late August, or early September, four weeks after transplanting your fall brassicas, especially cabbage and broccoli, but also kale and collards, broadcast a mix of clovers: 1 oz (30 g) Crimson clover, 1 oz (30 g) Ladino white clover and 2 oz (60 g) Medium red clover per 100 sq ft (9 m2). Crimson clover is a winter annual and will be the biggest and the first to flower, in April. Medium red clover is a biennial and will be the next to flower. White clover is perennial and will take over the plot as the others subside. Be sure to get the medium red clover, not the Mammoth kind that dies when mowed. Likewise, for maximum benefit, get the tall Ladino white clover, not the low-growing “wild” type. In March, mow down the old brassica stumps and let the clovers flourish. You will be mowing this patch about once a month from March to October next year to prevent the crimson clover and the annual weeds from seeding.

Cover crops to sow soon after your first frost date

I’ll say more about this next month, and because I want this website to be useful to a geographically wide range of growers, I’m including a preview here. If the area is ready for cover crops up to 10 days past the frost date, sow winter wheat or winter rye with hairy vetch or Austrian winter peas. Winter rye is hardier than any other cover crop and can take later planting dates. But it is a bit harder than wheat to incorporate in the spring. Sow winter rye from 14 days before to 28 days after first fall frost. See Working with the time you have left in the Summer Cover Crops post. Austrian winter peas can be sown later than other legumes, it’s too late for clovers.

My book Sustainable Market Farming has a chapter on cover crops and many pages of charts about particular options.

The book Managing Cover Crops Profitably (third edition) from the Sustainable Agriculture Research & Education Program (SARE), is the best book I know on the subject. You buy the book for $19 or download it as a free PDF from SARE.

Cover Crops for July: Millet and Sorghum-Sundangrass

Focus Cover Crops for July:                     Millet and Sorghum-Sudangrass (Sudex)

In July we are very much looking for cover crops that will grow in hot weather. Last month I wrote about sunn hemp, with some notes about other hot weather legumes. This month we are turning our attention to good hot weather grass cover crops. Warm weather grass cover crops we use include Sorghum-Sudan hybrid (Sudex), and the millets.

Next month I will discuss winter-killed winter cover crops before early spring vegetables and undersowing winter-hardy cover crops in standing vegetable crops.

Also see my post Cover Crops in Summer for much more information, including  making space, sowing small spaces and finding time for the work.

Don’t sow a winter cover crop yet. If sown too early, oats head up in the fall and even drop seed. Only sow oats or barley in July in southern regions if you are sure you can get them turned under before they seed. They will not mow-kill.

Cover crop seeds are usually easier to find and cheaper at a local feed store, rather than online. Also you’re likely to find a regionally-adapted variety. But, of course, you may not find USDA Organic seed there. Be careful buying feed-grade seeds (rather than seed-grade), as they can contain weed seeds including GMO canola.

Variation in spring field corn root development at harvest following various summer cover crops. Preceding cover crop from left to right: sunn hemp (residues mulched in), pearl millet without fertilizer, sorghum-sudangrass, corn, pearl millet with fertilizer, and sunn hemp (residues harvested). Credit: Z. J. Grabau, UF/IFAS

Types of millet

Millet has good insect resistance and is relatively free of diseases. Read the descriptions below and especially the final height, the days to maturity and whether or not it mow-kills at any stage. See Working with the time you have left in the post Cover Crops in Summer. Avoid the trouble I had battling pearl millet with small walk-behind equipment, having to mow it frequently to keep it manageable.

German/Foxtail Millet

Foxtail millet seed heads.
Photo Eden Brothers Seed Supply

Setaria italic, 3-4’ (1-1.3 m) Fast growing annual (60-70 days). 75-90 days to seed formation. Not frost hardy. Mow-kills or roll-kills reliably after heading.

Sowing date: Needs warm soil. From 2 weeks after last frost onwards. Growth is considerably less if sown after summer solstice – day-length sensitive.

Sowing rate (US): Drilled:20 #/ac, Broadcast:30 #/ac, B:1-1.5 oz/100 ft2

Sowing rate (metric): D:22 kg/ha, B:34 kg/ha, B:3-4.5 gm/m2,

Uses and Cautions: Can be followed by late summer and fall crops. Fairly well-behaved – unlikely to become a weed. Easier than most other millets to incorporate.

Notes: Small seeds need good seedbed and few weed seeds. Fairly drought-tolerant once established. Shallow roots.

Japanese Millet

Japanese Millet
Photo Hancock Seed Company

Echinochloa esculenta 3-5’ (1-1.6 m). Fast growing annual, 45 days. Not frost hardy. Foxtail type, grown more in the north. Cannot mow-kill or roll-kill reliably.

Sowing date: From corn planting date. May and early June are best in the mid-Atlantic. Can be sown until early July, but growth is considerably less if planted after summer solstice because it is day-length sensitive.

Sowing rate (US): D:15-25 #/ac, B:35 #/ac, In mix with soy:54 # soy:12 # millet/ac, B:2 oz/100 ft2

Sowing rate (metric): D:17-28 kg/ha, B:40 kg/ha, In mix with soy:60 kg soy:14 kg millet/ha, B:6 gm/m2

Uses and Cautions: Can be followed by summer and fall vegetable crops.

Notes: For best re-growth, mow at 60 days when 3’ (1m) tall, and before heading. Cut to 3-8” (8-20 cm) and repeat every 40 days after that. If cut after heading, it will flower again in 2-4 weeks on short stems and set seed. Tolerates drought and wet soils, including cold wet soils.

Browntop Millet

Urochloa ramosa (L.) Nguyen. 2-5’ (0.6-1.5 m). 50-60 days. Cannot mow-kill. Foxtail type, grown more in the south.

Sowing date if last frost is 4/30, first frost 10/14. May – August

Sowing rate (US): D:20-30#/ac, B:30-40 #/ac, B:3 oz/100 ft2

Sowing rate (metric): D:22-34 kg/ha, B:34-45 kg/ha, B:9 gm/m2

Notes: Tolerant of acidic soils, low fertility and flooding.

Proso/Broomcorn Millet

Jean Hediger grows Proso millet on her Nunn, Colorado farm
Photography courtesy of Jean Hediger

Panicum miliaceum, More than 5’ (1.6 m) 60-90 days to maturity.

Sowing: Optimum soil temperature ranges from 55°F-65°F (13°C-18°C)

Sowing rate (US): D:20 #/ac, B:30 #/ac

Sowing rate (metric): D:22 kg/ha, B:34 kg/ha

Uses and Cautions: Can be followed by summer and fall crops. Seed heads may shatter once the topmost seeds are mature.

Notes: Cut after 60 days before it gets tough. A much finer textured grass than Pearl or Japanese millets. Makes a good mulch.

Pearl/ Cattail Millet

Pearl Millet
Photo USDA i7wiafe6

Pennisetum glaucum or P. Americanum, 5-10’ (1.6-3.2 m) Fast growing, 60-75 days. Not frost hardy. To winter-kill and avoid seed formation, sow 60-85 days before expected frost.

Sowing date: From corn-planting date until 60 days before fall frost. Soil temperatures of 75-90°F (24-35°C) are ideal.

Sowing rate (US): D:15#/ac in 18” rows, B:25-40 #/ac, B:1-1.5 oz/100 ft2

Sowing rate (metric): in 45 cm rows, B:28-44 kg/ha, B:3-4.5 gm/m2

Uses and Cautions: Can be followed by summer and fall crops. Or by early spring crops, if winter-killed.

Notes: Does OK in poor soils. Does not tolerate water-logging. Excellent biomass, even if sown in late summer, as it is not day-length sensitive. Mow before heading for fast regrowth. After heading it is fairly easy to mow-kill, although not as easy as German and Japanese millets.

How to broadcast seeds

To broadcast seeds, you can buy a shoulder-bag with a hand-crank that shoots out the seeds. Or you can develop a manual technique: take a handful of seeds from a bag or bucket you carry with you. Fling the seed confidently in front of your body in a wide fanning motion. Use a loose touch, don’t grip the seeds! Aim for about two seeds per square inch—but don’t worry if you don’t hit that goal. Broadcasting seeds is an art, and the exact density and pattern doesn’t matter.

While you are learning, try seeding half the crop walking in one direction, and then repeat at 90˚ to your original direction. After seeding, rake the seeds in, aiming to bury most of them ½-1” (1-2 cm) below the surface, but don’t sweat the details. Next, especially if water is in short supply, roll or tamp the soil so that the seed is in good contact with the soil, which will help it get the water it needs rather than drying out in an air pocket. Then irrigate with rain, a hose or a sprinkler to keep the soil moist until germination. Drip irrigation won’t do it.

EarthWay push seeder.
Photo from EarthWay

Alternatively drill the seed by hand with a hoe, or using a seeder. See VABF Using Manually-Operated Seeders for Precision Cover Crop Plantings on the Small Farm. Don’t worry if the seed ends up deeper than ideal. It will still germinate.

Sowing millet in mixes

See No-Till summer cover crops. A mix of soybeans or southern peas and foxtail millet can be grown during the summer and mow-killed (after heading), before planting in the fall. Garlic perhaps?

Mixes can generally be sown at a depth of 1” (2.5 cm), regardless of seed size. Major ingredients for a summer mix could include soy, southern peas and buckwheat. Lesser ingredients could include pearl millet, proso millet, radish, turnips, sunflowers and sunn hemp.

  • When legumes and grasses are mixed, sow on the date for the grass.
  • When 2 grasses are mixed, reduce the seeding rate of each by a third.
  • Do not reduce the seeding rate of legumes in mixtures.


Taking down large cover crops

Don’t grow sorghum-sudangrass unless you have tractor-mounted equipment. Even some of the millets are quite large. Before planting, have a plan for how you will terminate the cover crop. You don’t want to delay getting your fall vegetable crops established.

On a small scale, most gardeners mow cover crops down, or pull them, and use for mulch (chop-and-drop) or compost. It may be that composting produces a more balanced soil amendment compared to incorporating raw residues directly into the soil.

If you avoid incorporating the cover crop, you can plant the next crop right away. Some cover crops (such as Sorghum-Sudangrass) produce allelopathic substances that can temporarily inhibit the germination of seeds, meaning you have to wait three weeks to plant.

If cover crop residues are left on the surface rather than incorporated, the rate of decomposition is slowed. Some N is lost to the air (denitrification), but the increased organic matter can increase the diversity of micro-organisms at the surface. 80% of the carbon from cover crops is below the top 8” (20 cm), where almost all soil data are collected. Remember the value of the roots!



Sorghum-Sudangrass hybrid/Sudex grows 5’-12’ (1.5-3.6 m) tall in 60-70 days and produces an impressive amount of biomass.

  • It cannot be mow-killed. It is not frost hardy.
  • Sow ½-1 ½” (1-4 cm) deep (less deep than corn). Plant seeds 1.5” (4 cm) apart in rows 8” (20 cm) apart (for best weed-suppression), or up to 36-42” (90-110 cm) apart.
  • Sowing date: From 2 weeks after corn-planting date (needs warm soil) and anytime onward until six weeks before frost. In zone 7, mid-May to late August. Plant earlier at your own risk – I think we’ve had some success despite the warnings.
  • Sowing rate (US): Drilled:25-40 #/ac, less if rows 36-42” apart. Broadcast:40-50#/ac. Broadcast:2 oz/100 ft2
  • Sowing rate (metric): Drilled:28-56 kg/ha, less if rows 90-110 cm apart Broadcast:45-56 kg/ha, Broadcast:6 gm/m2
  • Sorghum-sudangrass will smother weed competition, and make big improvements to the soil texture and the levels of organic matter.
  • Fast growing, deep rooting.
  • Good in preparation for new strawberry beds.
  • Suppresses root-knot nematodes, soybean cyst nematode and annual ryegrass.
  • After it’s established, sorghum-sudangrass is highly drought-resistant and thrives in summer heat
      • Sorghum-sudan cover crop after mowing to encourage regrowth.
      Photo Kathryn Simmons

    When the sorghum-sudangrass reaches 4’ (1.2 m) tall, cut it down to 6”-12” (15-30 cm) to encourage regrowth and more, deeper, root growth that will loosen compacted soil.

  • The cut tops make a good long-lasting mulch, in place, or to haul elsewhere.
  • Good ahead of fall crops if given 8-10 weeks of growth.
  • Winter-killed Sorghum-Sudan can be followed by early spring crops.
  • Sorghum-sudangrass roots exude allelopathic compounds that suppress pest nematodes and inhibit small seeds (weeds and crops) from germinating and even inhibit the growth of tomatoes, lettuce, and broccoli.
  • Wait at least 6 weeks after incorporating sorghum-sudangrass before planting a susceptible crop in the same spot.
  • Does well in mixes with buckwheat, soy, and/or viney legumes.
  • Can mix 10 # with 50 # southern peas/ac (11 kg with 56 kg southern peas /ha).
  • Sunn hemp and sorghum-sudangrass grow well mixed together. Try a 50:50 mix to start with. This mixture can increase overall biomass and the diversity benefits soil microorganisms and therefore nutrient cycling.
  • Be careful if feeding to livestock. Read up about prussic acid poisoning from this cover crop. Young plants (less than 24” (60 cm) tall) and those stressed by drought or killed by frost, can cause prussic acid poisoning. Ducks, geese and goats enjoy the forage.
  • It’s undeniable that Sorghum-Sudangrass can be hard to incorporate once tall, unless you have tractor-based equipment. It is too massive to tackle with small mowers or weed whips.


Secondary Cover Crops in July

In July, legumes such as sunn hemp, soybeans, southern peas, and Partridge pea are good legumes to consider. See June’s post for more about those and also Senna Ligustrina a native perennial legume. Another option is summer, if you have a weedy area or suffer with nematodes or foliar fungal diseases, is solarization.


Solarizing with clear plastic. Photo Pam Dawling

Solarization uses clear plastic (old hoophouse plastic is ideal) to kill pests, diseases and weed seeds near the surface of the soil by covering the soil for six weeks or more in hot weather.

If you are solarizing to kill weeds, you can see when they are dead. Bryan O’Hara poked a thermometer probe through solarization plastic and found a 50F degree (28C) difference between the outside air and the soil immediately under the plastic; a 10F (6C) difference at 1″ (2.5 cm) deep and little temperature gain lower than that. Solarization does not kill all the soil life below the surface level!

Extension offers Solarization and Tarping for Weed Management on Organic Vegetable Farms in the Northeast USA which can, of course, be modified for those of us in other regions.


I’ve written here before about our struggles with root knot nematodes in our hoophouse, and you can read everything I know about nematodes in my book The Year-Round Hoophouse. Nematodes are only active in warm weather, and we have not had problems with them outdoors yet, but of course, it’s warmer in the hoophouse!

Cucumber roots with nematodes (see circles).
Photo Pam Dawling

My article on nematodes in Growing for Market  in November 2014 describes our discovery of the beasties and our first attempts to deal with them. My most thorough blogpost about nematodes was for Mother Earth News  Managing Nematodes in the Hoophouse.

Good news – great hoeing weather! Bad news – more nematodes in the hoophouse August 2014 includes a photo of our first attempt at solarizing – a  bit of a How Not To! There is info on dealing with nematodes from Garry Ross in Hawaii, where nematodes are a fact of daily life, in my post Cold weather, snow, thinking about nematodes from February 2015.

Warm-weather cover crops that resistant or deter nematodes include sesame, Iron and Clay cowpeas, some OP French marigold varieties (but avoid Tangerine Gem or hybrid marigolds); chrysanthemum; black-eyed Susan; gaillardia (blanket flower, Indian blanket); a sesame/millet mix, sunnhemp, partridge pea, California poppy. Some of these require a long growing season to achieve much. See Solarization and crop choices to fight nematodes.

 Cover Crop Planning

My book Sustainable Market Farming has a chapter on cover crops and 9 pages of charts about particular options.

Cover of managing Cover Crops Profitably book from SARE

The book Managing Cover Crops Profitably (third edition) from the Sustainable Agriculture Research & Education Program (SARE), is the best book I know on the subject. You buy the book for $19 or download it as a free PDF from SARE.

See Harvey Ussery in  Four Outstanding Cover Crops for Summer.

“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.

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: The Amazing Underground Secret to a Better Garden). 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.”