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.
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.
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.
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.
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.
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
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.
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!
- 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
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.
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!
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.
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.”