Cover Crops for June – Sunn hemp


Sunn hemp flowering in November at Nourishing Acres Farm, NC. Photo Pam Dawling

Focus Cover Crop for June: Sunn hemp

Last month I wrote about buckwheat. For small areas that will be needed back in production soon, buckwheat continues to be a good choice, unless irrigation is in short supply. You don’t get much biomass from buckwheat in a drought!

In June we are mostly looking for cover crops that will grow in hot weather. One we have recently started to use in our hoophouse is Sunn hemp (Crotalaria juncea). It looks promising as an outdoor cover crop here too. Sunn hemp is not at all related to cannabis – don’t smoke it.

Sunn hemp, a tropical legume, can grow as tall as 9’ (2.7 m) in as many weeks.  It needs 8-12 weeks frost-free to grow to full productivity. In the US it is grown as a summer annual, except in Hawaii, where it can be grown to seed at lower elevations. With adequate moisture, temperature and fertility, researchers have recorded a growth rate of 1 foot per week. It comes from India, where it is grown for fiber production and as a forage, as well as a cover crop. It is recommended for warm season use in US hardiness zones 8-13, but has been successfully cultivated as far north as Washington State.

Sunnhemp cover crop at Nourishing Acres Farm, NC.
Photo Pam Dawling

Benefits of growing sunn hemp

If you sow sunn hemp in a summer gap between spring and fall vegetable crops, it will provide a nitrogen boost for the fall crop, because it is a legume. Also, as a branching vertical plant, it avoids the problems of sprawling legumes. In dense plantings, it can fix more than 120 lbs (54 kg) of nitrogen and 12 pounds of biomass per 100 sq ft (0.56 kg/sq m). It can fix over 100 lbs of nitrogen and produce over 5000 lbs of biomass per acre (112 kg/ha and 5604 kg/ha respectively).

Most studies of vegetables grown following sunn hemp have found higher vegetable crop yields, because of the nitrogen boost, and the large amount of biomass increasing the soil organic matter. The large production of biomass means it is useful as a way to sequester carbon. The well-developed root system with a strong tap root also provides erosion control.

Sunn hemp suppresses weeds and also conserves soil water, storing summer rainfall for fall crops, and reducing runoff. Sunn hemp may enhance soil microbiota. Sunn hemp residue contains allelochemicals, that inhibit or delay germination of weed seeds and small crop seeds. Because of this, do not sow small-seeded crops or cover crops for several weeks after incorporation.

Sunn hemp suppresses plant-parasitic nematodes such as root-knot (Meloidogyne incognita) and reniform (Rotylenchulus reniformis) nematodes by producing allelochemicals that disrupt nematode life cycles. It promotes the growth of both antagonistic microorganisms and beneficial nematodes. Pest nematode numbers can be reduced for several weeks after incorporating sunn hemp into the soil. (I was disappointed to read “weeks” rather than “months” as I was hoping, although the EDIS ENY-717(see resources section) offers “a few months”)

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

Sunn hemp can be grown as a wind break to protect sensitive vegetable or flower crops, or young trees. Mow or cut the sunn hemp at 60 days, or lop the tops, to prevent too much shading.

Sunn hemp seeds at the start of a seed germination test. Photo Pam Dawling

Sowing sunn hemp

Sow sunn hemp starting a week after your sweet corn sowing date, up to 9 weeks before your first fall frost, when it will die. A soil temperature of 68°F (20°C) or more is good. It tolerates a wide range of soils (but not if waterlogged), doing better in poor sandy soils than most crops. Water for the first two weeks of growth, but do not overwater.

Plant inoculated seed (use the same inoculant as for southern peas) up to 1” (2.5 cm) deep, with seeds 1.5” (4 cm) apart in the row, and with rows 6” (15 cm) apart. Sowing densely (as with all cover crops) will work better to smother the weeds. On a field scale, drill at 25-50 lbs/ac (28-56 kg/hectare), or broadcast at 40-60 lbs/ac (45-68 kg/ha).

Another opportunity is to sow sunn hemp in the late summer or fall, 7-9 weeks before a frost. The frost-killed mulch covers the surface for an early spring food crop planting.

Sunn hemp seeds germination test. Photo Pam Dawling

Sowing sunn hemp in mixes

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. Only grow this very tall cover crop mix if you have tractor-based equipment. It is too massive to tackle with small mowers or weed whips.

Sunn hemp can also be mixed with other legumes, such as American joint vetch (Aeschynomene americana), southern peas (Vigna unguiculata), hairy indigo (Indigofera hirsuta), and slender leaf rattlebox (Crotalaria ochroleuca). This list comes from Florida and may not apply where you live. The shorter height of northern sunn hemp varieties, such as day-neutral ‘AU Golden’ and ‘Ubon’, may work better for cover crop mixtures. ‘Crescent Sunn’ is a short-day variety which will carry on growing if sown out of season.

Growing sunn hemp

Sunn hemp is fairly drought-tolerant from two weeks after germination, and requires little care. For maximum growth, irrigate until 75% of the plants are flowering (perhaps at the end of the third month), then you can stop irrigating.

Cutting the crop back at less than 60 days after sowing stimulates branching (more biomass) and more root penetration (better drainage). In our hoophouse, we have used hedge shears to do this at a nice ergonomic elbow height. Cutting at around 60 days produces long-lasting mulches that increase soil carbon. After 60 days, the stems thicken and become fibrous and high in cellulose. It is best to mow Sunn hemp before 90 days, due to the toughness of the fiber, making it hard to incorporate.

On a field scale, a roller-crimper can be used to break the plant stems, leaving a layer of mulch suitable for no-till transplanting of fairly large transplants. The sunn hemp is killed by crimping, and does not regrow.

A long bed of sunn hemp in November at Nourishing Acres Farm, NC. Photo Pam Dawling

Challenges with sunn hemp

Sunn hemp is notorious for seeds that are high in toxic pyrrolizidine alkaloids. Ingesting lots of seeds can cause damage to the liver, lungs, heart and nervous system. Susceptibility depends partly on the animal species: pigs are most vulnerable, followed by chickens, horses, cattle and sheep. Goats have the lowest risk.

Tropic Sun has reduced levels of these alkaloids, and is non-toxic to poultry and livestock. This and other southern varieties may be less tolerant to cold climates than northern varieties such as AU Golden and Ubon. AU Golden may flower 5-6 weeks after sowing

In some states (in 2005, Arkansas for example), Sunn hemp is regarded as a noxious weed, so do check the rules where you are, at your local NRCS office, Extension Service or state agricultural service. Some Crotolaria are noxious weeds, but as sunn hemp will not set seed consistently north of 28° N latitude (slightly north of Corpus Christi, TX), it has little potential for becoming a weed.

Deer and rabbits may browse on sum hemp, and some moths and pod boring insects may attack the stems, leaves or seedpods.

Sunn hemp growing in southern Florida.
Credit: Qingren Wang, UF/IFAS

More resources on sunn hemp

USDA Sunn hemp Plant Guide, 2005:

Ask Ifas: Questions and Answers for Using Sunn Hemp as a Green Manure Crop

EDIS SL 306 Sunn Hemp – A Promising Cover Crop in Florida

EDIS ENY-717 Management of Nematodes and Soil Fertility with Sunn Hemp Cover Crop.

NRCS USDA 1999 Sunn Hemp: A Cover Crop for Southern and Tropical Farming Systems

USDA Plant Materials Program


Secondary Cover Crops in June

In my May post I mentioned secondary cover crops such as soy, mustard, sunn hemp, and southern peas. I explained why we don’t grow mustards as cover crops (too many brassica food crops, too many harlequin bugs).

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.

In June, legumes such as soybeans, southern peas, and Partridge pea are other good legumes to consider.

Soybeans as a cover crop

Soybeans are a great summer cover crop and they are also a legume, so they add nitrogen to the soil. They have good shade tolerance and tolerance to foot traffic (that is, people harvesting crops on either side). Because of this, we like using soy to undersow in sweet corn.

Iron and Clay southern peas as cover crop in the hoophouse, smothering weeds.
Photo Pam Dawling

Southern peas are another warm weather cover crop option. They are also a legume, and so will add nitrogen to the soil. Iron and Clay is the sprawly variety best known for cover crop use, but other varieties also work.

Senna Ligustrina, a native perennial legume, is another warm season cover crop possibility. I was given this suggestion last September by a reader in Florida. She suggested I look for a senna native to my region. Ernst Conservation Seeds sells Maryland Senna, which tolerates wetlands and dry roadsides. The idea here is to find a plant adapted to your region, meaning it will grow well. The other side of the coin is that if you are growing annual crops, you will need to pay attention and prevent self-seeding, unless you are able to cope with the chaos.

At Ernst Conservation Seeds, they “grow, process, and sell hundreds of species of native and naturalized seeds and live plant materials for ecological restoration, sustainable landscaping, reclamation, wetlands, and natural resources conservation.” If you are looking for some less usual cover crops seeds, this is the place to turn to.

Next month I will write about sorghum-sudangrass and the millets. If you are in a warmer climate than I am (central Virginia) or you want to consider more options, those are good hot weather grasses. Also see my post Cover Crops in Summer for much more information, including  making space, sowing small spaces and finding time for the work.

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.

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.

Harvey Ussery  Four Outstanding Cover Crops for Summer.

ATTRA Cover Crop Options for Hot and Humid Areas