Sunn Hemp- Crotolaria juncea is a fast growing nitrogen fixing legume. Sunn Hemp is used for green manure forage, organic soil building and cover crop applications. Sunn Hemp is non seed bearing in the North American climates. Sunn Hemp provides as much as 2 1/2 tons per acre of green tonnage with a maximum of 100 units of Nitrogen fixation per acre in as little as 60 days.
Sunn Hemp is much easier to get incorporated into the soil when needed than sorghum, cow peas, and many other cover crop varieties. Sunn Hemp is the next generation cover crop for the vegetable farmers.
Seed Rate: 30-50 lbs. per acre
Planting: After the last chance of frost in the spring.
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Cover Crop & Green Manure: Used as a cover crop, sunn hemp can improve soil properties, reduce soil erosion, conserve soil water, and recycle plant nutrients.
When grown as a summer annual, sunn hemp can produce over 5,000 pounds of biomass and 100 pounds of nitrogen per acre. It can produce this amount within 60 to 90 days, so it has the potential to build organic matter levels and sequester carbon in the soil. It is known to suppress nematodes.
Sunn hemp originated in India where it has been grown since the dawn of agriculture. It has been utilized as a green manure, livestock feed, and as a non-wood fiber crop.
As of 2005, Arkansas considered the genus Crotalaria as a noxious weed. Please consult the PLANTS Web site and your State Department of Natural Resources for this plant’s current status (e.g. threatened or endangered species, state noxious status, and wetland indicator values).
This plant may become weedy or invasive in some regions or habitats and may displace desirable vegetation if not properly managed. Please consult with your local NRCS Field Office, Cooperative Extension Service office, state natural resource, or state agriculture department regarding its status and use. Weed information is also available from the PLANTS Web site at plants.usda.gov. Please consult the Related Web Sites on the Plant Profile for this species for further information.
Legume family (Fabaceae). Branched, erect, herbaceous shrubby annual growing 3 to 9 feet high with bright green simple, elliptical leaves. It has deep yellow terminal flowers (open raceme to 10 inches long) and the light brown pods are small (1 inch long and 1/2 inch wide) and inflated. It has a well-developed root system, with a strong taproot. The number of seeds per pound is 15,000.
Sunn hemp is a tropical or sub-tropical plant that when grown in the continental United States performs like a summer annual. It can be planted year round in Hawaii below an elevation of 1,000 feet. However, it does not perpetuate itself well and is not found in the wild. Sunn hemp is adapted to a wide range of soils and performs better on poor sandy soils than most crops. It is for such situations that it has attracted attention. It grows best on well-drained soils with a pH from 5.0 to 7.5.
To establish a successful stand, seed should be broadcast or drilled and covered ½ to 1 inch deep into a well prepared, weed-free seedbed. If broadcasted, seed at a rate of 40 to 60 pounds of live seed per acre. If drilled, the rate should be 30 to 50 pounds per acre in 6-inch rows. The higher rates should be used if the crop will be terminated in less than 60 days or if severe weed competition is expected. Where weed competition is mild, drilled rates as low as 20 pounds of live seed per acre have been satisfactory. Inoculate with the cowpea-type rhizobia bacteria.
Using a winter cover crop/green manure is a conservation practice that provides soil-improving characteristics. A common problem, however, is that the relatively short period between cash crop harvest in the fall and planting the following spring can result in less than optimum biomass production of the cover crop. Sunn hemp, because of its rapid growth and relatively short growing season requirement, can be an excellent alternative. Where conditions are favorable, it can provide the benefits of a winter legume prior to a killing frost in the fall and also in the summer after the winter crop has been harvested.
Warm weather (frost-free) is needed for 8 to 12 weeks to provide biomass and nitrogen. Small grains following sunn hemp can utilize the symbiotically produced nitrogen, thus reducing or eliminating the loss of nitrogen. It must be plowed under before reaching the full bloom stage or it becomes too fibrous when using it as a green manure.
Pests and Potential Problems
Some species of Crotalaria, including Crotalaria juncea, contain toxic alkaloids, particularly the seeds and pods. ‘Tropic Sun’ is non-toxic and is resistant to root-knot and reniform nematodes. The genus Crotalaria has been known for its suppression of plant-parasitic nematodes.