Sunn Hemp has been cultivated in India since ancient times. This species is widely distributed throughout the tropics, being most numerous in eastern and southern Tropical Africa.
Traditionally Sunn Hemp has been considered a good source of fiber for the manufacture of twine and cord, and it has demonstrated potential for use in pulp and paper. Sunn Hemp is also an excellent green manure that can return more organic matter to depleted soil than most clovers or soybeans. Sunn Hemp fixes up to 67 kg (147 lb) of nitrogen per acre and may even suppress root-knot nematodes. In many parts of the tropics, Sunn Hemp is grown in rotation with rice, maize, tobacco, cotton, and other crops. It is sometimes grown with sugarcane, pineapples, and coffee, or it may be used as a cover crop in plantations and fruit orchards. Most varieties of Sunn Hemp contain poisonous alkaloids that render this crop toxic to livestock. In some areas, Sunn Hemp hay has been fed to cattle (no more than 10% of the diet), but it is not generally recommended for use as animal fodder.
Sunn Hemp is an erect, annual legume with rapid, vigorous growth. The plants are generally unbranched from the ground to approximately 0.7 m (2 ft), but above this height, many branches will develop if the plants are not crowded. When it is grown in dense stands, such as for a green manure crop, Sunn Hemp will have a single, spindly stem that can be 3 m (10 ft) tall. With vigorous later roots and a long taproot that can exploit subterranean water sources, Sunn Hemp plants do have some drought resistance. Although it is adapted to hot climates, this plant will endure slight frost. Sunn Hemp grows vigorously in well-drained soils with a pH of 5.0 to 7.5, and because it is a legume that nodulates freely, Sunn Hemp thrives even on poor soils with little or no nitrogen fertilizer. If grown as a fiber crop, however, Sunn Hemp should be grown on light, loamy soil as it will produce coarse fiber with low yields if grown on heavy clay. Innoculation is generally unnecessary as the cowpea-type rhizobia that nodulate Sunn Hemp are present in most soils. After broadcasting the Sunn Hemp seed, seedlings will appear in about 3 days and will rapidly produce a thick ground cover that smothers weeds. After planting, no care is needed until harvest time, although you should watch for lodging as the plants get taller. If being used as a green manure, the plants should be plowed into the soil at bud or early-bloom stage, which is about 60 days after planting. At this age, nitrogen content in the plant is high and decomposition is rapid. If allowed to grow too tall (over 1 m/3-4 ft) or become overmature, the plants become fibrous and are difficult to turn under.
Harvesting and Seed Production
Sunn Hemp will grow in warm temperate regions, but most varieties will set seed only in the tropics and subtropics. If growing Sunn Hemp for seed, the plants should be watered, if needed, until about 75% are flowering, then they should be allowed to mature, then dry. The seed crop should mature during the dry season, if possible. It takes about 5 months for the plants to fully develop and produce ripe seed.
Pests and Diseases
Many disease organisms and pests, including viruses, fungi, insects, and nematodes, have been found on Sunn Hemp, but they seem to cause little economic damage.