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Yuncong Li and Herbert H. Bryan Teresa Olczyk Scientific Name: Crotalaria juncea L. Family: pea family (Fabaceae, or Leguminosae). Common Name: Sunn hemp, Indian hemp, Madras hemp, brown hemp, and sunn Common Variety: 'Tropic Sun', released by the University of Hawaii Origin: Sunn hemp is native to India and Pakistan. In southeast Asia sunn hemp has been grown as a fiber and green manure crop for centuries and now is cultivated in many tropical regions. Sunn hemp is cultivated in Hawaii, California and to some extent in Alabama. Plant description: Sunn hemp is a tall, herbaceous annual, with erect fibrous ridged stems. The plant is covered with short, downy hairs. Its taproots are long and strong with many lobed nodules. The plants branch at about 20 inches above the ground when not crowded, but branching is suppressed somewhat in dense stands. Sunn hemp is photoperiod sensitive and flowers in response to shorter days. In Homestead flowering has occurred in 90 days. Flowers are bright yellow and showy. Uses: Sunn hemp grows to a height of 6 to 7 feet tall in just ten weeks in south Florida. No other cover crop is known to grow so rapidly. In Alabama sunn hemp is planted immediately after corn harvest to prevent erosion of the soil by heavy fall and winter rains. Since this plant is a legume, it can fix much of the nitrogen needed by the next crop. Further sunn hemp can be grown as a high-protein forage for late summer when pastures may perform poorly. In India cloth, twine and rope are made from the fiber of older plants. In some areas seeds are fed to pigs and horses without adverse effects. Sunn hemp serves as an excellent cover crop in rotation with vegetables. It suppresses weeds and reduces root-knot nematodes, controls erosion and may serve as a windbreak both for vegetables and for tropical fruit crops. Seeds germinate readily and seedlings rapidly produce a thick ground cover. Sunn hemp can produce between 5,000 and 12,500 lb/ac of dry biomass. In an experiment conducted in Homestead, sunn hemp yielded a total of 7,700 lb dry biomass (roots and shoots) and fixed 182 lb N per acre 3 months after seeding. Climate and soils: Sunn hemp grows well at mean annual temperatures from 47 to 82 ° F. Warm temperatures with moderate humidity are best. Growth may be slowed by cool seasons, and the plant is susceptible to freezing temperature below 28 ° F. Although sunn hemp tolerates poor soil, it productivity is enhanced on more fertile soils. Sunn hemp tolerates soil pH from 5.0-8.4. It is well adapted for well-drained calcareous soils and for acidic sandy soils. Cultivation: In order to establish a cover crop, sunn hemp should be sown at 20 to 40 lbs. seeds/acre. The lower seeding rates enable the plants to branch profusely. Seed should be planted less than one inch deep. At greater depths the germination is poor. Seeds can be inoculated with cowpea inoculant to improve the fixation of nitrogen. However in some instances, sunn hemp without inoculation has grown very well and developed many nodules. In south Florida with adequate soil moisture the seedlings emerge after three or four days and form a thick cover. Sunn hemp is drought tolerant and no irrigation is necessary during the summer in south Florida. Nevertheless in Miami-Dade County field demonstration have shown that irrigation just before and after seeding improves germination. Harvesting: Sunn hemp does not require mowing during the middle of summer. Mowing when the plant is 6 to 12 inch tall produces many lateral branches but the plants do not recover fully. The plants should be plowed down when flower buds have formed or during the early flowering stage. Mowing before disking and plowing facilitates soil incorporation and decay. For maximum immediate release of nitrogen to the subsequent vegetable crop, sunn hemp should be plowed within 60 days, when the nitrogen concentration is high and decomposition rapid. Pests: Whiteflies and thrips can be found on sunn hemp seedlings during the early summer. However no vegetable insects except stinkbugs and cucumber beetles have been found on mature plants in south Florida. Rotation of sunn hemp with corn is used to control worms in Hawaii. Since sunn hemp is resistant to nematodes, it is advantageous to grow it in rotation with nematode-susceptible crops. The extremely rapid early growth of sunn hemp enables it to shade out weeds very effectively. The sensitivity of sunn hemp to some herbicides (metribuzin, linuron and chlorimuron) has been reported in Taxes. Seed production: Shortages of seed have prevented sunn hemp from being grown extensively. Currently the seed costs $1.50 to $4.00 per pound. Probably seed production is more difficult in the subtropics than in the tropics. In the tropics, flowering can begin at six weeks, with maturity reached at four months or more. Worldwide seed yields range from 400-900 lb/ac of seed. There are approximately 15,000 seeds/lb. Seeds remain viable for a number of years. Evaluation of seed production in south Florida is in progress. |
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