'Tropic Sun' Sunn Hemp
Crotalaria juncea

Summary

  • Tropical erect, branching, annual legume
  • Green manure crop used to add nitrogen and organic matter
  • Reduces root knot nematodes
  • Provides weed suppression
  • Recommended in Hawai`i for green manure with vegetable crops and ornamentals
  • Must be plowed down before reaching full-bloom stage or becomes too fibrous
  • 'Tropic Sun' cultivar is non-toxic to poultry and livestock

Common Name

Its common name is sunn hemp (Rotar and Joy).

Scientific Name

The scientific name is Crotalaria juncea (L.) (Rotar and Joy).

Cultivars

The only cultivar to be described here is 'Tropic Sun,' a joint release by the University of Hawai`i and the USDA Soil Conservation Service (now known as Natural Resources Conservation Service). (Rotar and Joy)

Seed Description

Seeds are dark slate green and about 1/4 inch long (1/2 cm). There are approximately 15,000 seeds per lb. (30,000 - 35,000/kg). (Rotar and Joy)

Seedling Description

'Tropic Sun' sunn hemp seedlings are reported to have good seedling vigor. Damping-off disease of seedlings may occur during periods of high rainfall when the soil remains moist, although it has not been a serious problem (Rotar & Joy).

Mature Plant Description

'Tropic Sun' sunn hemp is an erect, branching, annual legume. Plants are generally unbranched from the ground to about 2 ft. (60 cm). Above this height many branches will develop if the plants are not crowded. Leaves are simple, elliptical, 21/2 - 5 inches (61/4 - 121/2 cm) long, 1/2 - 1 inch (11/4 - 21/2 cm) wide. Flowers are bright yellow, 18-20 developing on terminal racemes. The seedpods are cylindrical, 1 - 11/4 in (21/2 - 3 cm) long, 1/4 - 1/2 in (1/2 - 11/4 cm) wide, papery and inflated (Rotar and Joy).

Temperature

'Tropic Sun' sunn hemp is a warm season plant and may grow slower in winter than in summer (Rotar & Joy).

Origin and Geographic Distribution

The seed of 'Tropic Sun' sunn hemp was purchased in 1958 from a farmer who grew it as a cover crop on the island of Kauai. No other information is available regarding its origin. In the past, the Pineapple Research Institute did considerable work with the Crotalaria species and may have brought in this strain (Rotar & Joy).

Sunn hemp is indigenous to India and has been distributed throughout the tropics and subtropics (Chee & Chen). It is found in Indonesia, Rhodesia, Malaysia, Taiwan, Thailand, and China (Rotar and Joy).

Ecology

Sunn hemp is adapted to hot, semi-arid and arid areas and is drought resistant. It can tolerate light frosts. It can grow on poor soils, however production is improved with fertilization (Chee & Chen).

Water

'Tropic Sun' sunn hemp should receive a minimum of 1 inch (25 mm) of moisture per week for maximum growth, however it is quite drought tolerant. In a trial planting on Molokai it produced a yield of 1000 lb/ac (1120 kg/ha) green matter after 6 weeks with only 2 inches (50 mm) of water (Rotar & Joy). Sunn hemp does not tolerate waterlogging (Chee and Chen).

Nutrients

'Tropic Sun' sunn hemp can grow on infertile as well as fertile soils. For maximum yields, a soil test should be taken and fertilizer and soil amendments added as recommended (Rotar & Joy). Sunn hemp has great difficulty growing in soil with low calcium and/or low phosphorus (McHugh).

Soil pH

'Tropic Sun' sunn hemp grows best at a pH between 5.0-7.5 (Rotar & Joy). Sunn hemp has great difficulty growing in soil with low calcium and/or low phosphorus. In general thoseare soils that have a low pH (McHugh).

Soil Type

'Tropic Sun' sunn hemp will grow on a wide range of soils from coarse to fine textured, but grows best on well-drained soils.

Shade Tolerance

No information is available in this database on this topic.

Salinity Tolerance

Sunn hemp is not salt-tolerant (Chen and Chee).

Herbicide Sensitivity

No information is available in this database on this topic.

Life Cycle

'Tropic Sun' sunn hemp is a rapid, vigorous grower. It reaches a height of over 4 ft (1.2 m) in 60 days when grown under favorable conditions. It can attain a height of over 6-ft (1.8 m) in approximately 90 days (Rotar and Joy).

Seeding Rate

  • Broadcast: 40-60 lbs. pure live seed per acre (45-67 kg/ha).
  • Drilled: 30-50 lbs. pure live seed per acre (34-56 kg/ha) in 6-inch rows.
  • Use higher seeding rates if the crop will be incorporated within 30-45 days or if severe weed competition is expected. (Rotar & Joy)

Seeding Depth

Broadcast and cover or drill to a depth of 1/2 to 1 inch (11/4 - 21/2 cm) in a well-prepared weed free seedbed (Rotar & Joy).

Seeding Method

Broadcast and cover or drill.

Seeding Dates

  • Year round at elevations between 0-1000 ft.
  • Spring/summer at elevations between 0-2000 ft. (NRCS)

Inoculation

Inoculate with a cowpea-type of "EL" inoculant of Rhizobium (Rotar & Joy).

Seed Cost

$3.00 to $4.00 per lb (Oahu RC&D).

Seed Availability

Readily available.

808-622-9026

Days to Flowering

No information is available in this database on this topic.

Days to Maturity

No information is available in this database on this topic.

Seed Production

Hand harvesting:

Allow a small area of plants to mature and develop seedpods. Hand harvest when seeds rattle inside the pods (about 5 months from planting). Crush or crack pods and separate seed.

For commercial or large-scale seed production:

  • Drill 3 to 4 seeds per foot about 1/2 inch deep in 36 to 42 inch rows. This will give a seeding rate of about 3-4 lb/acre. Isolate the seed field from other Crotalaria plants.
  • Test soil and apply fertilizer and amendments per recommendations (broadcast before planting or banded next to seed at planting).
  • Apply a preemergence herbicide and irrigate with at least 1 inch of water to incorporate.
  • Irrigate if needed until about 75% of plants are flowering (usually occurs at end of the third month). Stop irrigating after the 75% bloom stage. 'Tropic Sun' is easily threshed when dry. To enable the plants to dry naturally, time the planting so that flowering, seed set, and harvest occur during the dry season.
  • Cultivate as needed to control weeds. Fields should be weed free at harvest to prevent contamination of crop. Remove any wild Crotalaria before harvest.
  • Observe for insect and disease buildup and spray as needed (see below for pests).
  • Combine when seeds rattle in the pods, about 5 months from planting. Raise the combine header as high as possible without leaving seed pods on the plants (which prevents excess straw from slowing combine). Initially set the concave clearance at 1/8 to 3/16 inches and the cylinder speed at 1150 to 1200 RPM. Adjust as needed according to crop conditions.
  • When seed crop is ready for harvest, the plants should be dry and self-defoliated. If crop is still green, desiccate by spraying 1-2 weeks before harvest.
  • Clean seed with standard commercial seed-cleaning equipment. Dry the combined material before cleaning if needed.
  • If commercial seed-drying and storage facilities are available, dry seed to below 10% moisture and store at low temperature and humidity.
  • Seed yields of over 2200 lb/ac (2470 kg/ha) may be achieved, varying with environmental conditions (Rotar and Joy).

Seed Storage

Dry to below 10% moisture content and store at low temperature and humidity (Rotar & Joy).

Growth Habit

'Tropic Sun' sunn hemp is an erect, branching, annual legume. It is a rapid, vigorous grower. It reaches a height of over 4 ft (1.2 m) in 60 days when grown under favorable conditions. It can attain a height of over 6-ft (1.8 m) in approximately 90 days. Plants are usually unbranched from the ground to 2 ft (60 cm) and many branches develop above this height. When planted at higher rates, branching begins higher on the plant and fewer branches develop (Rotar and Joy).

Maximum Height

'Tropic Sun' sunn hemp grows to a maximum height of 6 feet tall (Rotar & Joy).

Root System

Chen and Chee report that sunn hemp has a long strong taproot with many lateral roots and numerous irregularly branched and lobed nodules.

Establishment

By seed.

Maintenance

No information is available in this database on this topic.

Mowing

Mow with a rotary or flail mower (Rotar & Joy).

Incorporation

'Tropic Sun' sunn hemp should never be allowed to grow beyond the full-bloom stage if it is being grown as a green manure. Plow down the crop in the bud or early flowering stage (normally about 60 days from planting). If allowed to grow beyond this stage, the plants become fibrous and very difficult to plow under. Maximum nitrogen content is highest at or before early bloom stage. Home gardeners with light tillage equipment should incorporate when plants reach a height of 3 ft., within about 30-50 days (Rotar and Joy).

Harvesting

'Tropic Sun' sunn hemp pods can be harvested by hand when the seeds rattle in the pods, normally about 5 months after planting. Crush or crack the pods and separate the seeds (Rotar and Joy). For commercial seed harvesting, see above "Seed Production."

Equipment

For turning crop under as a green manure:

  • Mow with a rotary or flail mower, then plow as soon as possible so the material does not get too dry. The chopped material tends to pile and drag in front of the plow if too dry.
  • Plow the erect standing crop. A single plow will usually do a better job than multiple plows. Do not disk or lay crop down before plowing as the plant will become tangled in the plow.
  • Rototill with rear-tine machines when the plants are succulent. For crops over 4 ft tall, mow first.
  • Disk with a heavy-duty disk when the plants are succulent (Rotar & Joy).

Uses

  • Green manure crop used to add nitrogen and organic matter
  • Reduces root knot nematodes
  • Provides weed suppression
  • Recommended in Hawai`i for green manure with vegetable crops and ornamentals

NOTE: Other cultivars of sunn hemp are widely used for fiber production (India) to make twine and cord, paper and pulp. It is also used as a forage crop (in restricted amounts due to the presence of poisonous glucosides), as a green manure and as a cover crop (Chen and Chee).

Mixtures

No information is available in this database on this topic.

Biomass

Approximate dry matter yield 2.5 tons/acre (NRCS).

N Contribution

Approximate N content 51 lbs./T dry matter (NRCS).

Non-N Nutrient Contribution

No information is available in this database on this topic.

Effects on Water

No information is available in this database on this topic.

Effects on Soil

No information is available in this database on this topic.

Effects on Livestock

Seeds and forage of 'Tropic Sun' sunn hemp are nontoxic in laboratory tests and feeding trials. Other species of Crotalaria contain poisonous alkaloids and, under certain conditions, can be toxic to animals (Rotar & Joy).

Pest Effects, Insects

Aphids (Aphis spp.) and the bean butterfly (Lampides boeticus) have been the main insect pests of 'Tropic Sun' reported (Rotar & Joy).

Pest Effects, Nematodes

'Tropic Sun' is resistant to root knot nematodes (Meloidogyne spp.). Experiments have shown that the number of root knot nematodes in the soil can be reduced, probably because 'Tropic Sun' is not a suitable host (Rotar & Joy).

Pest Effects, Diseases

Powdery mildew (Oidium sp.) may appear on leaves during very humid weather on 'Tropic Sun' sunn hemp (Rotar & Joy).

Pest Effects, Weeds

'Tropic Sun' sunn hemp competes well with weeds once it is established. Plant at higher rates if severe weed competition is expected (Rotar & Joy).

Pest Effects, Vertebrates

No information is available in this database on this topic.

Uses in the Pacific Region

No information is available in this database on this topic.

Uses in Hawai`i

The Hawai`i Natural Resources Conservation Service Technical Guide includes 'Tropic Sun' sunn hemp. Their specification describes 'Tropic Sun' sunn hemp as follows:

  • Resistant to root knot nematodes
  • Minimum broadcast seeding rates of 40-60 lbs. pure live seed/acre;
  • Increase seeding rate to 60 lbs/A if incorporating early to produce finer stemmed material that is easier to till into the soil or if severe weed competition is expected;
  • pH range from 5.0-7.0;
  • Inoculant group: cowpea;
  • Approximate growing time 60 days;
  • Approximate dry matter yield 2.5 tons/acre;
  • Approximate N content 51 lbs./T dry matter;
  • Optimum planting period year round at elevations between 0-1000 ft.;
  • Optimum planting period spring/summer at elevations between 0-2000 ft.

Caswell, DeFrank, Apt and Tang conducted research on Crotalaria juncea and found it was a poor host for Rotylenchulus reniformis.

A farmer on Molokai reports using 'Tropic Sun' sunn hemp as a green manure and nematode suppressor with upland taro very successfully.

Dr. S. Schenck with HARC tested Crotalaria juncea with pepper and with tomato. Both crops were planted in fields with high counts of Meloidogyne. The Crotalaria was planted about 2 to 3 months prior to planting the crop plants and was plowed under just before the replant of the crops. Nematode populations were significantly reduced in Crotalaria plots as compared to fallow. Galling on the tomato roots was significantly reduced in tomato by the Crotalaria, but no yield data was taken. In pepper there was significant growth increase in the plots previously planted to Crotalaria (personal communication).

For More Information

References

Caswell, E.P., DeFrank, J., Apt, W.J., and Tang, C.S. 1991. Influence of Nonhost Plants on Population Decline of Rotylenchulus reniformis. Journal of Nematology 23(1):91-98.

Chee, Y.K. and Chen, C.P. 1992. Crotalaria juncea L. In: 't Mannetje, L. & Jones, R.M. (Editors): Plant Resources of South-East Asia No 4. Forages. Pudoc-DLO, Wageningen, the Netherlands. pp. 98-100.

Evans, Dale O., Joy, Robert J., & Chia, C.L., 1988. Cover Crops for Orchards in Hawaii. Hawaii Institute of Tropical Agriculture and Human Resources, University of Hawaii at Manoa, Honolulu, Hawaii, United States. 16 pp.

Rotar, Peter P. and Joy, Robert J., 1983. 'Tropic Sun' Sunn Hemp, Crotalaria juncea L. Hawaii Institute of Tropical Agriculture and Human Resources, University of Hawaii at Manoa, Honolulu, Hawaii, United States. 8 pp.

USDA Natural Resources Conservation Service, Hawai`i Field Office Technical Guide, Section IV, Code 340 "Cover and Green Manure Crop" May 1992. Pacific Islands Area Field Office Technical Guide (eFOTG) - East Area

Text last updated on 9/25/2009.

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These webpages were originally generated under a grant program from Western SARE entitled "Covering New Ground: Tropical Cover Crops for Improving Soil Quality" EW98-012 (1998-2002).