Sunn Hemp (Juncea)

The Sunn Hemp (Juncea) is generally described as an annual forb/herb. This is not native to the U.S. (United States) . Leaves are not retained year to year.

Uses of : Landscaping, Medicinal, Culinary, etc.

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.

General Characteristics

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.

Required Growing Conditions

For current distribution, please consult the Plant Profile page for this species on the PLANTS Web site.

Adaptation 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.

Cultivation and Care

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.


As of 2005, Arkansas considered the genus Crotalaria as a noxious weed.

General Upkeep and Control

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.

Seed Production -Drill 3 to 4 live seeds per foot about 1/2 inch deep in 36 to 42 inch rows. This will give a live seeding rate of about 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). -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. -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 inch 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 with an approved desiccant 1-2 weeks before harvest. -Clean seed with standard commercial seed-cleaning equipment. Dry the combined material before cleaning. -If commercial seed-drying and storage facilities are available, dry seed to below 10% moisture and store at low temperature and humidity. -Seed yields have ranged from 500 of over 2200 lb/ac, varying with environmental conditions and cultural practices.

Plant Basics
General Type Forb/herb
Growth Duration Annual
Plant Nativity Introduced to U.S.
Physical Characteristics
Displays Fall Colors No
Flower Conspicuousness No
Gardening Characteristics
Cold Stratification Required No
Toxic to Nearby Plants No
Toxic to Livestock No
After-Harvest Resprout Ability No
Responds to Coppicing No
Growth Requirements
pH Range 0–0 pH
Precipitation Range 0–0 inches/yr
Planting Density 0–0 indiv./acre
Minimum Frost-Free Days 0 day(s)
Sustainability & Use
Leaf Retention No
Fire Resistant No

Source: USDA, NRCS, PLANTS Database,
National Plant Data Center, Baton Rouge, LA 70874-4490 USA