Common Name: Sunn Hemp

General Information

Crotalaria juncea is an erect, laxly branched annual plant growing 1 - 3 metres tall with a strong and deep tap-root system[, ].
The plant has been cultivated since prehistoric times, especially in India, for its fibre[]. It is one of the earliest of the distinctly named fibres of India, being mentioned in early Sanskrit under the name of 'sana'[]. It is widely grown in many areas of the tropics and subtropics, including Africa, Asia and Australasia, as a green manure crop and for its fibre[, ]. It has become naturalized in places[].

Known Hazards

The presence of compounds which cause unpalatability and/or are poisonous under socertainme conditions is typical of this genus. Although this plant can often contain poisonous glucosides it is commonly used as a forage[].
The seeds of many Crotalaria species contain toxic pyrrolizidine alkaloids. Trichodesmine is the principal toxic alkaloid in Crotalaria juncea[].
Pyrrolizidine alkaloids have a cumulative effect upon the body and, unless concentrations in a plant are high, occasional use is generally completely safe. They are derived from amino acids including ornithine. Many pyrrolizidine alkaloids have pronounced hepatic toxicity, but the lungs and other organs may be affected as well. Mutagenic and carcinogenic activities of pyrrolizidine alkaloids have also been reported[].
The seeds are reported to contain trypsin inhibitors; they are said to be poisonous to cattle, and they can poison both horses and pigs[].

Botanical References

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Range

A frequently cultivated fibre plant, the original range is unknown but was probably southern Asia.

Habitat

Sparse forests, along trails; at elevations below 100 - 200 metres[].

Properties

Edibility Rating
Medicinal Rating
Other Uses Rating
Habit	Annual
Height	1.60 m
Pollinators	Bees, Insects
Self-fertile	No
Cultivation Status	Cultivated, Wild

Cultivation Details

Sunn hemp is often cultivated in the dry to moist tropics and subtropics, where it can be grown at elevations up to 1,500 metres. It grows best in areas where annual daytime temperatures are within the range 20 - 30°c, but can tolerate 4 - 40°c[]. Mature plants can be killed by temperatures of -2°c or lower, whilst young growth will be severely damaged at -1°c[]. It prefers a mean annual rainfall in the range 500 - 1,500mm, but tolerates 200 - 4,300mm[]. Sun hemp is generally sensitive to photoperiod. Long day lengths favour vegetative growth and reduce seed set, although selections exist that are neutral to day length[].
The plant is widely adaptable to different soil types, and tolerant of low fertility[, ]. For fibre production, a light, loamy well-drained soil is preferred; on low-lying or clay soils plants achieve vigorous growth, but the bast fibre is coarser and yields are lower[]. Plants are not very tolerant of salty conditions[]. Prefers a pH in the range 6 - 7, tolerating 5 - 8.4[]. Established plants are drought resistant[].
When grown as a fibre crop, the plants are usually harvested at the flowering stage or when the stems turn yellow. When the main priority is seed production, the crop is harvested when the seeds are ripe; the fibre is extracted from the stems afterwards[].
The production of green plants is normally 40 - 60 tonnes/ha yielding 2.5 - 4.5 tonnes/ha of dry fibre[].
There are no significant differences in strength and quality of fibre obtained from plants retted at flowering time and those retted when seeds are fully mature[].
The retting period depends on water temperature, locality, time of year, weather conditions, depth and source of water, thickness of stalks, and quantity of straw in relation to volume of water. Cut straw with a yellowish tinge requires 10 - 21 days to bleach out sufficiently to have fibre of a satisfactory colour. Stems cut while green will bleach out when exposed directly to the sun but have to be turned at least twice[].
This species has a symbiotic relationship with certain soil bacteria, these bacteria form nodules on the roots and fix atmospheric nitrogen. Some of this nitrogen is utilized by the growing plant but some can also be used by other plants growing nearby[]. Soybean soils must contain the proper nitrogen-fixing bacteria. When grown on the same land for 2 - 3 successive years, increasing yields are obtained year after year[]. Seed can be purchased that has been treated with this rhizobium, it is unnecessary on soils with a pH below 5.5 but can be helpful on other soils[]. When removing plant remains at the end of the growing season, it is best to only remove the aerial parts of the plant, leaving the roots in the ground to decay and release their nitrogen.

Edible Uses

The flowers are pickled[].

Medicinal

The seeds are said to purify the blood and are used to treat impetigo and psoriasis[].

Agroforestry Uses:

The plant is widely grown in the tropics as a green manure. As well as producing high organic-matter yields, it can reduce the build-up of root-knot nematode populations[].
It is often grown as a cover crop and in rotation with tobacco, vegetables, dry grains, rice, corn, cotton, sugarcane, pineapples, coffee, and orchard crops[].
Due to the shade of its dense canopy the plant can be used as a cover crop to suppress weed populations[].

Other Uses

A valuable, high quality, strong fibre is extracted from the stems and bark[, ]. It is used to make twine and cord; canvas and fishing nets; and paper and pulp[]. Sunn hemp fibre has greater tensile strength and is more durable under exposure than jute (Corchorus spp), though it is not as strong as hemp (Cannabis sativa)[]. The three main properties that make sunn hemp an excellent candidate for paper-making are 1) good yields of bleachable sulphate pulps, 2) pulp strength properties that are equal to or greater than those of mixed southern hardwood pulp, and 3) length-to-width ratio of bast fibre that is greater than that of wood fibres. Stems are composed of two types of fibre - the bast and the woody core. Bast fibres, located in the outer bark, are much longer than the core fibres, but the widths of both fibres are similar. The proportion of bark in the total stalk by dry weight ranges from 15% to 20%. Sunn hemp fibre is used in twine, rug yarn, cigarette and tissue papers, fishnets, sacking, canvas and cordage. The fibre is stronger when wet; it is fairly resistant to mildew, moisture and microorganisms in salt water[].

Propagation

Seed - sown in situ. Germination is rapid and seedlings emerge 3 days after sowing, soon producing a thick ground cover that smothers weeds303]. Too much moisture is harmful during the first 2 weeks after germination[].
Seeds are ready for harvest when pods begin to turn yellow and seeds rattle in them[].

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