Sunn Hemp

Overview

Sunn hemp is a fast-growing legume used as a cover crop. In a few months it produces lots of nitrogen-rich biomass, which you cut and leave on the soil. The ‘Tropic Sun’ variety was developed in Hawaiʻi and is known to suppress root-knot nematodes.

History

Native to South Asia, where it has been grown for fiber. ‘Tropic Sun’ was released in Hawaiʻi by the University of Hawaiʻi and USDA.

Growing on Kauaʻi

• Grows quickly in warm weather on most soils.
• Use it to rest and rebuild a bed between other crops.

Planting and propagation

• Broadcast seed thickly or sow in rows, about ½–1 inch deep, then water.

Care and maintenance

• Cut it at early flowering, before it sets seed, and leave it on the surface as mulch.

Role in the guild

Use sunn hemp to prepare new beds or between rows of young trees, then chop and drop it.

Harvest

Cut plants at early bloom, usually 2–3 months after sowing.

Uses

• Soil: nitrogen, organic matter, and nematode suppression.
• Bees: yellow flowers feed pollinators.

Problems to watch

• Spreading by seed if left to mature.
• Not for eating; the seeds can be toxic to animals.

Learn more

• Plant Pono: Crotalaria juncea

General guidance for Kauaʻi; conditions vary by site. Check any new plant at plantpono.org before planting.