Native to a broad arc stretching from Afghanistan through India to Indochina, Sunn hemp has been cultivated for millennia. Its species epithet, junceus, means "rush-like" in Latin, a nod to the tall, slender stems that can reach nine feet and were historically harvested for fiber — twisted into cordage, woven into nets, pressed into paper. Today it is valued primarily as a summer cover crop, capable of suppressing weeds through sheer density of canopy while fixing nitrogen at the root level. It grows quickly in warm, well-drained soils with a pH of 5 to 8 and thrives even in poor ground, though fertile soils will push it to its full height.

For best nitrogen fixation, seed should be inoculated with Rhizobium before sowing. Sunn hemp is not a host for root-knot nematodes, making it particularly useful as a rotation crop in gardens where those pests are a problem. It is killed by frost, and the long stems should be chopped before incorporation to help them break down. As a forage crop it has merit, but animals must be kept from grazing once flowering begins — the seeds carry toxins that accumulate at that stage. A summer sowing, a heavy canopy, and a frost-killed close: Crotalaria juncea does its best work quietly, below the surface.