Lablab purpureus aka Hyacinth Bean

Lablab purpureus, or hyacinth bean, belongs to the pea family. Kew lists Cape Verde, tropical and southern Africa, Madagascar, and India as its native range. Long use as a crop has blurred its origins. This fast-growing plant can live for more than one year. Gardeners also grow it as an annual where winters are cold. Its soft stems twine around supports as they grow.

Garden size varies with the source and growing conditions. RHS gives heights of 8–13 feet [2.5–4 metres]. NC State gives 10–25 feet [3–7.6 metres]. Tropical accounts report stems reaching 30 feet [9 metres]. Spread ranges from about 1.6–6 feet [0.5–1.8 metres]. The stems have a purple tint. Each alternate leaf has three broad, egg-shaped leaflets with smooth edges. Their upper faces are smooth, while short hairs cover the undersides. The green foliage falls away as the plant becomes dormant.

Scented, pea-shaped blooms form long clusters in summer and autumn. Flowers may be white, pink, or purple. They draw butterflies and hummingbirds, and insects pollinate them. The plant can also set seed with its own pollen. Flat pods follow the flowers and hold two to four seeds. Pods may be green, purple-tinged, or purple. Growers use the flowers and pods in cut arrangements. The vine can cover fences, walls, and trellises. It also suits containers, ground cover, and seasonal privacy screens.

Grow it in full sun with moist soil that drains well. It accepts several soil types and a broad pH range. Heat and humid air suit it, but cold limits growth. RHS rates it H1C; NC State lists USDA zones 10–11. Plant outdoors after frost risk has passed, using a strong support. A long summer helps the crop yield well. Heated glass offers another option in cool climates. Most plants have few pest or disease problems. Japanese beetles may chew holes in leaves, though vines can regrow.

People eat the seeds, young pods, leaves, flowers, and roots. Raw mature seeds are poisonous and contain cyanogenic compounds. Boil dried seeds thoroughly, changing the water several times. Cook young pods before eating them. Poisoning can cause vomiting, weakness, breathing trouble, and convulsions. The plant also supplies livestock fodder and green manure. Root bacteria help add nitrogen to the soil. Sources record traditional medicinal uses, but these do not establish safe treatments.