Hyacinth bean (Lablab purpureus, (L.) Sweet 1826)

Identity

Hyacinth bean (Lablab purpureus (L.) Sweet, 1826) is a climbing legume in the Fabaceae and the only species in the genus Lablab. Linnaeus first described it in Dolichos; Robert Sweet transferred it to its own genus in 1826, giving it its current scientific name. ^{1 7}

Regional names in French include dolique lablab, pois antaque on Réunion and pois boucoussou. The genus name comes from Arabic lablāb, a name for climbing plants that aptly describes this species’ habit. ⁷

Recognition

Twining stems, often flushed purple, wrap around available supports and can reach 6 m (20 ft) in a single season. The trifoliate leaves have broad, pointed leaflets, bright green or washed with purple depending on the cultivar. ^{3 6}

Upright flower clusters range from white through pink to deep purple and have a light fragrance. They develop into flattened green to deep purple pods, a few centimeters long, containing up to four oval seeds with a conspicuous white hilum, or seed scar. ^{2 3}

Below ground, an established plant has a taproot extending deeper than 2 m (6.6 ft). This helps hyacinth bean survive dry spells by reaching water unavailable to shallow roots. ⁸

Useful taxonomy

Only one species is recognized in the monotypic genus Lablab: Lablab purpureus, although several subspecies and varieties have distinct names. Names encountered in taxonomic accounts include:

• Dolichos lablab L., the basionym under which Linnaeus first described the species;
• Lablab purpureus subsp. bengalensis (Jacq.) Verdc., a subspecific name;
• Lablab purpureus subsp. uncinatus Verdc., the wild form closest to spontaneous African populations;
• Dolichos purpureus L., a historical name.

For identification, the accepted name Lablab purpureus provides the stable reference. Its many synonyms largely reflect the history of a plant long placed in Dolichos, rather than separate botanical species. ^{1 5}

Habitat and range

Sub-Saharan Africa, especially its eastern and southern regions, forms the native range, with populations also attributed to Cape Verde, Madagascar and India. Very ancient domestication makes the origin of some populations, particularly Indian ones, difficult to establish. ^{1 4}

Beyond this range, hyacinth bean is almost always deliberately cultivated, although it sometimes naturalizes on tropical waste ground and roadsides without becoming a serious weed. In Hawaii, for example, it grows outside cultivation without a major documented impact. ^{3 6}

Where winters bring frost, a single night below 0 °C (32 °F) destroys the plant and prevents naturalization in that setting. It therefore grows as a warm-season annual, sown after the last frost and killed by the first cold weather. ²

Life cycle and reproduction

Seeds sown in warmed soil after all frost risk has passed germinate within one to two weeks. Growth is then rapid: the twining stem climbs its support as trifoliate leaves develop, followed by flower clusters whose timing depends strongly on day length. ^{8 3}

Pod maturity takes 70–120 days after sowing, depending on cultivar and climate. Pods are harvested young and tender as a green vegetable, or left to dry on the plant for seed. Agronomic cultivars such as Rongai flower abundantly only when day length falls below a certain threshold; others, including Highworth, were selected to flower early in short growing seasons. ⁸

Propagation is exclusively by seed. Where soil lacks suitable rhizobia, a specific inoculant markedly improves root nodulation and nitrogen fixation: hyacinth bean does not always associate well with naturally occurring local strains. ⁸

Uses and risks

Food and forage are its principal uses: green pods are eaten as vegetables, dry seeds supply protein and foliage feeds livestock. Root nodules also fix atmospheric nitrogen, making hyacinth bean an effective green manure; on slopes, its dense foliage helps protect soil against erosion. ^{3 8}

The hazard lies in raw seeds and pods, which contain cyanogenic glycosides capable of releasing substantial amounts of cyanide. Thorough cooking, with several changes of water for dry seeds, destroys these compounds and makes the plant edible for people and livestock. ^{4 6}

As an ornamental, the vine readily climbs pergolas, wire fences and stakes, forming a dense screen of flowers and purple pods in a single season. This role is distinct from its traditional use as a food crop. ²

Varieties, look-alikes and related plants

Rongai is a late forage cultivar with white flowers and pale brown seeds, introduced from Kenya to Australia in 1962 for forage production. Highworth followed from India in 1973 for seed production; it is early and twining, with purple flowers and black seeds bearing a white hilum. The ornamental selection Ruby Moon stands out for its deep purple flowers and pods. ^{8 2}

Other climbing legumes grown for food or ornament are the most frequent look-alikes:

• Vigna unguiculata, cowpea, produces much longer, cylindrical pods than the flattened pods of hyacinth bean;
• Clitoria ternatea, butterfly pea, has bright blue flowers, unlike the white, pink or purple flowers of hyacinth bean;
• Phaseolus vulgaris, common bean, has similar flowers but narrower pods, without the purple coloring seen on hyacinth bean stems and pods.

Flattened pods, a prominent white seed hilum and often purple-tinged stems remain the most useful features for recognizing Lablab purpureus. ^{3 5}

Main sources

• Plants of the World Online / Kew, Lablab purpureus (L.) Sweet
• Royal Horticultural Society, Lablab purpureus - hyacinth bean
• North Carolina State Extension, Lablab purpureus
• Plants For A Future, Lablab purpureus
• USDA PLANTS Database, Lablab purpureus (L.) Sweet
• Useful Tropical Plants Database, Lablab purpureus
• INPN / MNHN, Lablab purpureus (L.) Sweet, 1826
• Tropical Forages, Lablab purpureus cultivar comparison