Lablab purpureus (L.) Sweet (hyacinth bean, bonavist)

Dolichos lablab L.

Map 1759

Plants annual (perennial herbs farther south). Stems 200–600 cm long, trailing or climbing and twining (erect and bushy in some cultivated forms), branched, unarmed, usually purplish-tinged, glabrous or sparsely pubescent with more or less downward-curved or -angled hairs, these not hooked at the tip. Leaves alternate, pinnately trifoliate, the petiole 7–15 cm long. Stipules 3–7 mm long, lanceolate-triangular with an asymmetric base, tapered to a sharply pointed tip, shed early; stipels 3–4 mm long, narrowly oblong-elliptic, persistent. Leaflets 4–10 cm long, 3–9 cm wide, broadly ovate, the terminal leaflet sometimes somewhat rhombic, broadly angled to nearly truncate at the base, short-tapered to a sharply pointed tip, the margins entire, the surfaces glabrous or sparsely and inconspicuosuly hairy, with 3 main veins from the base, but otherwise more or less pinnately veined. Terminal leaflet with the stalk 25–35 mm long, symmetric at the base; lateral leaflets with the stalk 3–5 mm long, asymmetric at the base. Inflorescences axillary, narrow racemes 10–40 cm long, with 8 to numerous flowers in clusters of 2–5 per node, the stalk 50–70 mm long, the bracts 4–5 mm long, narrowly ovate, shed early, each flower with a stalk 3–4 mm long, the bractlets 3–4 mm long, ovate-elliptic, densely hairy along the margins, more or less persistent. Calyces short-hairy along the margins, usually strongly purplish-tinged or dark purple, the tube 3–4 mm long, broadly bell-shaped, 2-lipped, the lobes 2–3 mm long, unequal, the upper 2 lobes fused into a broadly ovate, slightly hooded structure, the lower 3 lobes shorter, similar, narrowly ovate-triangular, sharply pointed at their tips. Corollas papilionaceous, purple or white, the banner 12–16 mm long, 12–14 mm wide, the expanded portion curved or bent backward, broadly oblong-ovate to nearly circular, notched at the tip, with a pair of thickened areas toward the base, the wings 14–16 mm long, 6–8 mm wide, obliquely oblong-obovate, curled over the keel, the keel 12–14 mm long, 3–4 mm wide, boat-shaped, fused to above the midpoint, bent or curved upward abruptly near the midpoint, the tip bluntly to sharply pointed. Stamens 10, in 2 alternating, slightly shorter and longer series, 9 of the filaments fused and 1 free, the fused portion 11–13 mm long, bent upward at its tip, the free portion 5–7 mm long, the anthers small, attached at the base, yellow. Ovary 7–9 mm long, hairy, the style 10–12 mm long, bent or curved upward toward the tip, somewhat flattened, hairy on the inner side, the stigma small, terminal, oblique. Fruits legumes, 6–9 cm long, 16–20 mm wide, oblong, short-tapered asymmetrically to a beak, flattened, sometimes slightly constricted between the seeds, straight or more commonly curved upward, dehiscing tardily by 2 valves, the upper margin beaded with small, warty outgrowths, purple to brownish purple at maturity (more or less green in some cultivated races), glabrous or nearly so, often somewhat shiny, mostly 2–6-seeded. Seeds 10–12 mm long, 6–8 mm wide, oblong in outline, the surface white to dark purple, brown, or black, smooth, somewhat shiny, with a white aril. 2n=22. July–September.

Introduced, uncommon, sporadic in southern Missouri (native of Africa, Asia, now cultivated and introduced throughout the tropics; introduced sporadically in the eastern U.S.). Alleys, roadsides, and open disturbed areas.

Lablab purpureus was first collected as an escape in Missouri by Paul Redfearn in 2001 from Greene County and by Bill Summers in 2002 from Howell County. It probably does not persist long outside of gardens. Hyacinth bean has been cultivated for thousands of years as a food for people and livestock, especially in India, China, and Japan (Tindall, 1983). The plants are hardy and do well in warm, dry climates. The young leaves can be picked and eaten like spinach. Some forms produce an edible tuber. The beans can be eaten, but only after being cooked in several changes of water because of the presence of toxic compounds. The species also is used as a cover crop and green manure. In the United States, it is planted mainly as an ornamental.

The more erect, bushy forms are sometimes called var. lignosus (Prain) Kumari, but these are better treated as cultivars until genetic relationships among the wild and cultivated races can be studied in more detail. Verdcourt (1970a), in his studies of Lablab for the Flora of Tropical East Africa Project, divided the species into three subspecies. He treated the native populations as ssp. uncinatus Verdc. and segregated an unusual African cultivated race with slender fruits as ssp. benghalensis (Jacq.) Verdc. The remaining polymorphic group of cultivated forms were included in ssp. purpureus.