Description from Flora of China
Mucuna nigricans (Loureiro) Steudel var. hainanensis (Hayata) Wilmot-Dear; M. nigricans var. hongkongensis Wilmot-Dear; M. suberosa Gagnepain.
Climbing vines, to 5 m. Young stems glabrous or with sparse fine adpressed hairs. Leaves widely varying in size, 7-25 cm; petiole 4.8-11.5 cm, glabrous or sparsely finely hairy; stipels 2-6 mm; leaflets papery or leathery, almost glabrous on both surfaces, lateral veins 4 or 5(-7) pairs, gently curved; terminal leaflet obovate-elliptic or elliptic, 4.5-10.5(-16) × 2.5-6(-9) cm, base rounded, apex caudate to abruptly shortly acuminate; lateral leaflets 5-8(-11) cm, ratio of abaxial to adaxial halves ca. 1.5:1. Inflorescence axillary, 6-40 cm, with 5-18 nodes, basal part without flowers but often with a few long acuminate bracts 2-3 cm; bracts at flowering nodes large, oblong or broadly ovate to elliptic with rounded often hooded apex, 1-2 cm, hairy; bracteoles linear-ovate or narrowly elliptic, to 13 × 4 mm; pedicels 8-10 mm, densely silky adpressed hairy. Calyx with dense silky short hairs and irritant bristles; tube 7-10 × 10-14 mm; lobes narrow, lowest 8-10 mm, laterals 5-6 mm. Corolla deep purple (rarely white); standard 2.5-3.5 cm; wings 4.5-5.5 × 1-1.3 cm; keel 4.8-5.7 cm. Legume asymmetrically oblong or ovate-oblong, 9-18 × 4.5-5.5 cm, ca. 1 cm thick, leathery, with red-brown irritant hairs, with narrowly rounded base and acute apex often with persistent style, both margins with pair of wings ca. 1 cm wide, both surfaces with 8-14 markedly oblique crowded winglike lamellae uniformly 4-5 mm high, mostly not interrupted nor branched, some extending onto marginal wings of fruit. Seeds 2-4, black, oblong or reniform, 1.7-2.5 × ca. 1.5 cm, 5-7 mm thick; hilum length 1/2-3/4 of seed circumference. Fl. Jan-Mar, fr. Mar-May.
Mucuna hainanensis is distinguished by its fairly small usually purple flowers with a relatively long wing and its oblong legume with simple uninterrupted oblique lamellae of almost uniform height and sometimes running into the marginal wings. It is vegetatively almost identical to M. interrupta and M. revoluta, both of which have fruit lamellae T-shaped in cross section; M. revoluta also has distinctive minute spreading indumentum on the inflorescence. Mucuna lamellata has a similar but narrower fruit with less oblique lamellae, seeds only half the size, and rhombic-ovate leaves. Mucuna championii has a similar fruit and small flowers but a distinctive red indumentum. Mucuna "sp. D" of Wilmot-Dear (Kew Bull. 39: 57. 1984) is a white-flowered variant of M. hainanensis.
All of the material from the Flora area belongs to the typical subspecies. The other subspecies, Mucuna hainanensis subsp. multilamellata Wilmot-Dear, is found in the Philippines and the E and N Indian subcontinent and is distinguished mainly by its fruit with dense short spreading hairs and 20-25 scarcely oblique lamellae and its leaflets sometimes longer.
Forests, thickets, valleys, mountain slopes, dry or swampy soil; sea level to low elevations, rarely to 1000 m. ?Guangdong, ?Guangxi, Hainan, Yunnan [Vietnam].