Description from Flora of China
Phaseolus vexillatus Linnaeus, Sp. Pl. 2: 724. 1753; Vigna vexillata var. pluriflora Franchet; V. vexillata var. yunnanensis Franchet.
Perennial herbs, twining. Root woody, fusiform. Stems with spreading brown bristly hairs, glabrescent. Stipules ovate to ovate-lanceolate, 3-5 mm, cordate or auriculate at base, ciliate; petiole 1-11 cm; leaflets membranous, variable in shape, ovate to lanceolate, 4-9(-15) × 2-5(-8) cm, brown or gray pubescent on both surfaces, base rounded to cuneate, margin entire, sometimes slightly 3-lobed, apex acute or acuminate. Racemes axillary, 2-6-flowered, subumbellate; peduncles 5-20 cm. Bracteoles subulate, ca. 3 mm, caducous. Calyx with brown or white bristly hairs, rarely glabrescent; tube 5-7 mm; lobes linear or linear-lanceolate, 2-5 mm, upper 2 connate at base. Standard pink, purple, or partly yellow, sometimes with yellow or purple spots inside at base, 2-3.5 × 2-4 cm, emarginate; keel whitish or purplish, falcate, with beak incurved through 180°. Legumes erect, linear-terete, 4-14 cm × 2.5-4 mm, bristly. Seeds 10-18, yellowish, black, or brown to scarlet with black spots, oblong or oblong-reniform, 2-4.5 mm. Fl. Jul-Sep.
This species is used medicinally.
Vigna vexillata var. tsusimensis Matsumura (Bot. Mag. (Tokyo) 16: 93. 1902) differs only by having larger seeds, ca. 5 × 4 mm. It has been recorded from Taiwan (see T. C. Huang & H. Ohashi, Fl. Taiwan, ed. 2, 3: 393. 1993).
Thickets, open forests. Anhui, Fujian, Gansu, Guangdong, Guangxi, Guizhou, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Shaanxi, Sichuan, Yunnan, Zhejiang [widely distributed in tropical and subtropical regions].