Description from Flora of China
Crotalaria benghalensis Lamarck; C. sericea Willdenow (1802), not N. L. Burman (1768), nor Retzius (1788); C. tenuifolia Roxburgh ex Hornemann.
Herbs, erect, 50-100 cm tall. Branches terete, ribbed, appressed silky pubescent. Stipules linear, ca. 2 mm, caducous. Leaves simple; petiole 3-5 mm; leaf blade oblong to linear-lanceolate, 6-12 × 0.5-2 cm, both surfaces pilose but abaxially more densely so, base attenuate, apex acuminate and mucronate. Racemes terminal, 10-20-flowered; bracts linear, 3-4 mm. Pedicel 5-8 mm; bracteoles inserted at base of calyx tube, shorter than bracts, densely pubescent. Calyx 2-lipped, 1-1.5 cm, parted almost to base, densely rusty pilose; lobes lanceolate, curved. Corolla yellow; standard oblong, 1.5-2.5 cm, base with 2 appendages; wings obovate-oblong, 1.5-2 cm; keel subangular, slightly longer than wings, apex with a long slightly incurved twisted beak exserted beyond calyx. Legume subsessile, cylindric, 2-4 × 1.2-1.7 cm, 10-15-seeded, rusty pubescent. Seeds smooth to papillose around hilum. Fl. Aug-Dec, fr. Jan-May.
Crotalaria juncea is cultivated as a fiber crop ("Sunn Hemp") and as a green manure. It is widely naturalized in Africa, tropical and subtropical America, Asia, Australia, and Papua New Guinea. Its original distribution is probably S Asia.
Sparse forests, along trails; below 100-2000 m. Cultivated or naturalized in Fujian, Guangdong, Guangxi, Jiangsu, Shaanxi, Shandong Sichuan, Taiwan, Yunnan, and Zhejiang [Bhutan, Cambodia, India, Indonesia, Laos, Malaysia, Myanmar, Thailand, Vietnam].