Orobus lathyroides Linnaeus, Sp. Pl. 2: 728. 1753, not Vicia lathyroides Linnaeus (1753); Ervum unijugum (A. Braun) Alefeld; Lathyrus messerschmidii Franchet & Savatier; Vicia unijuga f. albiflora Nakai; V. unijuga var. angustifolia Nakai; V. unijuga var. breviramea Nakai; V. unijuga var. ciliata H. Lé­veillé; V. unijuga var. integristipula H. Léveillé; V. unijuga var. kaussanensis H. Léveillé; V. unijuga var. lobata Seok & B. H. Choi; V. unijuga subsp. minor (Nakai) Y. N. Lee; V. unijuga f. minor Nakai; V. unijuga var. ouensanensis H. Léveillé.

Herbs perennial, (15-)40-100(-180) cm tall. Stem sub­shrubby, pilose when young. Leaves paripinnate; stipules hastate or sublanceolate, 8-20 × 3-5 mm, margin unequally toothed; leaflets 1-paired, ovate to lanceolate or rhombic-ellip­tic, (15-)30-90(-115) × (6-)15-40(-50) mm, both surfaces pi­lose, apex sometimes acuminate; tendril rarely present, usually replaced by a mucro. Raceme rarely branched, ± cylindric, 4.5-7 cm, usually obviously longer than leaf, densely 8-20-flow­ered. Calyx campanulate or obliquely so, glabrescent. Corolla deep or light blue to purple to red, rarely white; standard violin-shaped, constricted at middle, 11-15 mm; wings 13-14 mm, longer than keel. Ovary linear, glabrous; ovules 2-8. Legume oblong, flat, 20-35 × 5-7 mm, glabrous, apex beaked. Seeds 3-7, oblate-spheroid. Fl. Feb (in S Yunnan), Jun-Sep, fr. Jul-Oct. 2n = 12, 14, 24.

Vicia unijuga may be the commonest species of Vicia in China, or at least it is the most often collected. The morphological variation is considerable. Several specimens from Hubei are distinctive in having leaflets lanceolate, large (55-115 × 25-45 mm), with a long-acuminate apex (A. Henry 6596, BM!, E!, K!; E. H. Wilson 1230, K!; E. H. Wilson 2036, E!; E. H. Wilson 3486, BM!, E!, K!; and Silvestri 1185, K!). A specimen apparently from the Beijing area ("Fl. Pekinensis," Bret­schneider 209, K!) is similar, as is one from S Korea (Chung In-Cho’s collector 9555, E!). Two specimens from Shandong ([German name] 218, K!; E. Faber 245, K!) have leaflets narrowly elliptic and unusually narrow (30-50 × 6-12 mm). A specimen from S Yunnan (J. F. Rock 2548, K!) has leaves approaching this narrowness.

Vicia unijuga var. trifoliolata Z. D. Xia (Acta Phytotax. Sin. 34: 433. 1996; V. unijuga f. trifoliolata (Z. D. Xia) Y. Endo & H. Ohashi), described from Shaanxi, Shanxi, and Sichuan, was diagnosed in the protologue as "A typo foliolis 3 cirrho 4-5 cm longo, floribus minoribus laxioribusque differt" (differs from the type [by having] leaflets 3 with tendril 4-5 cm long, with flowers smaller and laxer). Further study is needed to ascertain its status.

"Vicia unijuga var. waldeniana" (S. Y. Hu in B. M. Walden & S. Y. Hu, Wild Fl. Yunnan Centr. China, 131. 2000) belongs here but was not validly published because no type was indicated (Vienna Code, Art. 37.1).

Forests, forest margins, thickets, scrub, hedges, mountain and hill slopes and summits, damp valleys, pastures, grassy places, river and stream banks, ravines, rocky places on slopes, roadside banks, field margins; below 4000 m. Anhui, Gansu, Guizhou, Hebei, Heilongjiang, Henan, Hubei, Jiangsu, Jiangxi, Jilin, Liaoning, Nei Mongol, Qinghai, Shaanxi, Shandong, Shanxi, Sichuan, Xizang, Yunnan, Zhejiang [Japan, Korea, Mongolia, Russia (Far East, Siberia)].