Description from Flora of China
Asclepias canescens Willdenow, Ges. Naturf. Freunde Berlin Neue Schriften 3: 418. 1801; Cynanchum glaucum Wallich ex Wight; Vincetoxicum canescens (Willdenow) Decaisne; V. glaucum (Wallich ex Wight) K. H. Rechinger; V. hirundinaria Medicus subsp. glaucum (Wallich ex Wight) H. Hara.
Herbs, most parts canescent or puberulent. Rhizomes with dense fascicles of roots. Stems erect, to 50 cm, flexuous, pale or glaucous. Petiole 2-15 mm; leaf blade ovate to oblong, or lanceolate, 2.5-8 × 1.5-5 cm, leathery, base rounded to cordate, apex acuminate or apiculate. Inflorescences usually sessile, umbel-like, pubescent. Pedicel 2-7 mm. Sepals ca. 0.7 × 0.4 mm, puberulent. Corolla yellow or green fading to greenish orange; tube ca. 1 mm; lobes ovate, 2-3 × ca. 1.5 mm, glabrous outside, sparsely hirsute inside, apex subacute. Corona lobes broadly ovate, fleshy, apex rounded, broader than long. Follicles slender, 5-10 cm. Seeds broadly ovate, 5-7 mm, comose. Fl. May-Aug, fr. Aug-Dec. 2n = 22.
Extensive field work is needed to establish the extent of variation in the characters used to separate members of the Cynanchum vincetoxicum complex, including species 34-37. Two species groups, separated arbitrarily by the presence or absence of corolla hairs, are recognized. The group with hairy corollas includes C. forrestii, C. bicampanulatum, and C. rockii. Although most material with glabrous corollas belongs to C. inamoenum, some collections from widely scattered localities between the Himalayas and Taiwan have been named as C. vincetoxicum. It is not clear that these are discontinuous with C. inamoenum and that they represent true C. vincetoxicum. For the present, all material with glabrous flowers is included in C. inamoenum.
Open woodland, thickets; ca. 2500 m. Sichuan, Xizang, Yunnan [Afghanistan, Bhutan, India, Kashmir, Nepal, Pakistan, Russia; SW Asia]