Pale or European swallow-wort, dompte-venin de Russie

Cynanchum rossicum Kleopow ex A. W. Hill in B. D. Jackson et al., Index Kew., suppl. 8, 67. 1933

Stems erect proximally, twining distally. Leaves: petiole 0.5–2 cm; blade pinnipalmately veined, elliptic or lanceolate to ovate, 6.5–12 × 2.5–7 cm, membranous, base truncate to rounded, with 2–8 laminar colleters, margins ciliate, apex acuminate, surfaces pilosulous on veins abaxially, glabrous adaxially. Inflorescences solitary at nodes, simple or compound cymes, 5–20-flowered; peduncle 1.5–2.5 cm, pilosulous in a single line. Pedicels 3–7 mm, pilosulous. Flowers: calyx lobes narrowly lanceolate, 2 mm, apex attenuate, glabrous; corollas pinkish tan to reddish brown, campanulate, not fleshy, lobes spreading, apically twisted, lanceolate, 3–5 mm, 1.5–2 times longer than wide, apex obtuse to acute, glabrous, gynostegial corona of 5 prismatic, basally united segments exceeding style apex, pinkish tan to brick red; style apex depressed, umbonate, green. Follicles 3.5–7 × 0.5–0.6 cm, apex attenuate to acuminate. Seeds 10–20, brown, lenticular, lanceolate to ovate, 4–6.5 × 2.5–3 mm; coma white, 2–3 cm. 2n = 22.

Flowering May–Jul(–Aug); fruiting Jun–Sep. Dis­turbed areas, gardens, cemeteries, old fields, pastures, powerline rights of way, coastal bluffs, basalt outcrops, limestone, shallow to deep soils, forest edges, woods, thickets, alvars, grasslands; 0–400 m; introduced; N.B., Ont., Que.; Conn., Ind., Mass., Mich., N.H., N.J., N.Y., Pa.; se Europe (sw Russia, Ukraine); introduced also in nw Europe (Norway).

The taxonomy of Vincetoxicum rossicum in the flora area has long been confused, as described above. This species has been reported from North America as V. hirundinaria (Cynanchum vincetoxicum) and V. medium (C. medium). Vincetoxicum hirundinaria is cultivated in the flora area and may persist ephemerally but appears not to have naturalized. Vincetoxicum rossicum has not always been distinguished from V. hirundinaria. Thus, early reports of C. vincetoxicum or V. hirundinaria mostly apply to V. rossicum. Even when the two species were distinguished, plants of V. rossicum were commonly called C. medium or V. medium; however, this name is now considered misapplied to V. rossicum and to be a synonym of V. hirundinaria. There have been numerous reports of V. hirundinaria in the northern United States and southern Canada, but the great majority of these have been shown to represent V. rossicum, either through examination of herbarium specimens or interpretation of descriptions of corolla color.

Vincetoxicum rossicum is the more frequently encountered and better established species of the genus in Canada. In addition to the provinces listed above, V. rossicum is known from cultivation near Halifax, Nova Scotia, but is not yet known to have escaped in that province. Like V. nigrum, V. rossicum has been shown to be facultatively autogamous, via in situ pollinia germination, a rare condition in Apocynaceae (A. DiTomasso et al. 2005).