Black swallow-wort, dompte-venin noir

Asclepias nigra Linnaeus, Sp. Pl. 1: 216. 1753; Cynanchum louiseae Kartesz & Gandhi

Stems erect to prostrate prox­imally, twining distally. Leaves: petiole 0.5–1.5 cm; blade pinnipalmately veined, lance­olate to broadly ovate, 3–12 × 1–6.5 cm, membranous, base truncate, rounded, or subcordate, with 2–8 laminar colleters, margins ciliate, apex attenuate to acuminate, surfaces pilosulous on veins abaxially, glabrous adax­ially. Inflorescences solitary at nodes, simple or com­pound cymes, 4–10-flowered; peduncle 0.5–1.5 cm, pilosulous. Pedicels 5–6 mm, pilosulous. Flowers: calyx lobes lanceolate to deltate, 1–1.5 mm, apex acute, pilosulous to glabrate, margins ciliate; corolla very dark purple, rotate to rotate-campanulate, fleshy, lobes apically planar, deltate, 1.5–3 mm, ± as wide as long, apex acute, glabrous abaxially, pilosulous to hispidulous adaxially, gynostegial corona a thick, shallowly 5-lobed or crenulate ring exceeding style apex, reddish purple to dark purple; style apex depressed, umbonate, green. Follicles 4–8 × 0.7–1 cm, apex attenuate to acuminate. Seeds 7–15, brown, ovate, 6–8 × 3–4.5 mm; coma white, 2–3 cm. 2n = 44.

Flowering May–Aug(–Nov); fruiting (Jun–)Jul–Oct. Disturbed areas, gardens, fences, old fields, pastures, roadsides, streamsides, ravines, slopes, beaches, rail­roads, limestone, igneous substrates, rocky soils, thickets, woods, grasslands; 0–400 m; introduced; Ont., Que.; Conn., Ill., Ind., Kans., Ky., Maine, Md., Mass., Mich., Mo., N.H., N.J., N.Y., Ohio, Pa., R.I., Vt., Wis.; sw Europe (France, Italy, Portugal, Spain).

Vincetoxicum nigrum is more widely known in North America by the illegitimate name Cynanchum nigrum (Linnaeus) Persoon, a later homonym of C. nigrum Cavanilles [basionym of the Mesoamerican species Gonolobus niger (Cavanilles) R. Brown ex Schultes]. It is the more frequently encountered and better estab­lished of the two Vincetoxicum species in the United States. One of the earliest and best documented escapes from cultivation occurred in Cambridge, Massachusetts, where garden plantings at Harvard University (when Asa Gray was herbarium director in the late 1800s) and elsewhere were documented as serving as invasion foci for surrounding areas. The species is still commonly encountered on fences in many places in Cambridge. In addition to the states listed above, V. nigrum is known from cultivation near Minneapolis, Minnesota, but is not yet known to have escaped in that state. Chromosome numbers of 2n = 22 and 2n = 44 have been reported from the native range in Europe, and it is unclear whether the introduced populations are exclusively tetraploid or also include diploids (A. DiTomasso et al. 2005). Unlike most species of Apocynaceae that have been studied, V. nigrum has been shown to be capable of autogamous pollination through the in situ germination of pollinia within anther thecae (DiTomasso et al.).