Flora of North America species comparison

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.).

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Species ca. 28–35 (2 in the flora).

Vincetoxicum has been treated in many regional floras in North America as a synonym of a large and polyphyletic Cynanchum; however, these genera have been shown to be rather distantly related (S. Liede and A. Täuber 2002; Liede et al. 2012). The circumscription adopted here corresponds in part to the Vincetoxicum clade of Liede et al. (2016). The later homonym Vincetoxicum Walter refers to distantly related species of Gonolobus and Matelea (subtribe Gonolobinae) and is a source of some taxonomic confusion because numerous American species of those genera were described under Vincetoxicum.

Vincetoxicum is well supported as belonging to the Tylophorinae K. Schumann (Asclepiadeae Duby). S. Liede et al. (2012, 2016) recommended uniting Vincetoxicum with Tylophora R. Brown and all other genera of Tylophorinae, except Pentatropis R. Brown ex Wight & Arnott, in order to resolve the non-monophyly of Tylophora and Vincetoxicum. This extreme proposal does not affect species in the flora area, but it seems prudent to exclude Tylophora from the synonymy of Vincetoxicum until a more considered approach to a revision of Tylophorinae is undertaken, weighing the advantages and disadvantages of broadly versus narrowly circumscribed genera.

Reports of Vincetoxicum hirundinaria Medikus [synonyms: Cynanchum medium (Decaisne) K. Schumann, not R. Brown, C. vincetoxicum, V. medium, V. officinale] are based upon plants not persisting outside cultivation or misapplication to plants of V. rossicum. Vincetoxicum hirundinaria was occasionally cultivated in the late nineteenth and early twentieth centuries at scattered sites in the northern United States and southern Canada, but none of the occurrences outside of cultivation appears to have persisted and no extant populations are known. Inconsistent recognition of the specific distinction between V. hirundinaria and V. rossicum, coupled with misapplication of the name C. medium to plants of V. rossicum in the Americas, has led to many erroneous reports of naturalized populations of V. hirundinaria in the flora area. For example, the concept of V. hirundinaria of H. A. Gleason and A. Cronquist (1991) combines attributes of both species. Vincetoxicum hirundinaria can be distinguished most readily from the two naturalized species in the flora area by the cream to yellowish cream corollas.

The two naturalized species in the region, Vincetoxicum nigrum and V. rossicum, are considered invasive and to be threats to native forest understory vegetation. Means to control these species are an active area of research (A. DiTomasso et al. 2005) and the continuing use of V. nigrum as a horticultural species should be curtailed. Monarch butterfly larvae (Danaus plexippus) do not complete development on V. nigrum or V. rossicum; however, there is equivocal evidence for monarch oviposition on Vincetoxicum (DiTomasso et al.). An unusual reproductive characteristic of many species of Vincetoxicum, shared by V. nigrum and V. rossicum, is polyembryonic seeds (DiTomasso et al.).

(Discussion copyrighted by Flora of North America; reprinted with permission.)