Flora of North America species comparison

Asclepias verticillata is parapatric with the closely related and morphologically similar A. linearis, A. pumila, and A. subverticillata. It can be difficult to distinguish from these relatives where their ranges overlap. Similarities with A. linearis and A. subverticillata are discussed under those species; no definitive hybrids between A. verticillata and these species have been documented. The characteristic marginal corona segment tooth is often reduced to a shallow lobe in western populations of A. verticillata, which complicates distinguishing this species from A. subverticillata, and which suggests past introgression. A widely disjunct collection of A. verticillata was made in Arizona, well within the range of A. subverticillata, for Plants of the Hopis (Millspaugh 176 [F]); persistence of the species in Arizona has not been documented by additional collections. Hybrids with A. pumila are usually readily detected because the parental species are distinct in leaf arrangement and internode length (whorled and distant nodes in A. verticillata versus alternate and congested in A. pumila). These hybrids often have mixed phyllotaxy and have been documented in Kansas, Montana, South Dakota, and Texas. Asclepias verticillata is strongly rhizomatous and forms dense colonies on roadsides and in prairies; in forests, however, genets are small and solitary stems are common. Like several other milkweed species, it is rare and declining at the northeastern terminus of its range (in Delaware, Massachusetts, New Jersey, Pennsylvania, Rhode Island, and Vermont). It is also considered rare in Saskatchewan, where it has been documented by few specimens. Reports from Wyoming (Crook County) all seem to pertain to A. pumila (B. Heidel, pers. comm.).

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

Asclepias elata is one of the few milkweeds in the flora area that possesses a mostly Mexican distribution but does not belong to the major clade that contains most of such species (species 23–27 here). Rather it is the sole representative in the flora, and the most northerly distributed member, of a small clade of glaucous, vegetatively homogeneous species ranging as far south as Costa Rica (M. Fishbein et al. 2011). Asclepias elata has been inconsistently recognized as distinct from A. glaucescens since the time of A. Gray et al. [1878–1897, vol. 2(1)]. However, at least since J. N. Rose (1892), the distinction between these species has been understood, and was presented clearly by R.E. Woodson Jr. (1954). Although indistinguishable in the absence of flowers, A. elata differs by fully pendent (versus erect to pendent) flowers, fewer flowers per umbel, longer corolla lobes, and corona segments that spread away from the gynostegium and are exceeded by the style apex, with the appendage merely a crest included in the segment (versus segments that are strict, overtop the style apex, and bear exserted appendages in A. glaucescens). Both species have extensive distributions, but they are largely allopatric, except in Mesoamerica. The northernmost populations of A. glaucescens are in Nayarit and San Luis Potosí. Asclepias elata is one of a cohort of species reaching their northwestern limits in Arizona that indicate biogeographic affinity of that region to the eastern slope of the Sierra Madre Occidental and Altiplano, rather than the western slope of the Sierra Madre Occidental. Other examples from the milkweed flora include A. brachystephana, A. involucrata, A. nummularia, A. oenotheroides, and A. quinquedentata. In Arizona, A. elata is known from Cochise, Graham, Pima, and Santa Cruz counties, in New Mexico from Eddy, Hidalgo, and Sierra counties, and in Texas from Brewster, Culberson, Jeff Davis, and Presidio counties.

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