Flora of North America species comparison

Asclepias mexicana Decaisne was misapplied to A. fascicularis in the past; this is the legitimate name of a related species endemic to southern and eastern Mexico. The common name Mexican (whorled) milkweed stems from this past confusion. Compared to its close relatives A. angustifolia, A. linearis, A. pumila, A. subverticillata, and A. verticillata, the leaves of A. fascicularis are not particularly narrow (despite the implication of another common name). However, very narrow leaves are found in A. fascicularis when it is growing at relatively dry sites, especially at the eastern limit of its range in southeastern Idaho. Such specimens (for example, Mumford 272 [MO], Atwood 28495 [NY]) have been attributed in the past to A. subverticillata in error. Asclepias fascicularis is completely allopatric with its Incarnatae clade relatives (species 7–16). Like these species, it is easily cultivated, and its seeds are widely available. In Washington, the range of A. fascicularis is restricted largely to the valleys of the Columbia and Spokane rivers and in Idaho to the Snake and Weiser rivers.

(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.)