Flora of North America species comparison

Asclepias syriaca is surely the most familiar milkweed in North America, and one that evokes ambivalence. It has been considered an undesirable species because of its prolific rhizomatous spread and ability to invade and thrive in cultivated land. However, it has been used as a food plant by indigenous and colonizing peoples, and its pleasantly fragrant and nectariferous flowers are avidly sought by diverse insects, highlighting the ecological importance of A. syriaca. Moreover, it has come to be appreciated because of its importance as one of the most commonly utilized host plants of the monarch butterfly, Danaus plexippus. There is some evidence that its population in agricultural lands in the upper midwestern United States has dramatically declined in the last several decades. However, its range and abundance prior to European settlement are not well understood and may have been much lower than in historical times, particularly in deforested areas of the eastern United States. It is considered to have been introduced to Newfoundland, Alabama, Georgia, Louisiana, Mississippi, and South Carolina and has been reported, but without documentation, from Texas. It has been documented from Salem, Oregon, but appears to not be established there (R. Halse, pers. comm.). It is considered to be of conservation concern at the northwesternmost edge of its range in Saskatchewan, where a single population is known in Estevan Municipality. Asclepias syriaca is the most promiscuous of milkweeds, as it is known to hybridize with at least seven other species (A. amplexicaulis, A. exaltata, A. ovalifolia, A. purpurascens, A. speciosa, A. sullivantii, A. tuberosa). Hybrids with A. speciosa are most frequent, as discussed under that species. Hybrids with A. exaltata are not infrequently encountered in the Appalachian Mountains and elsewhere. Other hybrids are highly localized. In all cases, putative hybrids are inferred from intermediate floral and vegetatative traits.

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

Asclepias nummularia is the only milkweed in the flora area with small stature and obovate to orbiculate leaves that are bluish under dense tomentum until late in the season. The plants often bear an uncanny resemblance to tiny cabbages. Fruit set may fail across broad regions in some years, perhaps due to drought, although the adequacy of pollination in this species has not been studied. Unlike many species of Asclepias, post-reproductive and non-reproductive plants of A. nummularia often persist until frost. Coupled with the early flowering of this species, vegetative plants are observed and collected far more often than reproductive ones. Some plants persisting into the fall months have much larger leaves and longer stems, especially in shady or moist habitats; these have often been mistaken for A. latifolia, the range of which only barely overlaps that of A. nummularia in Grant County, New Mexico. Asclepias nummularia is not common in New Mexico, where it has been found in Grant, Hidalgo, and Sierra counties, and is only locally common in Texas in the Davis Mountains (Jeff Davis County), and additionally in Brewster and Presidio counties. Populations from San Luis Potosí and south with narrower leaves have been segregated as A. nummularioides W. D. Stevens; recognition of this taxon requires further evaluation.

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