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 cutleri is an edaphic endemic, limited to deep red, pink, and orange sand deposits developed on geologic units of sedimentary origin of the Colorado Plateau. Its range is restricted to Apache, Coconino, and Navajo counties in Arizona and Grand and San Juan counties in Utah. The majority of occurrences are on the Navajo reservation. A report from San Juan County, New Mexico, needs confirmation. Although rarely collected, it is highly cryptic due to its small stature and slender habit, and it is probably more common than it appears in its preferred habitat. The herbage has a bluish hue when fresh that turns green on drying, similar to A. brachystephana and A. cryptoceras. It is often erroneously described to be an annual because the very deep roots are almost never extricated, and the slender subterranean stem appears to be the root. Paired fruits from a single flower appear to be unusually common compared to other species of Asclepias. The flowers are remarkably similar to those of A. brachystephana and the so-called dwarf milkweeds, A. eastwoodiana, A. ruthiae, A. uncialis, and the sympatric A. sanjuanensis, but these species are only distantly related to A. cutleri (M. Fishbein et al. 2011).

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