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 feayi is one of a trio of very slender milkweeds in the southeastern United States, along with A. cinerea and A. viridula. These species are divergent in floral morphology, but without flowers they are very difficult to distinguish (even in fruit), and they appear to be close relatives. However, A. feayi occurs primarily in peninsular central and southwestern Florida, from Lake to Collier counties, most commonly in scrub. A single disjunct population has been documented from Clay County in the northeastern part of the state (Hall 1896 [FLAS]). Asclepias cinerea and A. viridula are found in northern Florida or further north, in flatwoods. All three species are cryptic in the absence of flowers and appear to respond positively to fire and rainfall events. They are likely to be more common than is apparent because they are inconspicuous and emerge episodically. Nonetheless, numerous historical locations for A. feayi have been developed and are no longer capable of supporting populations, and its conservation status merits evaluation. An unusual putative hybrid with A. pedicellata represented by a single collection is documented from Marion County (Judd 2639 [FLAS]), suggested by the exactly intermediate floral morphology.

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