Flora of North America species comparison

Asclepias engelmanniana is usually a tall herb with drooping leaves and spherical umbels of greenish yellow flowers rising above surrounding grassland plants. In spite of its distinctive appearance, it is quite similar to its close relatives, A. rusbyi and A. stenophylla. The yellowish green coronas, squat flowers, upcurved fruiting pedicels, and drooping leaves distinguish A. engelmanniana from A. stenophylla, which has more slender flowers with creamy coronas, straight pedicels in fruit, and spreading to ascending leaves. Despite ranges with only little overlap and few if any mixed populations, these two species are often confused, especially in the absence of flowers. Compared to its close relative, A. engelmanniana is distributed further west, in mixed- and short-grass prairies. A report of A. engelmanniana from South Dakota has not been confirmed, and reports from Iowa appear to have been based on misidentified specimens of A. stenophylla. Reports from Arkansas are unconfirmed and also very likely to be based on misidentifications. Asclepias engelmanniana is considered to be of conservation concern in Wyoming, where it has been recorded only from Goshen County. Asclepias rusbyi has been inconsistently distinguished from A. engelmanniana (for example, E. Sundell 1994), although the differences elucidated by R. E. Woodson Jr. (1954) are sound. These species are readily distinguished by the characters in the key and appear to have allopatric ranges. Reports of A. engelmanniana from Arizona, western New Mexico, Nevada, Utah, southwestern Colorado, and Sonora, Mexico, all pertain to A. rusbyi.

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

Asclepias uncialis is by far the most widespread of the four diminutive, red-violet-petaled milkweeds of western North America (species 57–60). It typically has much narrower leaves than the other three species, and is extraordinarily cryptic in the absence of flowers in its characteristic short-grass prairie habitat, where its leaves closely mimic dominant grama grasses, particularly Bouteloua gracilis. Although it is widespread, it is encountered commonly only in southeastern Colorado and has only been recorded at single sites in Oklahoma (Cimarron County) and Texas (Andrews County), where the species should be considered to be of conservation concern. It is considered to be of concern in Colorado and New Mexico, but it is possible that this cryptic species is more common than has been recorded. Nonetheless, it appears that it has declined in northern Colorado. An 1873 specimen (C. C. Parry 246 [GH]) from Wyoming is the only documented record from that state and is from a highly disjunct location (attributed to Sweetwater County). It is possible that the reported location was in error; otherwise, it appears that A. uncialis has been extirpated from Wyoming, which is excluded from the range of the species in this treatment.

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