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 hallii is the only milkweed likely to be found above 2500 m in the Rocky Mountain and Intermountain regions, where it is found mostly on rocky canyon slopes. It appears to be a shorter-statured derivative of A. speciosa, which it greatly resembles, and which is the only species with which it commonly co-occurs. The range of A. hallii extends into the plains along river valleys and there overlaps with A. speciosa. Hybrids between these species have been documented only rarely in Colorado, and one of the syntypes of A. curvipes from Wyoming may represent this hybrid. Compared to A. speciosa, A. hallii is shorter statured, the leaves are narrower and are regularly subopposite or alternate rather than strictly opposite, and the corona segments are much shorter and erect. Despite overall similarities, it appears that these two species may not be close relatives (M. Fishbein et al. 2018).

The somewhat cruciform distribution of Asclepias hallii is odd and may be relictual. It extends from the eastern slope of the Rocky Mountains in Colorado west to the ranges of central Nevada, and from the Wasatch Range in northern Utah south to the Sierra Ancha in central Arizona. The species is fairly common at lower elevations of the Rocky Mountains in Colorado, particularly on the eastern slope, but is quite rare across the rest of its range, and it is considered to be of conservation concern in Utah. There are few documented occurrences in Arizona (Coconino, Gila, and Navajo counties), Nevada (Elko, Eureka, Lander, and White Pine counties), and New Mexico, where it has been documented by only a single specimen from Colfax County and may be extirpated. Its conservation status in these states merits evaluation. Asclepias hallii appears not to have been collected in Wyoming since 1958 and has been presumed to be extirpated; however, there is recent photo documentation from Albany County.

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