Flora of North America species comparison

Asclepias longifolia has sometimes been treated to include A. hirtella as conspecific, as discussed under that species. As circumscribed here, the distribution of A. longifolia extends along the Atlantic Coastal Plain south of Delaware to Florida and westward to the Mississppi River. Reports of A. longifolia from west of the Mississippi River in Louisiana and in Texas are assigned here to A. hirtella. Hence, the range of A. longifolia in Louisiana is extremely limited, and the conservation status of this species warrants assessment in that state (Livingston, Saint Helena, Saint Tammany, Tangipahoa, and Washington parishes). Reports of A. longifolia from Maryland and West Virginia have not been confirmed, and the species is considered extirpated from Delaware. Otherwise, it is rare at the northern extent of its range and considered to be of conservation concern in North Carolina and Virginia (Greensville and Prince George counties).

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

Asclepias subverticillata and A. verticillata are amply distinct away from the zone of contact from Texas to Montana. The most reliable characters for distinguishing these species are the absence of a marginal corona segment tooth and presence of arrested vegetative branches in A. subverticillata. The greater frequency of multistemmed plants and completely glabrous leaves are also characteristic of A. subverticillata. However, absence of vegetative branches in A. subverticillata is common, especially in young or poorly developed plants. It can be difficult to confidently identify incomplete specimens or immature plants in the narrow zone of parapatry. Gene flow between the species has not been investigated, and the muddy species boundaries could be attributable to past or ongoing introgressive hybridization. The identity of some populations in New Mexico, the Texas Panhandle, and western Kansas have been debated, and further study is needed to determine whether they belong to one of the parental species or are advanced generation hybrids. Asclepias subverticillata barely enters Kansas (Grant, Hamilton, Morton, Seward, Stanton, and Stevens counties) and Oklahoma (Beaver and Cimarron counties). It also appears to be rare at the northeastern end of its range in Wyoming (Carbon County), where it is thought to be extirpated. There are few occurrences of A. subverticillata disjunct from the main range and within the range of A. verticillata. There is a single, adventive population along a railroad in St. Louis, Missouri, documented in 1962 and last observed in 1970 (V. Mühlenbach 1979); it is unknown whether this population persists. Asclepias subverticillata hybridizes with A. pumila. These hybrids are usually readily detected because the parental species are distinct in leaf arrangement and internode length (whorled and distant nodes in A. subverticillata versus alternate and congested in A. pumila). Such hybrids often have mixed phyllotaxy and have been documented in northern New Mexico. Reports of A. subverticillata from Idaho are based on misidentifications of A. fascicularis and are discussed under that species. Searches for A. subverticillata in southeastern Idaho have documented only A. fascicularis in that region (Lynn Kinter, Idaho Game and Fish, pers. comm.). Like other southwestern milkweed species with cream flowers (for example, A. nyctaginifolia, A. subulata), tarantula hawk wasps (Pompilidae, Pepsinae) are avid floral visitors to A. subverticillata, in spite of tiny flowers presenting minute quantities of nectar.

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