Asclepias tonkawae
Asclepias tonkawae, the skirt milkweed, is a species of flowering plant in the family Apocynaceae. It is endemic to the U.S. state of Texas, where it occurs in sandy upland habitats.
Taxonomy
The species was formally described in 2025 by Daniel P. Duran and collaborators following a phylogenomic study of Asclepias tomentosa. The species was previously included within Asclepias tomentosa, but genetic and morphological evidence supported its recognition as a distinct species.
Phylogenomic analyses revealed that Texas populations formed a deeply divergent lineage, reciprocally monophyletic with respect to eastern populations of A. tomentosa. Population genetic analyses showed high levels of differentiation (FST ≈ 0.296–0.357), supporting species-level separation.
Description
Asclepias tonkawae is a perennial herb growing up to about tall from a tuberous rootstock. Stems are pubescent and may be unbranched or branch after damage. Leaves are variable in shape, ranging from narrowly lanceolate to ovate, with pubescence on both surfaces and pinnate-reticulate venation.
The inflorescences consist of terminal or upper axillary umbels bearing 12–25 flowers. The flowers have a green to pinkish or purplish corolla with strongly reflexed lobes that are appressed to the pedicel, a key feature distinguishing the species from related taxa. Fruits are elongated follicles up to long.
Distribution and habitat
The species is restricted to eastern Texas, where it has been recorded from several counties including Anderson, Franklin, Henderson, Houston, and others. It occurs in xeric upland habitats associated with sandy soils, particularly the Carrizo and Sparta formations.
Asclepias tonkawae is typically found in post oak savannas and sandhill habitats, often along sand ridges and ancient marine terraces. It can persist in disturbed areas and is capable of vegetative regeneration from root fragments.
Etymology
The specific epithet tonkawae honors the Tonkawa people, whose historic and present-day lands overlap with the species' distribution. The species has been given the common name "skirt milkweed" in reference to its distinctive corolla morphology.
Conservation
Because of its limited geographic range and habitat specificity, Asclepias tonkawae has been assessed as critically imperiled (G1/S1) using NatureServe criteria.
Threats to the species include habitat conversion to pasture, urban development, forestry practices such as pine monoculture, herbicide use, and suppression of natural fire regimes. Some populations occur within protected areas, where appropriate management may support their persistence.