The Rarest Milkweed in Texas Just Got An Endangered Species Listing…
The words “Prostrate Milkweed” don’t roll off the tongue so easy, but I guess it’s easier for most than “Asclepias prostrata”. Whatever you call it, it’s one of the rarest and most elusive members of the genus in North America, and it’s specialized to growing on the sandy, dry soils of the thornscrub landscape of the South Texas Borderlands.
The genus Asclepias has 140 species in it - some are common, some are rare. Asclepias welshii, from the coral pink sand dunes of Southern Utah, is another bizarre one. Then there is the clade of 4 species of “Dwarf” milkweeds from the Four Corners Region. Asclepias meadii is another extremely rare one from the midwest, which has lost a lot of ground to habitat destruction (ie conversion to agriculture, human sprawl, and cancer-like growth). But Asclepias prostrata is certainly one for the evolutionary record books. Why exactly did it evolve this creeping habit? Where is the adaptive benefit of staying so low to the ground? How about those undulating leaf margins? Do they serve some purpose in mitigating leaf exposure to the hot South Texas sun? And what about those underground tuberous roots that enable it to go dormant during bust cycles of long drought and heat? Why does it sometimes disappear for two years, apparently laying dormant, only to suddenly re-emerge with the onset of cool rains?
The stems and leaves of this plant are covered in tiny hairs. This pubescence of course mitigates leaf temps and increases boundary layer humidity, preventing evapotranspirative moisture loss from leaf stomata. Like most milkweed species, I assume the pollinator to be some kind of large bee or wasp, drawn in by the nectar in those five hoods (the teeth-looking appendages) and the sweet and fragrant smell they produce. A foot of one of these insects must slip into that trapdoor of the stigmatic slit, pulling a pollinium out with it and transporting it to another flower. Milkweed flower anatomy is a whole other world, replete with its own vernacular. I did a video on it a few years back for the curious.
Anyway, does the endangered species listing MEAN much? Not really, not in the state of Texas, where property rights trump everything and many people will see little point in keeping anything around that doesn’t directly benefit humans, at least in the short term. Yet it still draws attention to this plant, and this plant needs all the attention it can get. Recently, botanists from San Antonio Botanic Garden collected fruits and seeds of this plant before it got its endangered species listing, and had a contract grower for US Fish and Wildlife Service grow them out. Plants were then accessioned at the botanic garden. A few other native plant growers in the region of South Texas have also propagated this plant and so far, have had luck with it. Sadly, like many species in the region, the true hope in protecting this species now seems to lie in its ability to tolerate human cultivation, as ex-situ conservation seems to have a lot more potential to save the species than preserving habitat. Habitat for this plant is certainly NOT protected, and populations of the plant have recently been destroyed for both border fence construction as well as by inadvertent road-grading before the plants were able to set seed. Making more people aware of the plant, and showing them how cool it is and why its worth being proud of and trying to protect, is all that can really be done at the moment since habitat is on the chopping block as the population of the region grows and commercial retail and tract house sprawl spreads out.
