Excerpts from Jim Conrad's
from the July 6, 2014 Newsletter issued from the Frio Canyon Nature Education
Center in the valley of the Dry Frio River in northern Uvalde County, southwestern Texas, on the southern border of the Edwards Plateau, USA
Beside the little road running up the Dry Frio Valley, growing up through a pile of Ashe Juniper posts eventually to be used for building a fence, an herbaceous plant caught my eye because its leaves were similar to our abundant roadside weed the Silverleaf Nightshade, but its flowers weren't nightshade flowers and, once I looked closer, the leaves were two to a node, or "opposite," instead of the typical one leaf per node, or "alternate," of nightshades. You can see the whole thing below:
On hands and knees I could see from the flowers' very distinctive form that this was a species of milkweed I'd never seen. A flower close-up appears at the top of this page.
Flowers of most milkweed species are numerous and closely packed in ball-like inflorescences, but here each cluster included only three to seven or so loosely held flowers. Also, many milkweed species hold their inflorescences at the tips of their branches, above their leaves, but on this plant flower clusters arose lower down, from where leaves attached to the stems.
Milkweed flowers are so unusual that to get to know them you must learn special milkweed-flower concepts, such as those of the corona, horn, and gynostegium. We provide a page just on milkweed-flower anatomy at http://www.backyardnature.net/fl_milkw.htm.
If you understand typical milkweed-flower anatomy, then you'll see in our flower picture that this species' corona limbs, or "hoods," are exceptionally slender and rise far above the gynostegium. Also, most milkweed flowers are some other color than green, especially pinkish, red or white.
Our roadside milkweed was such an atypical milkweed that it was pretty easy to figure out. It's variously called the Sidecluster Milkweed, Zizotes Milkweed, Texas Milkweed, Longhorn Milkweed, Primrose Milkweed, Lindheimer's Milkweed, and other names. It's ASCLEPIAS OENOTHEROIDES, a mostly tropical plant distributed from Costa Rica north through Mexico, entering the US in southern Arizona, New Mexico, Texas and Oklahoma.
A 2008 paper by Ana Mercedes Fernández entitled "Usos de las Especies del género Asclepias" reports that in Mexico backcountry folks apply the milky latex produced by Sidecluster Milkweed to painful teeth, and despite the latex containing certain toxic alkaloids, the fruit is reported as edible. I wouldn't try eating them without cooking them and pouring off the juice, though. Elsewhere I read that traditionally Native Americans made a poultice from the plant for skin rashes.
Sidecluster Milkweed is described as growing in sandy, rocky conditions, prairies, ditches and fields, to which we can now add roadsides.