Prostrate milkweed

Herbs. Stems 2–7, prostrate to decumbent, sometimes branched, 15–30 cm, pilosulous to tomentulose, not glaucous, rhizomes absent. Leaves oppo­site, petiolate, with 1 stipular colleter on either side of petiole; petiole 2–3 mm, pilosulous to tomentulose; blade linear-lanceolate to deltate, 1.8–5 × 0.4–1.8 cm, chartaceous, base truncate to rounded or subcordate, margins crisped, apex acute, mucronate, venation eucamptodromous, surfaces pilosulous to tomentulose, margins ciliate, laminar colleters absent. Inflorescences extra-axillary, pedunculate, 3–8-flowered; peduncle 0.4–2 cm, densely pilosulous to tomentulose, with 1 caducous bract at the base of each pedicel. Pedicels 8–14 mm, densely pilosulous to tomentulose. Flowers erect; calyx lobes lanceolate, 3–4 mm, apex acute, pilosulous; corolla green, lobes reflexed, elliptic, 8–11 mm, apex acute, pilosulous abaxially, minutely hirtellous at base adaxially; gynostegial column 3–3.5 mm; fused anthers brown, obconic, 2–2.5 mm, wings right-triangular, closed, apical appendages ovate, erose; corona segments cream, tinged dorsally yellow, green, or pinkish, subsessile, conduplicate-tubular, 5–7 mm, greatly exceeding style apex, apex truncate, glabrous, internal appendage falcate, exserted, sharply inflexed over style apex, papillose; style apex shallowly depressed, green to yellowish. Follicles pendent on lax pedicels, ovoid, 3.5–5.5 × 1–1.5 cm, apex acuminate, muricate-ridged, tomentulose. Seeds broadly ovate, 7–8 × 5–6 mm, margin corky-winged, erose, faces very sparsely papillose; coma 1–1.8 cm.

Flowering Mar–Oct; fruiting Jul–Oct(–Dec). Arroyos, flats, hills, caliche, sandy, gravel, silty, and calcareous, often compacted soils, thorn scrub; of conservation concern; 50–200 m; Tex.; Mexico (Tamaulipas).

Asclepias prostrata is one of the most unusual and poorly known milkweeds in the flora. It was first collected by A. Schott in 1853 during the United States-Mexico border survey, along the Rio Grande (Río Bravo) between Laredo and Ringgold barracks (near Rio Grande City). However, it was not described until much later, from a collection made in Tamaulipas. The species remains rarely collected in both the United States and Mexico, and it is considered extremely rare in Texas (Starr and Zapata counties) and of conservation concern. Many historically known populations in the lower Rio Grande valley have not been relocated in recent years and are presumed extirpated (A. Strong, Texas Parks and Wildlife Department, pers. comm.). Several populations are known to have been eliminated by the widening of highways; others are thought to have been impacted by the spread of the invasive grass, Cenchrus ciliaris Linnaeus. The prostrate habit of A. prostrata cannot be confused with any other species of Asclepias. However, A. prostrata exhibits a remarkable similarity in all vegetative traits, including habit, to two co-occurring asclepiads, Matelea brevicoronata and M. parvifolia, as well as species of Acleisanthes (Nyctaginaceae), particularly the ubiquitous Acleisanthes longiflora. These species form a rather curious assemblage for which there is no hypothesized explanation involving convergent evolution. Since the description of Asclepias prostrata, prostrate species of Matelea occasionally have been misidentified as this species, even far outside its range.