Sperry’s milkweed

Subshrubs, densely cespitose. Stems few–numerous, erect to ascending, branched at base, 5–20 cm, minutely puberulent with curved trichomes to gla­brate, not glaucous, rhizoma­tous. Leaves opposite, sessile, with 1 stipular colleter on each side of leaf base; blade filiform, 2–5 × 0.05–0.1 cm, membranous, base cuneate, margins revolute, apex acute, mucronate, venation obscure, sur­faces glabrous, margins minutely ciliate at base, laminar colleters absent. Inflorescences extra-axillary, sessile, 1–2-flowered, with 1 caducous bract at the base of each pedicel. Pedicels 4–10 mm, minutely puberulent with curved trichomes in a line. Flowers erect; calyx lobes ovate, 2–3 mm, apex acute, glabrous; corolla pale green­ish yellow tinged with purple, lobes spreading, ovate, 4–5 mm, apex acute, glabrous; gynostegium sessile; fused anthers brown, cylindric, 1.5–2 mm, wings del­toid, apical appendages broadly ovate; corona segments pale greenish yellow to white (sometimes tinted purple), sessile, chute-shaped, sharply inflexed, 2–3 mm, equaling to exceeded by style apex, apex truncate, subcaudate, hirtellous, internal appendage an included crest, hirtel­lous; style apex shallowly depressed, green. Follicles erect on upcurved pedicels, fusiform, 5–7 × 0.5–0.7 cm, apex attenuate, smooth, glabrous. Seeds ovate, 5–6 × 3–4 mm, margin thickly winged, faces rugulose; coma 2–2.5 cm.

Flowering Apr–Sep; fruiting Aug–Sep. Slopes, ridges, canyons, limestone, rocky soils, desert scrub and desert grasslands; 1300–1800 m; Tex.; Mexico (Coahuila).

Asclepias sperryi is one of the least known milkweeds. It is exceedingly cryptic and easily mistaken for a small clump of grass, perhaps of Bouteloua; it is likely to be far more common than documented. However, very few observations or collections have been made in recent decades, and conservation assessment is warranted. Its range is restricted to limestone mountains of the Chihuahuan Desert in the Big Bend region of Texas, in Brewster and Pecos counties, and adjacent areas of Coahuila, entirely within the range of its sister species, A. macrotis. It is the only species of Asclepias that regularly produces a single flower per umbel.