Wheel or dwarf milkweed

Herbs. Stems 1–13, decumbent, unbranched or branched near base, 4–10 cm, puberulent with curved trichomes, not glaucous, rhizomatous. Leaves opposite and alternate, petiolate, with 1 stipular colleter on each side of petiole; petiole 1–5 mm, puber­ulent with curved trichomes to pilosulous; blade linear to lanceolate, 1.7–5 × 0.2–1 cm, chartaceous, base cuneate, margins entire, apex atten­uate, venation obscure, surfaces puberulent on midvein with curved trichomes, margins densely ciliate, laminar colleters absent. Inflorescences terminal and extra-axil­lary at upper nodes, sessile, 3–7-flowered, bracts few. Pedicels 10–18 mm, densely puberulent with curved trichomes. Flowers erect; calyx lobes elliptic, 2–2.5 mm, apex acute, puberulent with curved tri­chomes; corolla red-violet, lobes reflexed with spreading tips, oval, 3–5 mm, apex acute, glabrous; gynostegial column 0.2–0.5 mm; fused anthers brown, cylindric, 1–1.5 mm, wings narrowly right-triangular, apical appendages ovate, erose; corona segments red-violet dorsally, white to orange proximally, sessile, cupulate, 1–2 mm, exceeded by style apex, apex truncate with a proximal tooth on each side, glabrous, internal append­age lingu­late, barely exserted from cavity, glabrous; style apex shallowly depressed, pink to red-violet. Follicles erect on upcurved pedicels, ovoid, 3–5 × 0.8–1.5 cm, apex acuminate, smooth, minutely puberulent with curved trichomes. Seeds broadly ovate, 7–8 × 5–6 mm, margin winged, erose, faces rugulose, lepidote; coma 1.5–2 cm.

Flowering Mar–Jun; fruiting Apr–Jun. Plains, hills, ridges, canyons, bajadas, shale, alluvium, clay, sandy, and rocky soils, prairies, desert grasslands, juniper woodlands; 900–1800 m; Ariz., Colo., N.Mex., Okla., Tex.; Mexico (Sonora).

Asclepias uncialis is by far the most widespread of the four diminutive, red-violet-petaled milkweeds of western North America (species 57–60). It typically has much narrower leaves than the other three species, and is extraordinarily cryptic in the absence of flowers in its characteristic short-grass prairie habitat, where its leaves closely mimic dominant grama grasses, particularly Bouteloua gracilis. Although it is widespread, it is encountered commonly only in southeastern Colorado and has only been recorded at single sites in Oklahoma (Cimarron County) and Texas (Andrews County), where the species should be considered to be of conservation concern. It is considered to be of concern in Colorado and New Mexico, but it is possible that this cryptic species is more common than has been recorded. Nonetheless, it appears that it has declined in northern Colorado. An 1873 specimen (C. C. Parry 246 [GH]) from Wyoming is the only documented record from that state and is from a highly disjunct location (attributed to Sweetwater County). It is possible that the reported location was in error; otherwise, it appears that A. uncialis has been extirpated from Wyoming, which is excluded from the range of the species in this treatment.