Dwarf milkweed

Herbs. Stems 1–15, decumbent, unbranched or branched near base, 5–18 cm, puberulent with curved trichomes to pilosulous, not glaucous, rhizomatous. Leaves opposite or subopposite to alternate, petiolate, with 1 or 2 stipular colleters on either side of petiole; petiole 1–2 mm, pilo­sulous to glabrate; blade linear to narrowly lanceolate, 1.5–12 × 0.2–0.8 cm, chartaceous, base cuneate to truncate, margins sometimes crisped, apex acute, mucro­nate, venation obscure to faintly eucamptodromous, surfaces sparsely pilosulous to glabrate, midvein puberulent with curved trichomes to pilosulous, margins densely ciliate, laminar colleters absent. Inflorescences terminal, sessile, 6–35-flowered, bracts few. Pedicels 12–19 mm, puberulent with curved trichomes to pilo­sulous. Flowers erect to spreading; calyx lobes elliptic, 2.5–3 mm, apex acute, pilosulous; corolla green, some­times tinged pink or red (especially abaxially), lobes reflexed, elliptic, 4.5–6 mm, apex acute, glabrous; gynostegial column 0.5–1 mm; fused anthers brown, cylindric, 1.5–2 mm, wings right-triangular, slightly open at tip, apical appendages ovate; corona segments cream, usually with a pink or red dorsal stripe, subsessile, conduplicate, 3.5–4.5 mm, slightly exceeding style apex, apex truncate with a spreading tip, glabrous, internal appendage falcate, exserted, sharply inflexed towards the style apex, glabrous; style apex shallowly depressed, cream or greenish cream to pink. Follicles erect on upcurved pedicels, ovoid, 4.5–5.5 × 1.5–2 cm, apex acuminate, rugose, faintly striate, minutely puberulent with curved trichomes to pilosulous. Seeds ovate, 7–8 × 5–6 mm, margin thickly winged, faces papillose and rugulose, lepidote; coma 1.5–2 cm.

Flowering Mar–Jul; fruiting May–Aug. Hills, slopes, ridges, canyons, arroyos, valleys, playas, flats, dunes, limestone, sandstone, basalt, calcareous, rocky, sandy, silty, and clay soils, alluvium, prairies, mesquite, shrubby, and desert grasslands, chaparral, oak, juniper, and pinyon-juniper woodlands, pine forests, pastures; 1000–2200 m; Ariz., Colo., Kans., N.Mex., Okla., Tex.; Mexico (Chihuahua, Durango, Nuevo León, Sonora).

Despite the common name dwarf milkweed, Asclepias involucrata is larger than the sympatric A. uncialis, to which it bears a great similarity in the absence of flowers or fruit. In such conditions, A. involucrata is highly cryptic among the short-statured bunch grasses with which it grows. It senesces typically by summer’s end, contributing to the impression that the species is not common. It has occasionally been circumscribed to include A. macrosperma (for example, E. Sundell 1994), although the distinctions made by R. E. Woodson Jr. (1954) were sound. Nonetheless, where the ranges of these largely parapatric species meet, in an arc from northwestern New Mexico to central Arizona, plants of intermediate morphology can be found. It is unknown whether these represent relics of the speciation process or examples of recent hybridization. Asclepias involucrata is rare and of conservation concern in Colorado (Baca, Bent, and Las Animas counties) and Oklahoma (Cimarron County). It is probably extirpated from Kansas (Stevens County), where it is known from a single, historical record. Reports from northern Arizona, southwestern Colorado, and Utah are based on records of A. macrosperma.