Slim milkweed

Herbs. Stems 1–4, erect, often branched, arrested vegetative branches absent, 30–75 cm, puberulent with curved tri­chomes in lines, not glaucous, rhizomatous. Leaves opposite, sessile, with 1 or 2 stipular col­leters on each side of leaf base on a ciliate interpetiolar ridge; blade linear, 3–8 × 0.1–0.3 cm, chartaceous, base cune­ate, margins entire, apex acute, mucronate, venation obscure, surfaces glabrous abaxially, puberulent with curved trichomes adaxially, especially on midvein, to glabrate, margins ciliate, laminar colleters absent. Inflorescences extra-axillary at upper nodes, peduncu­late, 7–20-flowered; peduncle 1–1.5 cm, puberulent with curved trichomes on 1 side, with 1 caducous bract at the base of each pedicel. Pedicels 6–8 mm, puberulent with curved trichomes on 1 side. Flowers erect to pendent; calyx lobes linear-lanceolate, 1.5 mm, apex acute, pilo­sulous; corolla green, tan, or pink, tinged red or brown, lobes reflexed with spreading tips, elliptic, 3–4 mm, apex acute, glabrous abaxially, minutely papillose at base adaxially; gynostegial column 1–1.2 mm; fused anthers green, columnar, 1.2–1.5 mm, wings narrowly right-triangular, closed, apical appendages deltoid; corona segments cream, stipitate, cupulate, dorsally flat­tened, 1.5 mm, exceeded by style apex, apex rounded, margin with a proximal tooth, glabrous, internal appendage acicular, exserted, arching over style apex, glabrous; style apex shallowly depressed, cream. Follicles erect on straight pedicels, narrowly fusiform, 5–8.5 × 0.4–0.7 cm, apex acuminate to attenuate, smooth, sparsely pilosulous to glabrate. Seeds ovate, 6–7 × 4–5 mm, margin winged, faces smooth; coma 2–3 cm.

Flowering (Mar–)Apr–Oct; fruiting Jul–Oct. Plains, ditches, valleys, marshes, alluvium, sandy and clay soils, prairies, savannas, mesquite grasslands, pastures; 0–200 m; Tex.; Mexico (Tamaulipas).

Although the range of Asclepias linearis is quite restricted, it is very common in grassland habitats on the coastal plain of southeastern Texas. Disjunct populations occur in the Dallas-Fort Worth area. It is similar in most respects to A. verticillata with the normally whorled leaves of that species the most reliable character distinguishing it from A. linearis. Although the ranges of these species overlap in the vicinity of Houston and Dallas-Fort Worth, and mixed populations have been reported, putative hybrids have not been documented, although they would be difficult to discern.