Sand milkweed

Herbs. Stems 1–8, spreading or decumbent to erect, unbranched or rarely branched near base, 20–100 cm, tomentose, not glaucous, rhizomatous. Leaves opposite, petiolate, with 1 or 2 stipular colleters on each side of petiole plus 0–4 in axil; petiole 7–17 mm, tomentose; blade oblong or obovate to ovate or oval, 4.2–11.5 × 2.5–7.5 cm, subcoriaceous, base rounded or truncate to subcor­date, margins often undulate or crisped, apex truncate to rounded (rarely acute), sometimes emarginate, often mucronate, venation brochidodromous, surfaces tomen­tose to nearly glabrate, margins ciliate, 12–24 laminar colleters. Inflorescences extra-axillary (sometimes appear­ing terminal), sessile or pedunculate, 14–51-flowered; peduncle 0–2 cm, tomentose, with 1 caducous bract at the base of each pedicel. Pedicels 15–25 mm, densely tomentose. Flowers erect to pendent; calyx lobes lanceolate, 2.5–3 mm, apex acute, densely tomentose; corolla green to yellowish green, sometimes tinged reddish or purplish, lobes reflexed with spreading tips, oval, 7–8 mm, apex acute, glabrous abaxially, minutely papillose at base adaxially; gynostegial column 1–2 mm; fused anthers green, obconic, 2–2.5 mm, wings right-triangular, closed, apical appendages ovate; corona segments cream to greenish cream or ochroleucous, subsessile, conduplicate, flaring at base, 3.5–4 mm, exceeding style apex, apex truncate to rounded, emar­ginate, minutely papillate, proximal margin toothed, internal appendage falcate, exserted, sharply incurved over style apex, apex upturned, minutely papillose. Follicles erect on upcurved pedicels, lance-ovoid, 5.5–10 × 2–2.8 cm, apex acuminate, smooth, pilosulous. Seeds oval, 9–12 × 6–8 mm, margin winged, faces minutely rugulose; coma 2–3 cm.

Flowering May–Aug(–Oct); fruiting Jul–Sep(–Oct). Sandhills, dunes, sandy soils, prairies, pastures, grasslands, oak scrub, riparian areas; 100–1900 m; Colo., Kans., Nebr., N.Mex., Okla., S.Dak., Tex., Wyo.; Mexico (Chihuahua).

Asclepias arenaria is the milkweed most consistently associated with pure sand soils in the western Great Plains. It is predictably found at the bases of stabilized and semi-stabilized dunes. Flowers of this species are visited by a variety of Hymenoptera, notably several species of large wasps, including tarantula hawk wasps (Pepsinae, Pompilidae) and scoliid wasps (Scoliidae), as well as by Lepidoptera. Non-flowering shoots of A. arenaria may produce linear leaves; they are easily overlooked and not identified as belonging to this species unless one is aware of this variation, especially when they are produced on rhizomes distant from shoots with typical foliage. This trait is found in several other broad-leaved milkweeds (for example, A. erosa, A. welshii). Asclepias arenaria is rare and considered to be of conservation concern in Wyoming, where it has been recorded from only two sites in Goshen County.