Blunt-leaved or clasping or sand milkweed

Asclepias obtusifolia Michaux

Herbs. Stems 1 or 2+, erect, unbranched, 35–175 cm, glabrous, glaucous, rhizomes absent. Leaves opposite, sessile, with 1 stipular colleter on each side of leaf base; blade broadly ovate or oval to oblong, 6–14 × 3–7 cm, chartaceous, base cordate, clasping, margins often crisped, apex rounded to truncate, emarginate, or obtuse, sometime mucronate, venation eucamptodromous to brochidodromous, sur­faces glabrous, glaucous, margins minutely ciliate, 6–16 laminar colleters. Inflorescences terminal (extra-axillary at upper nodes), pedunculate, 18–53-flowered; peduncle occasionally branched, 5–40 cm, glabrous, glaucous, with 1 caducous bract at the base of each pedicel. Pedicels 20–55 mm, puberulent with curved trichomes. Flowers erect to spreading; calyx lobes narrowly lanceolate, 3–5 mm, apex attenuate, sparsely pilosulous to glabrate; corolla green, often tinged red, purple, or bronze, lobes reflexed, lanceolate, 8–11 mm, apex acute, glabrous; gynostegial column 1.5–2.5 mm; fused anthers green, obconic, 2.5–3.5 mm, wings right-triangular, open at base, apical appendages rhomboid; corona segments reddish purple to cream, stipitate, tubular, 4–6 mm, exceeding style apex, apex truncate, erose, glabrous, internal appendage falcate, exserted, sharply inflexed over style apex, glabrous; style apex shallowly depressed, green. Follicles erect on upcurved pedicels, fusiform to narrowly lance-ovoid, 9–16 × 1–2 cm, apex long-acuminate, smooth, pilosulous. Seeds ovate, 9–10 × 6–7 mm, margin winged, faces minutely rugulose; coma 2.5–3 cm.

Flowering Mar–Sep; fruiting (Apr–)May–Sep. Dunes, ridges, slopes, sand hills, ravines, sandstone, rarely limestone, sandy, rocky, or silty soils, meadows, pastures, fields, railroad embankments, sand prairies, wet prairies, river banks, open oak woods, barrens, pine-oak forests, pine flatwoods and savannas, forest edges; 0–800 m; Ala., Ark., Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Tex., Vt., Va., W.Va., Wis.

The common name sand milkweed refers to a strong association of Asclepias amplexicaulis with sand­stone substrates and sandy soils. The clasping leaves and long-peduncled terminal inflorescence of A. amplexicaulis are distinctive among all co-occurring milkweeds. Western populations of A. amplexicaulis, primarily from prai­ries, usually have paler flowers with creamy coronas, whereas those from forest openings in the eastern and southeastern United States usually have pink to maroon coronas. The species is rare on the northwestern and northeastern margins of its range, where it is considered to be of conservation concern in Minnesota, Nebraska, New Hampshire, Rhode Island, Vermont (only in Chittenden County), and West Virginia. Hybrids with A. exaltata, A. purpurascens, and A. syriaca are known, but are local and not documented often. Presumed hybrids can be recognized by possessing intermediate floral and vegetative characteristics. Asclepias × intermedia Vail probably applies to the hybrid with A. syriaca based on Vail’s protologue (A. M. Vail 1904), but the holotype (E. P. Bicknell s.n. [NY]) is damaged, making the assignment tentative. The homonym A. amplexicaulis Michaux was applied to A. humistrata in the past, resulting in some taxonomic confusion between these species and the misidentification of her­barium specimens.