Sullivant’s or prairie or smooth milkweed, asclépiade de Sullivant

Herbs. Stems solitary, erect, unbranched, 55–90 cm, gla­brous, glaucous, rhizomatous. Leaves opposite, sessile, with 1 or 2 stipular colleters on each side of leaf base; blade lanceolate or ovate to oblong, 6.5–15 × 1.5–9 cm, succulent, base cor­date, margins entire, apex obtuse to rounded or acute, sometimes emarginate, mucro­nate, venation brochidodromous, surfaces gla­brous, glaucous, margins eciliate, 4–12 laminar colleters. Inflorescences extra-axillary at upper nodes, sometimes appearing terminal, pedunculate, 9–29-flowered; peduncle 1–6 cm, glabrous, glaucous, with 1 caducous bract at the base of each pedicel. Pedicels 22–36 mm, glabrous, glaucous. Flowers erect; calyx lobes lanceolate, 4–5 mm, apex acute, glabrous, glaucous; corolla dark pink, pale at base of lobes, lobes reflexed, sometimes with spreading tips, elliptic, 8–12 mm, apex acute, glabrous; gynostegial column 1–1.5 mm; fused anthers truncately green, obconic, 2.5–3.5 mm, wings narrowly right-triangular, open at base, apical appendages ovate; corona segments pale to dark pink, subsessile, tubular, flattened dorsally, 5–7 mm, exceeding style apex, apex broadly obtuse, oblique, glabrous, internal appendage subulate, exserted, sharply inflexed over style apex, glabrous; style apex shallowly depressed, green. Follicles erect on upcurved pedicels, lance-ovoid, 7–11 × 1.5–3 cm, apex acuminate, sparsely muricate, sparsely pilo­sulous to glabrate, glaucous. Seeds ovate, 7–9 × 5–6 mm, margin winged, faces minutely rugulose; coma 3.5–4.5 cm. 2n = 22.

Flowering (May–)Jun–Aug(–Sep); fruiting Jun–Oct. Ditches, fields, streamsides, flood plains, alluvium, clay and sandy soils, prairies, wet prairies, shrubby grass­lands, forest openings, thickets; 100–700 m; Ont.; Ill., Ind., Iowa, Kans., Mich., Minn., Mo., Nebr., N.Dak., Ohio, Okla., S.Dak., Wis.

The distribution of Asclepias sullivantii is coexten­sive with the tallgrass prairie, where it favors moist sites. The broad, smooth, glaucous, clasping leaves, often with pink venation, are similar only to A. amplexicaulis in the region, but the latter species favors dry sites and has a long, terminal peduncle. Because of the tremendous reduction in the extent and quality of tallgrass prairie, A. sullivantii is presumed to be less common than in former times and is considered to be rare and of conserv­ation concern in Michigan, Minnesota, Wisconsin, and Ontario (Chatham-Kent, Elgin, Essex, Lambton, and Middlesex counties). It may be extirpated from North Dakota (historically in Cass and Richland counties) and South Dakota (historically in Clay, Lincoln, and Union counties). In Nebraska, it is limited to the eastern one-third of the state. Nonetheless, it is encountered commonly in suitable habitat along roadsides in the core of its range, in Illinois and Kansas. Asclepias sullivantii commonly co-occurs with and is often mis­taken for A. syriaca from a distance but is easily dis­tinguished by its smaller stature, ascending leaves, and the concentration of umbels at the apex of the stem, in addition to the glabrous and glaucous herbage that can be observed with closer examination. It is known to hybridize rarely with A. syriaca, and presumed hybrids can be recognized by intermediate vegetative and floral features.