Aquatic or white swamp or swamp or thin-leaf milkweed

Subshrubs or herbs, cespitose. Stems 1–5, erect, sparsely to moderately branched, especially towards base, 30–60 cm, puber­ulent with curved trichomes in a line to glabrate, not glaucous, rhizomatous. Leaves persistent or gradually caducous from the base, opposite, petiolate, with 1 or 2 stipular colleters on each side of petiole on a ciliate interpetiolar ridge; petiole 10–12 mm, ciliate; blade nar­rowly elliptic to oval or oblong, 5–14 × 0.3–3 cm, mem­branous or chartaceous, base cuneate, margins entire, apex acute to attenuate or acuminate, minutely mucro­nate, venation faintly brochidodromous to eucamp­todromous, surfaces sparsely puberulent with curved tri­chomes, more densely on veins, to glabrate, margins inconspicuously ciliate, laminar colleters absent. Inflorescences extra-axillary at upper nodes, sometimes appearing terminal, pedunculate, 12–30-flowered; peduncle 1.5–5 cm, puberulent with curved trichomes, sometimes only on 1 side, with 1 caducous bract at the base of each pedicel. Pedicels 7–13 mm, puberulent with curved trichomes, sometimes only on 1 side. Flowers erect to spreading; calyx lobes narrowly lance­olate to linear, 1.2–1.5 mm, apex acute, puberulent with curved trichomes; corolla white to pink-tinged, lobes reflexed with spreading tips, elliptic, 3–4 mm, apex acute to obtuse, glabrous abaxially, minutely papillose at base adaxially; gynostegial column 0.8–1.2 mm; fused anthers brown, cylindric, 1.5–2 mm, wings narrowly right-triangular, closed, apical appendages deltoid; corona segments white, sometimes faintly pink-tinged, stipitate, tubular, dorsally rounded, 1.5–2.5 mm, slightly exceeding style apex, apex obtuse to acute, glabrous, internal appendage acicular, exserted, arching over style apex, glabrous; style apex shallowly depressed, white, sometimes pink-tinged. Follicles pendulous on declined pedicels, lance-ovoid, 4–7 × 1–2.5 cm, apex long-acuminate, smooth, glabrous. Seeds broadly oval, 12–15 × 11–14 mm, margin broadly winged, faces smooth; coma absent.

Flowering (Mar–)Apr–Nov; fruiting Jun–Dec. Swamps, streamsides, ditches, bottomlands, flood plains, marshes, saturated or inundated clay, silty, and sandy soils, pine-oak, oak, and mixed hardwood for­ests, riparian woods, pine flatwoods; 0–300(–500?) m; Ala., Ark., Fla., Ga., Ill., Ind., Ky., La., Miss., Mo., S.C., Tenn., Tex.

Asclepias perennis is the most hydrophytic North American milkweed and is often found emerging from standing water in swamps and ditches. The pendulous fruits and hairless seeds are distinctive; it is the only milkweed in the United States with seeds lacking a coma, and only one of three such species in North America. It is widely distributed along the coastal plain; inland (Arkansas, Illinois, Indiana, Kentucky, Missouri, Tennessee), it is restricted to the valleys of the Mississippi and Ohio rivers and their tributaries.