3. Asclepias incarnata L. ssp. incarnata (swamp milkweed)

A. incarnata f. albiflora A. Heller

Pl. 220 g, h; Map 917

Plants with white latex and a fibrous rootstock. Stems 50–200 cm long, mostly several-branched, erect or ascending, glabrous or sparsely and minutely hairy in longitudinal lines, with 6 to numerous nodes. Leaves opposite, short-petiolate. Leaf blades 4–15 cm long, 0.5–4.5 cm wide, narrowly lanceolate to lanceolate or less commonly ovate, the base abruptly narrowed or rounded, occasionally shallowly cordate but not overlapping that of the opposite leaf, the tip gradually tapered to a sharp point, the margins flat or slightly curled under, glabrous or more commonly the undersurface sparsely and minutely hairy. Inflorescences 2–12, terminal or occasionally appearing lateral (at the tips of short branches), short- to long-stalked, with 10–40 flowers. Calyces reflexed, short-hairy on the outer surface, the lobes 1.5–2.5 mm long, lanceolate to ovate. Corollas reflexed, glabrous, pink or rarely white, the lobes 4–6 mm long, elliptic to oblanceolate. Gynostegium appearing stalked (the column visible below the bases of the hoods), pale pink, rarely white, the corona slightly shorter than to about as long as the tip of the anther/stigma head. Corona hoods 2.0–2.7 mm long, strongly ascending, attached near their bases, oblong-ovate in outline, the tips broadly rounded, the margins not toothed, the bases not pouched. Horns attached below the middle of the hoods, extended conspicuously beyond the tips of the hoods and incurved over the anther/stigma head, linear, not flattened, tapered to a sharp point at the tip. Fruits 5–9 cm long, erect or ascending from erect or less commonly deflexed stalks, lanceolate in outline, the surface smooth, usually minutely hairy. Seeds with the body 6.5–9.0 mm long, the margins relatively broadly winged, the terminal tuft of hairs white. 2n=22. June–August.

Scattered nearly throughout Missouri (eastern U.S. west to North Dakota and Arizona; Canada). Swamps, sloughs, marshes, margins of ponds and lakes, banks of streams and rivers, bottomland prairies, and occasionally bottomland forests; also ditches and railroads.

Swamp milkweed is gaining popularity as a garden ornamental; however, the young shoots and leaves are browsed by mammals such as rabbits and deer. Its roots are a food source for muskrats and other wetland mammals. Woodson (1954) separated a series of populations in other states to the east and south of Missouri characterized by less-branched stems, more conspicuous pubescence, and broader, less gradually tapered leaves as ssp. pulchra (Ehrh.) Woodson.