12. Asclepias syriaca L. (common milkweed)

A. syriaca var. kansana (Vail) E.J. Palmer & Steyerm.

Pl. 220 j, k; Map 926

Plants with white latex and deep-set rhizomes. Stems 50–200 cm long, unbranched or less commonly with a single branch at the tip, erect or ascending, sparsely to densely short-hairy, especially toward the tip, with 7–15 nodes. Leaves opposite (rarely the uppermost nodes with only 1 leaf), short-petiolate. Leaf blades 6–30 cm long, 3–11 cm wide, oblong to narrowly or broadly elliptic-ovate, less commonly elliptic-lanceolate, the base narrowed to broadly rounded, less commonly truncate, the tip narrowed to a blunt or sharp point or rounded but usually with a short, sharp point, the margins flat, the upper surface sparsely to moderately short-hairy, especially along the midvein, the undersurface densely short-hairy (felty). Inflorescences (1–)2–10, terminal and in the upper leaf axils, usually long-stalked, with 20 to more than 100 flowers. Calyces reflexed, moderately to densely short-hairy on the outer surface, the lobes 2.5–4.0 mm long, lanceolate to elliptic-lanceolate. Corollas reflexed, moderately to densely short-hairy on the outer surface and usually also on the inner surface near the base, green to lavender, usually tinged with pink and/or white, the lobes 7–10 mm long, elliptic-lanceolate. Gynostegium appearing very short-stalked (the column barely visible below the bases of the hoods), pale pink to pale purple, the corona conspicuously longer than the tip of the anther/stigma head. Corona hoods 3.5–5.0 mm long, spreading to ascending, attached near their bases, gradually narrowed from at or above the middle, the apical portion oblong to ovate in outline, the tip rounded, the margins with a pair of sharply triangular, ascending and incurved lobes at about the middle, the bases not pouched. Horns attached toward the hood bases, relatively short, bent or curved abruptly inward over the anther/stigma head and not extended past the tips of the hoods, sickle-shaped, relatively stout, slightly flattened, tapered to a sharp point at the tip. Fruits 7–12 cm long, erect or ascending from deflexed stalks, narrowly ovate to ovate and usually noticeably arched in outline, the surface with soft, narrow, warty tubercles, densely hairy (woolly). Seeds with the body 6–8 mm long, the margins narrowly winged, the terminal tuft of hairs white to light cream-colored or tan. 2n=22. May–August.

Common throughout Missouri (much commoner than the distribution map records) (eastern U.S. and adjacent Canada west to North Dakota and Texas; introduced farther west). Bottomland and upland prairies, openings and edges of mesic upland forests, and banks of streams and rivers; also pastures, old fields, margins of crop fields, ditches, roadsides, railroads, and open, disturbed areas.

This is the most abundant species of milkweed in the state and the one common in the largest variety of disturbed habitats. Forms with white flowers (f. leucantha Dore) or fruits lacking tubercles (f. inermis Churchill) are encountered locally in other parts of the range but have yet to be reported from Missouri. Rare putative hybrids between A. syriaca and A. amplexicaulis, A. speciosa, A. sullivantii, and A. verticillata, among other species, have been reported in the literature (Woodson, 1954; Kephart and Heiser, 1980; Adams et al., 1987; Klips and Vulley, 2004), but so far only the hybrid involving A. purpurascens has been documented from Missouri, based on a specimen from a mixed population at disturbed site in Iron County.