16. Asclepias viridiflora Raf. (green milkweed)

Acerates viridiflora (Raf.) Pursh ex Eaton

Asclepias viridiflora var. lanceolata (E. Ives) Torr.

Asclepias viridiflora var. linearis (A. Gray) Fernald

Pl. 222 e, f; Map 930

Plants with white latex and a thickened, somewhat woody rootstock. Stems 20–90 cm long, sometimes few-branched toward the tip, erect or ascending to less commonly spreading, sparsely to moderately short-hairy, sometimes in longitudinal lines or stripes, with 8 to numerous nodes. Leaves mostly opposite or subopposite, less commonly mostly alternate, sessile or very short-petiolate. Leaf blades very variable, 2–12 cm long, 0.3–6.0 cm wide, linear to lanceolate, ovate, oval, or nearly circular in outline, the base rounded, narrowed, or tapered, the tip rounded or narrowed or tapered to a blunt or sharp point, the margins flat, minutely hairy along the margins, glabrous or sparsely to moderately pubescent with minute hairs on the undersurface, especially along the midvein. Inflorescences 1–6 in the leaf axils, short-stalked or appearing sessile, with 20–80 flowers. Calyces reflexed, minutely hairy on the outer surface, especially along the margins, the lobes 2.0–3.5 mm long, lanceolate. Corollas reflexed, glabrous or sparsely hairy on the outer surface toward the tip, pale green, the lobes 5–7 mm long, elliptic-lanceolate. Gynostegium appearing sessile (the corona base touching the corolla or nearly so), pale green, the corona shorter than to nearly as long as the tip of the anther/stigma head. Corona hoods 4–5 mm long, erect, attached toward their bases, narrowly oblong-elliptic in outline, the tips rounded to bluntly pointed, the margins with a pair of short, triangular teeth or lobes below the middle, the bases pouched. Horns absent. Fruits 7–15 cm long, erect or ascending from usually deflexed stalks, narrowly elliptic-lanceolate in outline, the surface smooth, glabrous or more commonly minutely hairy. Seeds with the body 5.5–7.0 mm long, the margins narrowly winged, the terminal tuft of hairs light cream-colored or tan. 2n=22. May–August.

Scattered, mostly south of the Missouri River and in counties adjacent to the Big Rivers Division (eastern U.S. and adjacent Canada and Mexico west to Montana and Arizona). Upland prairies, sand prairies, savannas, glades, and exposed ledges and tops of bluffs, often on calcareous substrates; also roadsides.

Asclepias viridiflora exhibits a near continuum of variation in leaf shape, and although the extremes of long, narrow leaves vs. short, broad leaves appear very dissimilar, separation of the species into discretely defined varieties is not possible (Woodson, 1954).