Tall-green or green or prairie milkweed, asclépiade hérissée

Acerates hirtella Pennell, Bull. Torrey Bot. Club 46: 184. 1919; Asclepias longifolia Michaux subsp. hirtella (Pennell) J. Farmer & C. R. Bell

Herbs. Stems 1–3+, erect to spreading, unbranched (rarely branched), 30–125 cm, puberu­lent with curved trichomes, not glaucous, rhizomes absent. Leaves opposite or alternate, sessile or petiolate, with 1–3 stipular colleters on each side of petiole, also in axil; petiole 0–3 mm, puberulent with curved trichomes; blade linear to linear-lanceolate, 4–16 × 0.2–1.5 cm, chartaceous, base cuneate, margins entire, apex acute to attenuate, mucronate, venation faintly brochidodromous, surfaces scabridulous to puberulent with curved trichomes, especially on veins, margins ciliate, laminar colleters absent. Inflorescences extra-axillary, sessile or pedunculate, 34–112-flowered; peduncle 0–4 cm, hirtellous, with 1 caducous bract at the base of each pedicel. Pedicels 11–23 mm, hirtellous. Flowers erect to pendent; calyx lobes lanceolate, 1.5–2 mm, apex acute, hirtellous; corolla green to greenish cream, red-violet at tip, lobes reflexed, oblong, 3–5 mm, apex obtuse, glabrous; gynostegial column 0.5–1 mm; fused anthers green, cylindric, 1–1.5 mm, wings trapezoidal, closed, apical appendages oblate; corona segments cream to greenish cream, rarely pinkish lav­ender or brown, often with a dorsal red-violet stripe or at base, sessile, laminar, strongly dorsally compressed, margins incurved, appressed to column, saccate, 1.5–2 mm, not exceeding point of anther wings, greatly exceeded by style apex, apex truncate, glabrous, internal appendage absent or obscure, glabrous; style apex shal­lowly depressed, green. Follicles erect on upcurved pedi­cels, fusiform, 6–15 × 1–2 cm, apex attenuate, smooth, pilosulous to puberulent with curved trichomes. Seeds ovate, 10–11 × 7–8 mm, margin winged, faces smooth; coma 3.5–4 cm.

Flowering Apr–Oct; fruiting Jun–Oct. Plains, low hills, slopes, ditches, ravines, wet depressions, bottom­lands, limestone, shale, silty, sandy, clay, and rocky soils, prairies, glades, wet meadows, oak, oak-hickory, and pine-oak forests and edges, pastures; 70–400 m; Ont.; Ala., Ark., Ga., Ill., Ind., Iowa, Kans., Ky., La., Mich., Minn., Miss., Mo., Ohio, Okla., Tenn., Tex., W.Va., Wis.

Asclepias hirtella has been included sometimes in a broadly circumscribed A. longifolia. The species are parapatric and usually are readily distinguished by the hirtellous pedicels of A. hirtella and puberulent pedicels with curved trichomes of A. longifolia. Asclepias hirtella is also typically much taller with more numerous umbels compared to A. longifolia. However, populations along the Gulf Coastal Plain from eastern Texas to the Mississippi River and from southern Mississippi to central Georgia have proved challenging because they include plants with the growth form of A. longifolia and the pedicel vestiture of A. hirtella. Consequently, these populations have not been consistently assigned to one species or the other. However, the previously overlooked difference in corona segment length correlates perfectly with the pedicel vestiture and with geography. Thus, Gulf Coast populations west of the Mississippi River and north of the immediate coastline are here interpreted to represent short-statured A. hirtella. This hypothesis warrants investigation with population genetic data—it is quite possible that populations of A. hirtella in south­ern Arkansas, Louisiana, and Texas merit recognition as a subspecies. Asclepias hirtella is rare at the margins of its range and is considered to be of conservation concern in Georgia, Kentucky, Louisiana, Michigan, Minnesota (Mower County), Mississippi, Tennessee, West Virginia (Jackson, Mason, and Putnam counties), and Ontario (Essex County).