Green antelopehorn or green or spider milkweed

Acerates paniculata (Nuttall) Decaisne; Anantherix paniculata Nuttall; A. viridis Nuttall; Asclepiodora viridis (Walter) A. Gray; Podostigma viride (Walter) Elliott

Herbs. Stems 1–25, decumbent to erect, unbranched or rarely branched, 15–70 cm, incon­spicuously puberulent with curved trichomes to glabrate, not glaucous, rhizomes absent. Leaves alternate to sub­opposite, petiolate, with 1–4 stipular colleters on each side of petiole plus 2–4 in axil; petiole 2–6 mm, puberulent with curved trichomes to glabrate; blade oblong or ovate to oval, elliptic, or lanceolate, 3–13 × 1–6 cm, chartaceous, base rounded to subcordate, margins entire, apex obtuse to rounded, often emarginate, sometimes mucronate, venation eucamptodromous to faintly brochidodromous, surfaces puberulent with curved trichomes to glabrate, more densely so on veins, margins ciliate, 6–12 laminar colleters. Inflorescences terminal, pedunculate, 4–23-flowered; peduncle usually branched, 0.5–6 cm, pilosulous to puberulent with curved trichomes, with 1 caducous bract at the base of each pedicel. Pedicels (8–)14–23 mm, puberulent with curved trichomes to pilosulous. Flowers erect to spreading; calyx lobes linear-lanceolate, 4–5 mm, apex acute, pilosulous to puberulent with curved trichomes; corolla pale green, campanulate, lobes ascending and exceeding corona segments, oval, (9–)12–15 mm, apex acute to obtuse, glabrous; gynostegium sessile; fused anthers brown and green, turbinate, 2.5–3 mm, wings shallowly trapezoidal, closed, apical appendages ovate, erose; corona segments pale to dark purple, upper margin usually white, sessile, clavate-tubular, 3–5 mm, greatly exceeded by style apex, deflexed at base, margins connivent, apex ascending to incurved, rounded, upper margin and cavity hirtellous, internal appendage a low internal crest, hirtellous; style apex shallowly depressed, green. Follicles erect on upcurved pedicels, lance-ovoid to ovoid, 6–13 × (0.5–)1.2–3 cm, apex acuminate, shal­lowly rugose-ribbed, inconspicuously muricate apically on ribs, striate, pilosulous. Seeds broadly ovate, 5.5–7.5 × 4–6 mm, margin winged, obscurely erose at chalazal end, faces rugulose, minutely hirtellous; coma 2.5–4 cm.

Flowering Jan–Nov; fruiting (Apr–)May–Nov. Slopes, flats, glades, ravines, fields, pastures, hammocks, ditches, shale, limestone, granite, sandstone, silty, sandy, rocky, clay, and calcareous soils, prairies, mesquite-juniper grasslands, oak-hickory, pine-oak, and riparian woodlands, oak forests, forest edges and openings; 0–600 m; Ala., Ark., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Miss., Mo., Nebr., Ohio, Okla., Tenn., Tex.

Asclepias viridis is similar only to its sister species, A. asperula. The broader leaves with broader apices and corona segments that are less than half the length of the corolla lobes readily distinguish A. viridis from A. asperula. These species are also largely segregated edaphically: A. viridis on deeper, valley soils and A. asperula on rocky, upland soils. Hybrids between A. viridis and A. asperula subsp. capricornu have been documented at several locations in northern Texas and southern Oklahoma. Both species flower early in that region (April–May) and may re-flower sporadically through the summer in response to disturbance from fire or mowing, with a second peak of flowering in the fall when weather conditions are favorable. Asclepias viridis displays an unusual distribution. Outside of the tall- and mixed-grass prairies of the southern Great Plains, where it is most abundant, it occupies glade hab­itats across the eastern United States, extending to chalk prairies in the southeastern states and pine rocklands in southern Florida. It is rare at the margins of its range and is considered to be of conservation concern in Indiana (Clark and Harrison counties) and West Virginia (Jackson and Wirt counties). Recently, it has been documented to occur in Iowa, close to the Missouri state line in Ringgold County.