Slimleaf or narrow-leaved milkweed

Polyotus angustifolius Nuttall, Trans. Amer. Philos. Soc., n. s. 5: 201. 1836, not Asclepias angustifolia Schweigger 1812; Acerates angustifolia (Nuttall) Decaisne

Herbs. Stems 1 or 2 (rarely more), erect to spreading, rarely branched, 15–85 cm, puberulent with curved trichomes, not glaucous, rhi­zomes absent. Leaves alternate, subopposite, or oppo­site, sessile or petiolate, with 1 stipular colleter on each side of petiole; petiole 0–1 mm, spreading to ascending, glabrate; blade linear, conduplicate, 5–16 × 0.1–0.5 cm, chartaceous, base cuneate, margins entire, apex acute, venation faintly brochidodromous to obscure, surfaces sparsely puberulent with curved trichomes, especially on midvein, to glabrate, margins ciliate, laminar col­leters absent. Inflorescences extra-axillary, sessile or pedun­culate, 9–28-flowered; peduncle 0–1.3 cm, some­times branched at apex, puberulent with curved tri­chomes, with 1 caducous bract at the base of each pedicel. Pedi­cels 5–12 mm, puberulent with curved tric­homes to pilosulous. Flowers erect to pendent; calyx lobes lance­olate, 1.5–2.5 mm, apex acute, puberulent with curved trichomes to pilosulous; corolla pale green to greenish cream, lobes reflexed with spreading tips, elliptic, 3–5 mm, apex acute, minutely puberulent with curved trichomes at apex abaxially, glabrous adaxially; gynostegial column 0–0.5 mm, fused anthers green, truncately obconic, 1.5–2 mm, wings narrowly crescent-shaped, wide open at base, apical appendages deltoid; corona segments cream, often green-tinged, sessile, chute-shaped, margins incurved, appressed to anthers, 3–3.5 mm, equaling style apex, base saccate, auriculate, apex truncate, glabrous, internal appendage a short crest, the segment appearing 3-toothed, glabrous; style apex shallowly depressed, green. Follicles erect on straight pedicels, fusiform, 9–13 × 1–1.2 cm, apex long-acuminate, smooth, pilosulous. Seeds ovate, 6–7 × 4–5 mm, margin winged, faces minutely rugulose; coma 2.5–3 cm.

Flowering May–Aug; fruiting (Jun–)Aug–Oct. Hills, ridges, bluffs, slopes, flats, glades, sandhills, stream­sides, limestone, dolomite, rhyolite, sandy and clay soils, prairies, pastures, thickets, forest openings, pine savannas; 70–1900 m; Ark., Colo., Ill., Iowa, Kans., La., Minn., Mo., Mont., Nebr., Okla., S.Dak., Tex., Wyo.

Although Asclepias stenophylla is a distinctive species, it is difficult to distinguish from A. engelmanniana in the absence of flowers or fruits, where their ranges overlap in the Great Plains. The drooping leaves of A. engelmanniana can reliably distinguish that species from A. stenophylla. Asclepias stenophylla is also often mistaken for A. verticillata, but the nearly appendageless corona segments and alternate or opposite (versus whorled) leaves readily separate A. stenophylla from that species. Because of its slender habit, linear leaves, and small clusters of greenish cream flowers held close to the stem, it can be overlooked in its grassland habitats. Asclepias stenophylla is widespread and common in its core habitat of Ozark glades and dry sites in tallgrass in Missouri, and in mixed-grass prairies from South Dakota to Texas. It is quite rare at the margins of its range in Arkansas (Baxter County), Illinois (Adams, Calhoun, and Pike counties), Iowa (Guthrie, Plymouth, and Sioux counties), Louisiana (Winn Parish), Minnesota (Houston County), Montana (Carter County), and Wyoming (Crook and Weston counties). In Colorado, it exhibits an interesting disjunction between Yuma County in the east and the Front Range of the Rocky Mountains, where it is sporadic, but impacted by development and considered to be of conservation concern. A report from North Dakota has not been confirmed.