Engelmann’s milkweed

Acerates auriculata Engelmann ex Torrey in W. H. Emory, Rep. U.S. Mex. Bound. 2(1): 160. 1859, not Asclepias auriculata Kunth 1819

Herbs. Stems 1 or 2 (rarely more), erect, sometimes branched, 40–160 cm, glabrous, not glaucous, rhizomes absent. Leaves alternate, sessile, droop­ing, with 1 stipular colleter on each side of leaf base; blade linear, conduplicate, 5–19 × 0.15–0.3 cm, char­taceous, base cuneate, margins entire, apex acute, ven­ation faintly brochidodromous to obscure, surfaces sparsely puberulent with curved trichomes to glabrate, margins ciliate, laminar colleters absent. Inflorescences extra-axillary, sessile or pedunculate, 14–23-flowered; peduncle occasionally branched, 0–2 cm, pilosulous or puberulent with curved trichomes, with 1 caducous bract at the base of each pedicel. Pedicels 8–11 mm, pilose. Flowers erect to spreading; calyx lobes lanceolate, 3–4 mm, apex acute, pilosulous; corollas tan to russet abaxially, pale green to greenish cream or ochroleucous to tan adaxially, lobes reflexed with ascending tips, elliptic, 4–5 mm, apex acute, glabrous; gynostegial column 0.5–1.5 mm; fused anthers brown, broadly barrel-shaped, 2–2.5 mm, wings crescent-shaped and narrowly open throughout, apical appendages narrowly pandurate, conduplicate, not obscuring corpuscula; corona segments cream to tan or yellow, sessile, chute-shaped, 2–3 mm, equaling style apex, base saccate and auriculate, apex retuse to nearly truncate, glabrous, internal appendage absent or obscure, glabrous; style apex depressed, green to yellowish green. Follicles erect on upcurved pedicels, lance-ovoid, 6–10 × 1.2–2 cm, apex long-acuminate, smooth, pilosulous. Seeds ovate, 8–9 × 5–6 mm, margin winged, faces minutely papillose and rugulose; coma 2–2.5 cm.

Flowering (May–)Jun–Sep; fruiting Jul–Oct(–Nov). Hills, slopes, plains, valleys, arroyos, canyons, stream­sides, ditches, sandhills, dunes, shale, sandstone, lime­stone, gypsum, igneous substrates, sandy, gravelly, clay, calcareous, and rocky soils, prairies, shrubby and mesquite grasslands, pastures, pinyon-juniper, juniper, oak, and oak-juniper woodlands, riparian forests; 200–2300 m; Colo., Kans., Nebr., N.Mex., Okla., Tex., Wyo.; Mexico (Chihuahua, Coahuila).

Asclepias engelmanniana is usually a tall herb with drooping leaves and spherical umbels of greenish yellow flowers rising above surrounding grassland plants. In spite of its distinctive appearance, it is quite similar to its close relatives, A. rusbyi and A. stenophylla. The yellowish green coronas, squat flowers, upcurved fruiting pedicels, and drooping leaves distinguish A. engelmanniana from A. stenophylla, which has more slender flowers with creamy coronas, straight pedicels in fruit, and spreading to ascending leaves. Despite ranges with only little overlap and few if any mixed populations, these two species are often confused, especially in the absence of flowers. Compared to its close relative, A. engelmanniana is distributed further west, in mixed- and short-grass prairies. A report of A. engelmanniana from South Dakota has not been confirmed, and reports from Iowa appear to have been based on misidentified specimens of A. stenophylla. Reports from Arkansas are unconfirmed and also very likely to be based on misidentifications. Asclepias engelmanniana is considered to be of conservation concern in Wyoming, where it has been recorded only from Goshen County. Asclepias rusbyi has been inconsistently distinguished from A. engelmanniana (for example, E. Sundell 1994), although the differences elucidated by R. E. Woodson Jr. (1954) are sound. These species are readily distinguished by the characters in the key and appear to have allopatric ranges. Reports of A. engelmanniana from Arizona, western New Mexico, Nevada, Utah, southwestern Colorado, and Sonora, Mexico, all pertain to A. rusbyi.