Corn-kernel or broad-leaf milkweed

Asclepias obtusifolia Michaux var. latifolia Torrey, Ann. Lyceum Nat. Hist. New York 2: 217. 1827

Herbs. Stems 1–10, erect, unbranched, 25–100 cm, puber­ulent with curved trichomes or thinly tomentose to glabrate, sometimes glaucous, rhizoma­tous. Leaves opposite, sessile or petiolate, with 1–4 stipular colleters on each side of petiole, sometimes also in axil; petiole 0–4 mm, thinly tomentose to glabrate; blade oval or oblong to ovate or orbiculate, 5.5–14 × 3–14 cm, subsucculent to coriaceous, base cordate, sometimes clasping, margins entire, apex truncate to rounded, sometimes emarginate, mucronate, venation brochidodromous, surfaces thinly tomentose to glabrate, sometimes glaucous, margins minutely ciliate to glabrous, 24–80 laminar colleters. Inflorescences extra-axillary, sessile or pedunculate, 20–59-flowered; peduncle 0–2.5 cm, puberulent with curved trichomes to pilosulous, with 1 caducous bract at the base of each pedicel. Pedicels 15–35 mm, densely tomentose to glabrate. Flowers erect to pendent; calyx lobes lanceolate, 4–5 mm, apex acute, tomentose to glabrate; corolla green, lobes reflexed, elliptic to oval, 7–9 mm, apex acute, glabrous abaxially, papillose at base adaxially; gynostegial column 1–1.5 mm; fused anthers green, cylindric, 3–3.5 mm, wings right-triangular, closed, apical appendages ovate; corona seg­ments cream, sometimes dorsally yellow, aging yellow, stipitate, conduplicate, dorsally rounded, 3–5.5 mm, equaling to slightly exceeding style apex, apex truncate, oblique, papillose, internal appendage falcate, exserted, sharply inflexed over style apex, papillose; style apex shallowly depressed, green. Follicles erect on upcurved pedicels, ovoid, 6.5–9.5 × 2–3 cm, apex obtuse to apic­ulate, smooth, minutely pilosulous to thinly tomentu­lose. Seeds ovate, 7–8 × 5–6 mm, winged, faces minutely rugulose to smooth; coma 3–4 cm. 2n = 22.

Flowering May–Sep; fruiting Jun–Oct. Plains, hills, slopes, dunes, canyons, arroyos, terraces, springs, ditches, limestone, shale, sandstone, caliche, silty, clay, sandy, rocky, and gravel soils, prairies, shrubby and mesquite grasslands, pastures, desert scrub, pinyon-juniper, juniper, and riparian woodlands, pine forests; 400–2300 m; Ariz., Colo., Kans., Nebr., N.Mex., Okla., Tex., Utah; Mexico (Chihuahua, Coahuila).

Asclepias latifolia is a distinctive species of the western Great Plains and Colorado Plateau, rising above short grasses and appearing as squat, leafy pagodas. It is most likely to be confused with A. arenaria (which is restricted to sandy substrates) due to the overlapping leaf shapes and floral colors of these species. Asclepias latifolia favors clayey, often rocky soils, but can be found also on sandy soils, especially on the Colorado Plateau, outside the range of A. arenaria. These species can be distinguished by habit (erect in A. latifolia versus erect to decumbent in A. arenaria), vestiture (more uniformly and persistently hairy in A. arenaria), petioles (absent or nearly so in A. latifolia versus present in A. arenaria), and the flower and seed characters included in the key. Asclepias speciosa in the absence of reproductive structures is also commonly confused with A. latifolia, but the leaves of A. speciosa are distinctly petiolate, persistently hairy, and typically taper to the apex. There is an apparent gap in the distribution of A. latifolia on the eastern Colorado Plateau, in northwestern New Mexico and southwestern Colorado, but the disjunct portions of the range are not accompanied by phenotypic divergence. Asclepias latifolia is limited in Nebraska to southwestern counties (Deuel, Dundy, Franklin, and Hayes), but it is apparently not uncommon there. Like­wise, it is common in its limited range in Utah (Garfield, Grand, Kane, San Juan, and Wayne counties).