Large-seed or dwarf or Eastwood’s milkweed

Asclepias involucrata Englemann ex Torrey var. tomentosa Eastwood, Zoë 4: 120. 1893

Herbs. Stems 1–12, decumbent, unbranched or branched near base, 6–15 cm, densely puber­ulent with curved trichomes, not glaucous, rhizomatous. Leaves opposite or subopposite to alternate, petiolate, with 1 stipular colleter on either side of petiole; petiole 1–5 mm, densely puberulent with curved trichomes; blade lanceolate to linear-lanceolate or lance-ovate, 2.5–7 × 0.5–2 cm, chartaceous, base cuneate to truncate, margins crisped, apex attenuate, mucronate, venation obscure to faintly eucamptodromous, surfaces sparsely pilosulous to glabrate, midvein puberulent with curved trichomes to pilosulous, margins densely ciliate, laminar colleters absent. Inflorescences terminal, sessile, 12–40-flowered, bracts few. Pedicels 9–19 mm, densely pilose. Flowers erect to pendent; calyx lobes elliptic, 2.5–3 mm, apex acute, pilose; corolla green, tinged red (especially abaxially), lobes reflexed, oval, 4–5.5 mm, apex acute, glabrous; gynostegial column 0.2–0.8 mm; fused anthers brown, cylindric, 1–1.5 mm, wings right-triangular, slightly open at tip, apical appendages ovate; corona segments yellow to ochroleucous, subsessile, tubular, 2–3 mm, equaling or slightly exceeding style apex, apex truncate, glabrous, internal appendage falcate, exserted, sharply inflexed towards or over style apex, glabrous; style apex shallowly depressed, green or pink. Follicles erect on upcurved pedicels, ovoid, 5–6.5 × 1.2–2 cm, apex acuminate, rugose, faintly striate, densely pilosulous. Seeds ovate, 8–12 × 6–8 mm, margin thickly winged, faces densely rugulose; coma 1.5–2 cm.

Flowering Apr–Jun; fruiting May–Jul. Arroyos, hills, ridges, canyons, dunes, sandstone, limestone, sandy soils, juniper woodlands, shrubby grasslands, desert scrub; 900–1800 m; Ariz., Colo., N.Mex., Utah.

Asclepias macrosperma has not been consistently recognized as distinct from A. involucrata, as discussed under that species. It is quite homogeneous across its range on the Colorado Plateau, and intermediates with A. involucrata only occur where the ranges contact on the southern and eastern margins of the Plateau. It is readily distinguished from typical A. involucrata by broader leaves with crisped margins and smaller corona segments that are less compressed, less flared apically, and nearly uniformly yellow, as opposed to cream with a dark dorsal stripe. Also, A. macrosperma is largely confined to sandy, often unstabilized substrates, whereas A. involucrata occurs on stable, rocky, clay to sandy soils across most of its range. Asclepias macrosperma has been documented at few sites in Colorado (Montezuma County) and New Mexico (San Juan County) and should be considered to be of conservation concern in these states. It appears secure in Utah and on Navajo lands in Arizona.