Mohave milkweed

Herbs. Stems 1–10, spreading or decumbent to erect, unbranched or rarely branched near base, 15–40 cm, puberulent with curved trichomes or hir­tellous, not glaucous, rhizoma­tous. Leaves persistent or grad­ually caducous from base, oppo­site, petiolate, with 1 or 2 stip­ular colleters on each side of petiole plus 0–4 in axil; petiole 6–25 mm, puberulent with curved trichomes to hirtellous; blade ovate to lanceolate, 4.5–15 × 1.5–7.5 cm, chartaceous, base cuneate or obtuse to truncate or subcordate, margins sometimes crisped, apex obtuse to acute, venation eucamptodromous to faintly brochido­dromous, surfaces hirtellous, rarely conduplicate, 0–12 laminar colleters. Inflorescences extra-axillary, sessile or pedunculate, 5–28-flowered; peduncle 0–1 cm, hirtellous, with 1 caducous bract at the base of each pedicel. Pedicels 17–30 mm, hirtellous. Flowers erect; calyx lobes lanceolate, 3–5 mm, apex acute, hirtellous; corolla green, sometimes tinged reddish or purplish abaxially, lobes reflexed, elliptic to lanceolate, 9–13 mm, apex acute, minutely hirtellous throughout or glabrous at tips abaxially, glabrous adaxially; gynostegial column 0.3–0.5 mm; fused anthers brown, obconic, 1.7–2 mm, wings triangular, widest at middle, closed, apical appendages ovate; corona segments cream to green with cream apex, fading yellow, sessile, tubular, slightly sinuous, relatively stout, 8–11 mm, greatly exceeding style apex, apex slightly flared, truncate, minutely papillose to glabrate, internal appendage lingu­late, sharply incurved, barely exserted, greatly exceeded by segment margin and exposing cavity, minutely papillose. Follicles erect on upcurved pedicels, lance-ovoid, 6.5–10 × 1.5–3 cm, apex acuminate, smooth, sometimes faintly striate, puberulent with curved tri­chomes or hirtellous. Seeds ovate to oval, 6–8 × 4.5–6.5 mm, margin winged, faces smooth; coma 2–4 cm.

Flowering Apr–Sep(–Nov); fruiting May–Nov. Arroyos, canyons, mesas, hills, slopes, bajadas, ridges, plains, valleys, limestone, sandstone, granite, andesite, rhyolite, volcanic ash, sandy, silty, and gravel soils, desert scrub, mesquite and oak grasslands, oak and oak-juniper, and pinyon-juniper woodlands, chaparral, pine-oak forests; 300–1800(–2000) m; Ariz., Calif., Nev., N.Mex.; Mexico (Sonora).

Asclepias nyctaginifolia is a western counterpart of the more widespread A. oenotheroides and differs primarily in larger leaves and more robust corona segments. Differences are discussed under the latter species. The most widely used common name, Mohave milkweed, is somewhat misleading as the species is mainly distributed along the northern and eastern margins of the Sonoran Desert and barely enters the Mohave Desert. The species is common throughout the southwestern half of Arizona and is rare in California (San Bernardino County), Nevada (Clark County), and New Mexico (Catron, Grant, and Hidalgo counties).