Desert milkweed

Herbs. Stems 1–12, erect to decumbent, unbranched, 40–250 cm, thinly tomentose to glabrate, not glaucous, rhizo­matous. Leaves opposite, sessile or petiolate, with 0 or 1 stipular colleter on each side of petiole; petiole 0–6 mm, thinly tomen­tose to glabrate; blade ovate to lanceolate, 7.5–25 × 2.5–15 cm, succulent, base trun­cate to cordate, margins minutely erose, apex attenuate to acuminate, venation eucamptodromous, surfaces tomen­­tose to glabrate, margins ciliate, minutely erose, laminar colleters absent. Inflorescences terminal and extra-axillary, sometimes branched, pedunculate, 12–50-flowered; peduncle 2–10 cm, tomentose, with 1 caducous bract at the base of each pedicel. Pedicels 20–45 mm, densely tomentose to glabrate. Flowers erect to pendent; calyx lobes lanceolate, 4–5 mm, apex acute, tomentose to glabrate; corolla green, lobes reflexed, tips sometimes spreading, oval, 6–9 mm, apex acute, tomen­tose towards tips abaxially, glabrous adaxially; gyno­stegial column 1–1.5 mm; fused anthers green, cylindric, 2.5–3 mm, wings right-triangular, closed, apical appendages ovate; corona segments cream to ochro­leucous, stipitate, conduplicate, dorsally rounded, 3–5.5 mm, slightly exceeding style apex, apex truncate with a proximal tooth on each side, glabrous, internal append­age falcate, exserted, sharply inflexed over style apex, glabrous; style apex shallowly depressed, cream to greenish cream. Follicles erect on upcurved pedicels, lance-ovoid to ovoid, 6.5–10 × 2–3.5 cm, apex acuminate to apiculate, smooth, thinly tomentose. Seeds ovate, 8–13 × 5–10 mm, margin narrowly winged, faces minutely rugulose, ridges papillose; coma 2–2.5 cm. 2n = 22.

Flowering Mar–Nov; fruiting Apr–Nov. Dunes, arroyos, canyons, ridges, slopes, bajadas, flats, granite, gypsum, gravel, alluvium, volcanic substrates, sandy, saline, and silty soils, desert scrub, riparian scrub, shrubby grasslands; -50–2000 m; Ariz., Calif., Nev., Utah; Mexico (Baja California, Sonora).

Asclepias erosa is one of the most remarkable milk­weeds. It inhabits the driest regions in the flora area, yet it is one of the tallest and largest-leaved species of Asclepias. Its leaves attain a greater size than any other sympatric milkweed, and it may possess the largest leaves of any co-occurring vascular plant species in its range. It is found most commonly in desert arroyos, and it is assumed to be deep rooted and to access reliable sources of water, which would explain its anomalously large size. Like several other milkweeds inhabiting the American deserts, A. erosa has white coronas and is commonly visited by tarantula hawk wasps (Pompilidae, Pepsinae). It is considered rare in Utah, where it enters the state only in Washington County. Asclepias erosa is often con­fused with A. eriocarpa, another robust species with an overlapping range in southern California. Asclepias erosa has strictly opposite, sessile to shortly petiolate leaves with erose margins and corona segments that are level at the apex and only rarely pinkish, whereas A. eriocarpa has leaves that may be opposite, alternate, or whorled, with longer petioles, and entire margins, and corona segments with oblique apices and that are often pinkish.