Lemmon’s or big-leaf milkweed

Herbs. Stems 1–3, erect to ascending, unbranched, very stout, 100–150 cm, densely hirsute, not glaucous, rhizomes absent(?). Leaves opposite, pet­iolate, with 1 stipular colleter on each side of petiole; petiole 1–5 mm, hirsute; blade oval or oblong to ovate, 7–22 × 3–14 cm, subsucculent, base truncate to subcordate, margins entire, apex obtuse to truncate or emarginate, mucro­nate, venation brochidodromous, secondary veins nearly orthogonal, surfaces hirsute, margins ciliate, 8–16 laminar colleters. Inflorescences terminal, paired, and extra-axillary, pedunculate, 21–53-flowered; peduncle 6–13 cm, densely hirsute, with 1 caducous bract at the base of each pedicel. Pedicels 13–22 mm, densely hirsute. Flowers erect to pendent; calyx lobes lanceolate, 3.5–6 mm, apex acute, hirsute; corolla cream to greenish cream or ochroleucous, sometimes tinged pink, lobes reflexed with spreading tips, elliptic, 9–11 mm, apex acute, glabrous; gynostegial column 0.5–1 mm; fused anthers greenish brown, cylindric, 2.5–3 mm, wings right-triangular, closed, apical appendages oval; corona segments cream to ochroleucous, sometimes tinged pink, shiny, subsessile, conduplicate, 6–8 mm, equaling or exceeding style apex, apex truncate, spreading and tapering, glabrous, internal appendage laterally compressed, erect, barely exserted, glabrous; style apex shallowly depressed, green or pink. Follicles erect on upcurved pedicels, lance-ovoid, 9.5–13.5 × 2–3 cm, apex attenuate, smooth, densely hirsute. Seeds ovate, 6–7 × 4–5 mm, margin winged, faces minutely rugulose; coma 4–4.5 cm.

Flowering Jun–Sep; fruiting Aug–Oct. Canyons, slopes, streamsides, rocky and clay soils, pine-oak, pine, and riparian forests, oak woodlands, marshes; 1200–2200 m; Ariz.; Mexico (Chihuahua, Durango, Jalisco, Sonora, Zacatecas).

A highly distinctive species, Asclepias lemmonii just barely enters the United States in southern Arizona (Cochise, Pima, and Santa Cruz counties), where it inhabits canyons in pine-oak clad sky-island ranges. Asclepias elata is a common co-inhabitant of these can­yons. Asclepias lemmonii has been documented from the Baboquiviri, Chiricahua, Huachuca, and Santa Rita mountains, and it is not common in any of these. It is considered to be of conservation concern in Arizona. The large, hirsute leaves with nearly orthogonal venation and robust, hirsute stems of A. lemmonii are unmatched among American milkweeds. Plants may reach heights over 2 m in the main range of the species in the northern Sierra Madre Occidental.