San Juan milkweed

Herbs. Stems 1–8, decumbent, unbranched or branched near base, 4–13 cm, densely puber­ulent with curved trichomes, not glaucous, rhizomatous. Leaves opposite and alternate, petiolate, with 0 or 1 stipular colleter on either side of petiole; petiole 2–5 mm, margins puber­ulent with curved trichomes; blade lanceolate to ovate, 2–5 × 0.5–2 cm, chartaceous, base obtuse to cuneate, margins entire, apex acute to attenuate, venation eucamptodromous, surfaces sparsely pilosulous to gla­brate, persistent on veins, margins densely ciliate, lam­inar colleters absent. Inflorescences terminal and extra-axillary at upper nodes, sessile, 3–8-flowered, bracts few. Pedicels 11–24 mm, densely pilosulous. Flowers erect; calyx lobes elliptic, 2–2.5 mm, apex acute, pilose; corolla red-violet, lobes reflexed with spreading tips, oval, 4–5 mm, apex acute, glabrous; gynostegial column 0.2–0.5 mm; fused anthers brown, cylindric, 1.5–2 mm, wings narrowly right-triangular, apical appendages ovate, erose; corona segments red-violet dorsally, white to orange proximally, sessile, cupulate, 1.5–2 mm, slightly exceeded by style apex, apex truncate with a proximal tooth on each side, glabrous, internal append­age falcate, barely exserted from cavity, glabrous; style apex shallowly depressed, pink to red-violet. Follicles erect on upcurved pedicels, ovoid, 3–5 × 0.7–1 cm, apex acuminate, smooth, pilosulous to tomentulose. Seeds broadly ovate, 8–9 × 6–7 mm, margin winged, erose, faces densely rugulose, lepidote; coma 1–1.5 cm.

Flowering Apr–May; fruiting May–Jun. Slopes, ridges, arroyos, sandstone, sandy, silty, and rocky soils, juniper and pinyon-juniper woodlands, shrubby grasslands, desert scrub; 1400–1800 m; Ariz., N.Mex., Utah.

As discussed under Asclepias eastwoodiana, A. sanjuanensis is part of a complex of four species sometimes recognized as a single species. The range of A. sanjuanensis is larger than initially realized, extend­ing across northwestern Arizona (Apache and Navajo counties) and barely entering southeastern Utah (San Juan County), as discussed under A. ruthiae. These two species are completely allopatric. In New Mexico, A. sanjuanensis is restricted to San Juan County.