Rusby’s milkweed

Acerates rusbyi Vail, Bull. Torrey Bot. Club 25: 37. 1898; Asclepias engelmanniana Woodson var. rusbyi (Vail) Kearney

Herbs. Stems 1 or 2 (rarely more), erect, sometimes branched, 50–100 cm, glabrous, not glaucous, rhizomes absent. Leaves alternate, sessile or petio­late, drooping, with 1 stipular colleter on each side of petiole; petiole 0–0.1 cm, glabrous; blade linear, con­duplicate, 9–15 × 0.2–0.3 cm, chartaceous, base cuneate, margins entire, apex acute, venation faintly brochi­dodromous to obscure, surfaces glabrous, margins cil­iate, laminar colleters absent. Inflorescences extra-axillary, sessile or pedunculate, 7–28-flowered; peduncle occasionally branched, 0–1.5 cm, pilosulous, with 1 caducous bract at the base of each pedicel. Pedicels 7–10 mm, pilose. Flowers erect to spreading; calyx lobes narrowly lanceolate, 3–4 mm, apex acute, pilosulous to glabrate; corollas abaxially russet or tan or bronze to pale green, adaxially pale green, lobes reflexed with ascending tips, elliptic, 4–6 mm, apex acute, glabrous; gynostegial column 0–0.5 mm; fused anthers green, obconic, 3 mm, wings crescent-shaped, connivent distally, wider and open at base, apical appendages deltoid, obscuring corpuscula; corona segments yellowish green to yellow or bronze, sessile, chute-shaped, 1.5–2.5 mm, exceeded by style apex, base saccate, auriculate, apex truncate, glabrous; internal appendage absent or a low crest, glabrous; style apex shallowly depressed, green to yellowish green. Follicles erect on upcurved pedicels, lance-ovoid, 9–12.5 × 1.5–2.5 cm, apex long-acuminate, smooth, faintly striate, sparsely pilosulous to glabrate. Seeds ovate, 6–8 × 4–6 mm, margins winged, faces minutely papillose and rugulose; coma 2–2.5 cm.

Flowering Jun–Aug; fruiting Aug–Oct. Ridges, arroyos, canyons, slopes, basalt, granite, sandstone, shale, sandy, rocky, clay, and gravel soils, oak and pinyon-juniper woodlands, pine, pine-oak, and riparian forests, forest edges; 1200–2300 m; Ariz., Colo., Nev., N.Mex., Utah; Mexico (Sonora).

As noted under Asclepias engelmanniana, A. rusbyi has been only inconsistently recognized as distinct from its more widespread relative. The species has an unusual distribution. In the north, A. rusbyi is encountered on the Colorado Plateau north of the Colorado River and extends west into pinyon-juniper woodlands and desert canyons at the upper margin of the Sonoran Desert. A few populations have been documented in Colorado (Archuleta, La Plata, Mesa, Montezuma, and Montrose counties), Nevada (Lincoln County), northern New Mexico (Rio Arriba and San Juan counties), and Utah (Garfield, San Juan, and Washington counties), and should be considered to be of conservation concern in these states. South of the Colorado River, A. rusbyi is found throughout eastern Arizona, from the southern edge of the Colorado Plateau, through the Mogollon Rim, to the sky-island mountains of the border region, where it is found in pinyon-juniper woodlands and pine-oak forests. In this region, it is rare in New Mexico (Catron and Grant counties).