Nodding milkweed

Asclepias glaucescens Kunth var. elata (Bentham) E. Fournier

Herbs. Stems 1 (rarely 2 or 3), erect, unbranched, 30–80 cm, glabrous, glaucous, rhizo­ma­tous. Leaves opposite, sessile, with 1 inconspicuous stip­ular colleter on each side of leaf base; blade oval or elliptic to oblong or lanceolate, 5.5–14 × 1.5–6 cm, subsucculent, base cordate, clasping, margins sometimes crisped, apex rounded to acute, mucronate, venation brochidodro­mous, surfaces glabrous, glaucous, margins minutely and remotely ciliate, laminar colleters absent. Inflorescences terminal and extra-axillary at upper nodes, pedunculate, 7–20-flowered; peduncle 5–20 cm, pilosulous in a line, glaucous, with 1 caducous bract at the base of each pedicel. Pedicels 20–32 mm, pilosulous, often in a line. Flowers pendent; calyx lobes elliptic to oval, 5–8 mm, apex acuminate, sparsely pilosulous to glabrate; corolla green, lobes reflexed with spreading tips, oval, 11–14 mm, apex acute, glabrous; gynostegium sessile; fused anthers green, obconic, 3–3.5 mm, wings narrowly right-triangular, thick, open just above base, apical appendages lanceolate; corona segments white, yellow to tan dorsally, subsessile, conduplicate, 4–6 mm, equal­ing style apex, apex truncate, glabrous, internal appendage a crest, included in segment, glabrous; style apex depressed, green to greenish cream. Follicles erect on upcurved pedicels, fusiform to lance-ovoid, 8–13 × 1–2 cm, apex long-attenuate, smooth, pilosulous. Seeds ovate to lanceolate, 6–7 × 3–6 mm, margin winged, some­times minutely erose, faces rugulose; coma 4–4.5 cm.

Flowering Jun–Sep; fruiting Aug–Oct. Canyons, arroyos, stream banks, slopes, igneous substrates, limestone, rocky, sandy, and clay soils, pinyon-juniper and oak woodlands, pine-oak and riparian forests, meadows; 1200–2200 m; Ariz., N.Mex., Tex.; Mexico; Central America (Guatemala).

Asclepias elata is one of the few milkweeds in the flora area that possesses a mostly Mexican distribution but does not belong to the major clade that contains most of such species (species 23–27 here). Rather it is the sole representative in the flora, and the most northerly distributed member, of a small clade of glaucous, vegetatively homogeneous species ranging as far south as Costa Rica (M. Fishbein et al. 2011). Asclepias elata has been inconsistently recognized as distinct from A. glaucescens since the time of A. Gray et al. [1878–1897, vol. 2(1)]. However, at least since J. N. Rose (1892), the distinction between these species has been understood, and was presented clearly by R.E. Woodson Jr. (1954). Although indistinguishable in the absence of flowers, A. elata differs by fully pendent (versus erect to pendent) flowers, fewer flowers per umbel, longer corolla lobes, and corona segments that spread away from the gynostegium and are exceeded by the style apex, with the appendage merely a crest included in the segment (versus segments that are strict, overtop the style apex, and bear exserted appendages in A. glaucescens). Both species have extensive distributions, but they are largely allopatric, except in Mesoamerica. The northernmost populations of A. glaucescens are in Nayarit and San Luis Potosí. Asclepias elata is one of a cohort of spec­ies reaching their northwestern limits in Arizona that indicate biogeographic affinity of that region to the east­ern slope of the Sierra Madre Occidental and Altiplano, rather than the western slope of the Sierra Madre Occidental. Other examples from the milkweed flora include A. brachystephana, A. involucrata, A. nummularia, A. oenotheroides, and A. quinquedentata. In Arizona, A. elata is known from Cochise, Graham, Pima, and Santa Cruz counties, in New Mexico from Eddy, Hidalgo, and Sierra counties, and in Texas from Brewster, Culberson, Jeff Davis, and Presidio counties.