Slim-pod milkweed

Asclepias rzedowskii W. D. Stevens

Herbs. Stems 1–5, erect to ascending, sometimes decum­bent at base, unbranched or branched near base, 10–60 cm, sparsely puberulent with curved trichomes to glabrate, not glau­cous, rhizomes absent. Leaves opposite, sessile or petiolate, with 1 or 2 stipular colleters on each side of petiole; petiole 0–1 mm, puberulent with curved trichomes to glabrate; blade linear, 6–14 × 0.2–0.6 cm, membranous, base cuneate, margins entire, apex acute, mucronate, venation obscure to faintly eucamptodromous, surfaces sparsely puberulent with curved trichomes on midvein to glabrate, margins ciliate, 0–2 laminar colleters. Inflorescences extra-axillary, the uppermost appearing terminal, pedunculate, 4–10-flowered (appearing greater because umbels are in close proximity); peduncle 1.1–2.7 cm, sparsely puberulent with curved trichomes, with 1 bract at the base of each pedicel. Pedicels 14–21 mm, puberulent with curved trichomes. Flowers pendent to spreading; calyx lobes lanceolate, 2.5–3.5 mm, apex acute, strigulose to pilosulous; corolla green, sometimes tinged red, lobes reflexed, exposing corona, oval, 4–6 mm, apex acute, pilose abaxially, glabrous adaxially; gynostegial column 0.5 mm; fused anthers tan to brown, cylindric, 1.5–2 mm, wings narrowly right-triangular, slightly open at base, apical appendages ovate; corona segments red or pink to red-violet or purple at base, white at apex, shiny, sessile, conduplicate, dorsally rounded, 3–4 mm, equaling to slightly exceeding style apex, apex truncate with a proximal tooth on each side, glabrous, internal appendage a crest, barely exserted from cavity; style apex shallowly depressed, white to greenish. Follicles erect on upcurved pedicels, fusiform, 8.5–16 × 0.5–1 cm, apex long-attenuate, smooth, faintly striate, pilosu­lous to glabrate. Seeds ovate, 4–5 × 3–4 mm, margin winged, faces smooth; coma 2–2.5 cm.

Flowering Jun–Aug; fruiting Jul–Nov. Slopes, can­yons, limestone, rhyolite, rocky soils, chaparral, pinyon-juniper and oak woodlands, pine and pine-oak forests; 1300–2600 m; Ariz., N.Mex.; Mexico (Chihuahua, Distrito Federal, Durango, San Luis Potosí, México).

Although it is widely distributed, Asclepias quinquedentata is rarely encountered. The plant is cryptic, even in flower, because of the slender, few-leaved habit and nodding inflorescences. Nonetheless, it appears to truly be rare, at least in the United States. It is considered to be of conservation concern in Arizona, and its status in New Mexico requires evaluation. It has been reported from Texas, based on the presumed type locality. However, M. Fishbein et al. (2008) concluded that the type collection most likely was made in Arizona. The population in the Valle de México has been segregated as A. rzedowskii based on a subtle variant of the corona; it is here considered a synonym (Fishbein et al.).