Hall’s milkweed

Asclepias curvipes A. Nelson

Herbs. Stems 1–50, erect to ascending, unbranched or occa­sionally branched in inflores­cence, 30–70 cm, puberulent with curved trichomes or pilose to glabrate, not glaucous, rhi­zomatous. Leaves alternate to subopposite (sometimes con­gested into pseudo-whorls), petiolate, with 1–4 stipu­lar colleters on each side of petiole; petiole 10–20 mm, puberulent with curved trichomes to pilosulous; blade narrowly lanceolate to ovate, 5–16 × 1.5–9 cm, chartaceous, base obtuse to truncate, often oblique or unequal, margins entire, apex acute to obtuse, mucro­nate, venation brochidodromous, surfaces pilosulous or tomentulose to glabrate, more densely so on veins, margins ciliate, 10–20 comparatively large laminar colleters. Inflorescences extra-axillary, sometimes branched, pedunculate, 9–29-flowered; peduncle 0.5–10.5 cm, pilose to tomentulose, with 1 caducous bract at the base of each pedicel. Pedicels 16–28 mm, densely pilose to tomentulose. Flowers erect to spreading; calyx lobes lanceolate, 3–4 mm, apex acute, pilose; corolla pale pink to red or green with pink or red tinge, lobes reflexed with spreading tips, lance-ovate, 7–8 mm, apex acute to obtuse, glabrous abaxially, minutely papillate at base adaxially; gynostegial column 0.5–1 mm; fused anthers brown, obconic, 2–2.5 mm, wings curved and widest at base to nearly right-triangular, closed, base distended, apical appendages ovate, erose; corona segments cream with red or pink to purple dorsal stripe, sessile, conduplicate, dorsally flattened, 5–6.5 mm, greatly exceeding style apex, apex acute, glabrous, internal appendage falcate, exserted, sharply inflexed over style apex, glabrous; style apex shallowly depressed, cream to pink. Follicles erect on upcurved pedicels, lance-ovoid, 8–12 × 0.7–1.5 cm, apex acuminate, smooth, pilosulous. Seeds narrowly ovate, 6–7 × 4 mm, margin winged, faces and wings rugulose; coma 2.5–3.5 cm.

Flowering Jun–Aug; fruiting Jul–Sep. Slopes, ridges, ditches, arroyos, field margins, shale, ash, gypsum, igneous substrates, talus, gravel, clay, silt, sandy, and rocky soils, pinyon-juniper woodlands, shrubby grasslands, meadows, pine forests; 1700–3000 m; Ariz., Colo., Nev., N.Mex., Utah, Wyo.

Asclepias hallii is the only milkweed likely to be found above 2500 m in the Rocky Mountain and Intermountain regions, where it is found mostly on rocky canyon slopes. It appears to be a shorter-statured derivative of A. speciosa, which it greatly resembles, and which is the only species with which it commonly co-occurs. The range of A. hallii extends into the plains along river valleys and there overlaps with A. speciosa. Hybrids between these species have been documented only rarely in Colorado, and one of the syntypes of A. curvipes from Wyoming may represent this hybrid. Compared to A. speciosa, A. hallii is shorter statured, the leaves are narrower and are regularly subopposite or alternate rather than strictly opposite, and the corona segments are much shorter and erect. Despite overall similarities, it appears that these two species may not be close relatives (M. Fishbein et al. 2018).

The somewhat cruciform distribution of Asclepias hallii is odd and may be relictual. It extends from the eastern slope of the Rocky Mountains in Colorado west to the ranges of central Nevada, and from the Wasatch Range in northern Utah south to the Sierra Ancha in central Arizona. The species is fairly common at lower elevations of the Rocky Mountains in Colorado, particularly on the eastern slope, but is quite rare across the rest of its range, and it is considered to be of conservation concern in Utah. There are few docu­mented occurrences in Arizona (Coconino, Gila, and Navajo counties), Nevada (Elko, Eureka, Lander, and White Pine counties), and New Mexico, where it has been documented by only a single specimen from Colfax County and may be extirpated. Its conservation status in these states merits evaluation. Asclepias hallii appears not to have been collected in Wyoming since 1958 and has been presumed to be extirpated; however, there is recent photo documentation from Albany County.