Western whorled or horsetail or poison or whorled milkweed

Asclepias verticillata Linnaeus var. subverticillata A. Gray, Proc. Amer. Acad. Arts 12: 71. 1876

Herbs. Stems 1–8, erect, some­times branched, few to many arrested vegetative branches usually present, 20–90 cm, puberulent with curved tri­chomes in a line to glabrate, not glaucous, rhizomatous. Leaves 3–4-whorled, sometimes opposite on vegetative branches, sessile, with 1 stipular colleter on each side of leaf base on a ciliate interpetiolar ridge; blade linear, 3–13 × 0.1–0.4 cm, chartaceous, base cuneate, margins entire, apex acute, mucronate, venation obscure, surfaces glabrous, margins ciliate or eciliate, laminar colleters absent. Inflorescences extra-axillary, pedunculate, 9–25-flowered; peduncle 0.7–3.5 cm, puberulent with curved trichomes on 1 side, with 1 caducous bract at the base of each pedicel. Pedicels 5–12 mm, puberulent with curved trichomes. Flowers erect; calyx lobes lanceolate, 2–2.5 mm, apex acute, puberulent with curved trichomes to glabrate; corolla pale green to cream, sometimes pink- or tan-tinged, lobes reflexed with spreading tips, elliptic, 3.5–4.5 mm, apex acute, glabrous abaxially, minutely papillose at base adaxially; gynostegial column 0.8–1.2 mm; fused anthers green, columnar, 1.5–2 mm, wings narrowly right-triangular, closed, apical appendages deltoid; corona segments cream, sometimes green- or pink-tinged or striped, stipitate, cupulate, dorsally flattened, 1.5–2 mm, exceeded by style apex, margins entire, apex obtuse, glabrous, internal appendage acicular, exserted, arching over style apex, glabrous; style apex shallowly depressed, green to greenish cream. Follicles erect on straight pedicels, narrowly fusiform, 6–8.5 × 0.5–0.9 cm, apex acuminate to attenuate, smooth, minutely puberulent with curved trichomes to glabrate. Seeds ovate, 5–8 × 3.5–5 mm, margin winged, faces smooth; coma 2–2.5 cm.

Flowering (Apr–)May–Oct; fruiting (Jun–)Jul–Dec. Hills, ridges, mesas, slopes, flats, depressions, ciénegas, wet meadows, pastures, canyons, streamsides, arroyos, pond and lake margins, playas, bajadas, limestone, igneous rocks, sandstone, gypsum, clay, sandy, silty, and gravel soils, prairies, desert scrub, mesquite, juniper, and desert grasslands, pine savannas, chaparral, oak, pine-oak, pinyon-juniper, and riparian woodlands, pine and mixed-conifer forests; 800–2700 m; Ariz., Colo., Kans., Mo., N.Mex., Okla., Tex., Utah, Wyo.; Mexico (Chihuahua, Coahuila, Durango, Guanajuato, Nuevo León, San Luis Potosí, Sonora, Zacatecas).

Asclepias subverticillata and A. verticillata are amply distinct away from the zone of contact from Texas to Montana. The most reliable characters for distin­guishing these species are the absence of a marginal corona segment tooth and presence of arrested vegetative branches in A. subverticillata. The greater frequency of multistemmed plants and completely glabrous leaves are also characteristic of A. subverticillata. However, absence of vegetative branches in A. subverticillata is common, especially in young or poorly developed plants. It can be difficult to confidently identify incom­plete spec­imens or immature plants in the narrow zone of parapatry. Gene flow between the species has not been investigated, and the muddy species boundaries could be attributable to past or ongoing introgressive hybridization. The identity of some populations in New Mexico, the Texas Panhandle, and western Kansas have been debated, and further study is needed to determine whether they belong to one of the parental species or are advanced generation hybrids. Asclepias subverticillata barely enters Kansas (Grant, Hamilton, Morton, Seward, Stanton, and Stevens counties) and Oklahoma (Beaver and Cimarron counties). It also appears to be rare at the northeastern end of its range in Wyoming (Carbon County), where it is thought to be extirpated. There are few occurrences of A. subverticillata disjunct from the main range and within the range of A. verticillata. There is a single, adventive population along a railroad in St. Louis, Missouri, documented in 1962 and last observed in 1970 (V. Mühlenbach 1979); it is unknown whether this population persists. Asclepias subverticillata hybridizes with A. pumila. These hybrids are usually readily detected because the parental species are distinct in leaf arrangement and internode length (whorled and distant nodes in A. subverticillata versus alternate and congested in A. pumila). Such hybrids often have mixed phyllotaxy and have been documented in northern New Mexico. Reports of A. subverticillata from Idaho are based on misidentifications of A. fascicularis and are discussed under that species. Searches for A. subverticillata in southeastern Idaho have documented only A. fascicularis in that region (Lynn Kinter, Idaho Game and Fish, pers. comm.). Like other southwestern milkweed species with cream flowers (for example, A. nyctaginifolia, A. subulata), tarantula hawk wasps (Pompilidae, Pepsinae) are avid floral visitors to A. subverticillata, in spite of tiny flowers presenting minute quantities of nectar.