Whorled or eastern whorled or horsetail milkweed, asclépiade verticillée

Herbs. Stems 1 (rarely 2 or 3), erect, sparingly branched, arrested vegetative branches absent, 35–90 cm, sparsely puberulent with curved tri­chomes in lines, not glaucous, rhizomatous. Leaves 3–6-whorled, sessile, with 1 stipular colleter on each side of leaf base on a ciliate interpetiolar ridge; blade linear, 1.5–7 × 0.1–0.2 cm, chartaceous, base cuneate, margins entire, apex acute, mucronate, venation obscure, surfaces glabrous abaxially, puberulent with curved trichomes adaxially, especially on midvein, to glabrate, margins ciliate, laminar colleters absent. Inflorescences extra-axillary at upper nodes, pedunculate, 7–28-flowered; peduncle 0.8–4 cm, puberulent with curved trichomes on 1 side, with 1 caducous bract at the base of each pedicel. Pedicels 6–10 mm, puberulent with curved trichomes. Flowers erect to spreading; calyx lobes lanceolate, 1.5–2 mm, apex acute, puberulent with curved trichomes to glabrate; corolla pale green to cream or ochroleucous, sometimes tan-tinged, lobes reflexed with spreading tips, elliptic, 3–4 mm, apex acute, glabrous abaxially, minutely papillose at base adaxially; gynostegial column 1–1.2 mm; fused anthers green, columnar, 1.2–1.5 mm, wings narrowly right-triangular, closed, apical append­ages deltoid; corona segments cream, stipitate, cupulate, dorsally flattened, 1.5–2 mm, exceeded by style apex, apex obtuse, margin shallowly lobed (sometimes obscure) to sharply toothed proximally, 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, 6–11 × 0.4–0.8 cm, apex acuminate to attenuate, smooth, glabrous. Seeds ovate, 6–7 × 4–5 mm, margin winged, faces smooth; coma 2.5–3.5 cm. 2n = 22.

Flowering Feb–Oct; fruiting Mar–Nov(–Dec). Ridges, slopes, flats, glades, bluffs, dunes, sandhills, streamsides, wet meadows and depressions, lake shores, sandstone, limestone, granite, serpentine, dolomite, shale, sandy, clay, and rocky soils, prairies, pine flatwoods and barrens, pine and oak scrubs, oak and oak-hickory woodlands, pine, pine-oak and pine-mixed-hardwood forests, forest edges; 0–1000 m; Man., Ont., Sask.; Ala., Ark., Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Pa., R.I., S.C., S.Dak., Tenn., Tex., Vt., Va., W.Va., Wis.

Asclepias verticillata is parapatric with the closely re­lated and morphologically similar A. linearis, A. pumila, and A. subverticillata. It can be difficult to distinguish from these relatives where their ranges overlap. Simi­larities with A. linearis and A. subverticillata are dis­cussed under those species; no definitive hybrids between A. verticillata and these species have been docu­mented. The characteristic marginal corona segment tooth is often reduced to a shallow lobe in western pop­ulations of A. verticillata, which complicates dis­tinguishing this species from A. subverticillata, and which suggests past introgression. A widely disjunct collection of A. verticillata was made in Arizona, well within the range of A. subverticillata, for Plants of the Hopis (Millspaugh 176 [F]); persistence of the species in Arizona has not been documented by additional collections. Hybrids with A. pumila are usually readily detected because the parental species are distinct in leaf arrangement and internode length (whorled and distant nodes in A. verticillata versus alternate and congested in A. pumila). These hybrids often have mixed phyllotaxy and have been documented in Kansas, Montana, South Dakota, and Texas. Asclepias verticillata is strongly rhizomatous and forms dense colonies on roadsides and in prairies; in forests, however, genets are small and solitary stems are common. Like several other milkweed species, it is rare and declining at the northeastern terminus of its range (in Delaware, Massachusetts, New Jersey, Pennsylvania, Rhode Island, and Vermont). It is also considered rare in Saskatchewan, where it has been documented by few specimens. Reports from Wyoming (Crook County) all seem to pertain to A. pumila (B. Heidel, pers. comm.).