Asclepias stenophylla
Asclepias verticillata
narrow-leaf milkweed, slimleaf milkweed
asclépiade verticillée, eastern whorled milkweed, horsetail milkweed, whorled milkweed
1 or 2 (rarely more), erect to spreading, rarely branched, 15–85 cm, puberulent with curved trichomes, not glaucous, rhizomes absent.
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.
alternate, subopposite, or opposite, sessile or petiolate, with 1 stipular colleter on each side of petiole;
petiole 0–1 mm, spreading to ascending, glabrate;
blade linear, conduplicate, 5–16 × 0.1–0.5 cm, chartaceous, base cuneate, margins entire, apex acute, venation faintly brochidodromous to obscure, surfaces sparsely puberulent with curved trichomes, especially on midvein, to glabrate, margins ciliate, laminar colleters absent.
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.
extra-axillary, sessile or pedunculate, 9–28-flowered;
peduncle 0–1.3 cm, sometimes branched at apex, puberulent with curved trichomes, with 1 caducous bract at the base of each pedicel.
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.
5–12 mm, puberulent with curved trichomes to pilosulous.
6–10 mm, puberulent with curved trichomes.
erect to pendent;
calyx lobes lanceolate, 1.5–2.5 mm, apex acute, puberulent with curved trichomes to pilosulous;
corolla pale green to greenish cream, lobes reflexed with spreading tips, elliptic, 3–5 mm, apex acute, minutely puberulent with curved trichomes at apex abaxially, glabrous adaxially;
gynostegial column 0–0.5 mm, fused anthers green, truncately obconic, 1.5–2 mm, wings narrowly crescent-shaped, wide open at base, apical appendages deltoid;
corona segments cream, often green-tinged, sessile, chute-shaped, margins incurved, appressed to anthers, 3–3.5 mm, equaling style apex, base saccate, auriculate, apex truncate, glabrous, internal appendage a short crest, the segment appearing 3-toothed, glabrous;
style apex shallowly depressed, green.
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.
ovate, 6–7 × 4–5 mm, margin winged, faces minutely rugulose;
coma 2.5–3 cm.
ovate, 6–7 × 4–5 mm, margin winged, faces smooth;
coma 2.5–3.5 cm.
erect on straight pedicels, fusiform, 9–13 × 1–1.2 cm, apex long-acuminate, smooth, pilosulous.
erect on straight pedicels, narrowly fusiform, 6–11 × 0.4–0.8 cm, apex acuminate to attenuate, smooth, glabrous.
= 22.
Asclepias stenophylla
Asclepias verticillata
Although Asclepias stenophylla is a distinctive species, it is difficult to distinguish from A. engelmanniana in the absence of flowers or fruits, where their ranges overlap in the Great Plains. The drooping leaves of A. engelmanniana can reliably distinguish that species from A. stenophylla. Asclepias stenophylla is also often mistaken for A. verticillata, but the nearly appendageless corona segments and alternate or opposite (versus whorled) leaves readily separate A. stenophylla from that species. Because of its slender habit, linear leaves, and small clusters of greenish cream flowers held close to the stem, it can be overlooked in its grassland habitats. Asclepias stenophylla is widespread and common in its core habitat of Ozark glades and dry sites in tallgrass in Missouri, and in mixed-grass prairies from South Dakota to Texas. It is quite rare at the margins of its range in Arkansas (Baxter County), Illinois (Adams, Calhoun, and Pike counties), Iowa (Guthrie, Plymouth, and Sioux counties), Louisiana (Winn Parish), Minnesota (Houston County), Montana (Carter County), and Wyoming (Crook and Weston counties). In Colorado, it exhibits an interesting disjunction between Yuma County in the east and the Front Range of the Rocky Mountains, where it is sporadic, but impacted by development and considered to be of conservation concern. A report from North Dakota has not been confirmed.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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.).
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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