Asclepias stenophylla
Asclepias ovalifolia
narrow-leaf milkweed, slimleaf milkweed
asclépiade à feuilles ovées, oval-leaf milkweed
1 or 2 (rarely more), erect to spreading, rarely branched, 15–85 cm, puberulent with curved trichomes, not glaucous, rhizomes absent.
1, erect, unbranched, 30–70 cm, densely puberulent with curved trichomes or pilosulous to tomentose, 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.
opposite, petiolate, with 1 or 2 stipular colleters on each side of petiole;
petiole 3–10 mm, densely pilosulous to tomentose;
blade broadly ovate to oval or narrowly elliptic, 3–9 × 1.5–4.5 cm, chartaceous, base obtuse or rounded to truncate, margins entire, apex acute to obtuse, mucronate, venation brochidodromous, surfaces pilosulous abaxially, sparsely so adaxially except on veins, margins ciliate, 4–16 laminar colleters.
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, sessile or pedunculate, 5–21-flowered;
peduncle 0–7 cm, densely pilosulous to tomentose, with 1 caducous bract at the base of each pedicel.
5–12 mm, puberulent with curved trichomes to pilosulous.
12–22 mm, densely pilosulous to tomentose.
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, 2–3 mm, apex acute, densely pilosulous;
corolla cream to yellowish, lobes reflexed with spreading tips, elliptic, 5–7 mm, apex acute, pilosulous abaxially at apex, glabrous adaxially;
gynostegial column 0.2–0.5 mm;
fused anthers green, cylindric, 1.5–2 mm, wings right-triangular, closed, apical appendages ovate;
corona segments cream to yellowish, subsessile, conduplicate, dorsally flattened, 3–4 mm, exceeding style apex, apex acute with proximal tooth on each side, glabrous, internal appendage falcate, exserted, sharply inflexed to ascending over style apex, glabrous;
style apex shallowly depressed, cream to pinkish.
ovate, 6–7 × 4–5 mm, margin winged, faces minutely rugulose;
coma 2.5–3 cm.
ovate, 5–6 × 3.5–4.5 mm, margin winged, faces rugulose;
coma 2.5–3 cm.
erect on straight pedicels, fusiform, 9–13 × 1–1.2 cm, apex long-acuminate, smooth, pilosulous.
erect on upcurved pedicels, lance-ovoid, 5–8 × 1.2–2 cm, apex acute to apiculate, smooth, densely pilosulous to tomentose.
Asclepias stenophylla
Asclepias ovalifolia
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 ovalifolia is the northernmost-ranging species in the genus, and over much of its range co-occurs with at most one other species of Asclepias. The quality of its habitat has been degraded by woody encroachment, presumably resulting from fire suppression. It appears to be secure in the core of its range in Minnesota, North Dakota, Manitoba, and Saskatchewan. Elsewhere there are conservation concerns, as in Illinois (Cook, Kankakee, Kendall, Lake, and McHenry counties), Michigan (Lake and Menominee counties), Montana (Carter and Sheridan counties), Wyoming (Crook County), and Ontario. Asclepias ovalifolia was collected in 1915 in British Columbia in a valley of the Columbia Mountains (Bain. s.n. [UBC]), far disjunct from the species’ range east of the Rocky Mountains. The occurrence has been considered to be adventive and not persistent (F. Lomer, pers. comm.). It has been reported from Nebraska based on the original determination of what became the type specimen of A. hallii; it is not known to have ever occurred in that state. Hybrids with A. syriaca are known, but appear to be rare, and can be recognized by possession of intermediate floral and vegetative traits.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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