Asclepias stenophylla
Asclepias welshii
narrow-leaf milkweed, slimleaf milkweed
Welsh's milkweed
1 or 2 (rarely more), erect to spreading, rarely branched, 15–85 cm, puberulent with curved trichomes, not glaucous, rhizomes absent.
1–20, erect to ascending, unbranched, 35–100 cm, densely tomentose to glabrate, 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.
subopposite to opposite (alternate), sessile or petiolate, stipular colleters absent;
petiole 0–8 mm, tomentose to glabrate;
blade oblong or obovate to oval or ovate, 4.5–14.5 × 2–8 cm, subsucculent, base rounded to cordate, margins entire, apex rounded or truncate to emarginate or acute, mucronate, venation brochidodromous, secondary veins orthogonal, surfaces tomentose to glabrate, margins eciliate, 0–10 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 from upper nodes, pedunculate, 22–80-flowered;
peduncle 2.5–6 cm, densely tomentose, with 1 caducous bract at the base of each pedicel.
5–12 mm, puberulent with curved trichomes to pilosulous.
10–15 mm, densely pilose 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 pendent;
calyx lobes linear, 5–6 mm, apex acute, densely pilose to tomentose;
corolla ochroleucous or tan to red-violet, lobes reflexed with spreading tips, oval to oblong, 5.5–6.5 mm, apex acute, densely tomentose abaxially, glabrous adaxially;
gynostegial column 1–1.5 mm;
fused anthers green to tan, obconic, 1.5 mm, wings triangular, widest at middle, closed, apical appendages ovate;
corona segments cream to ochroleucous, sometimes green at base, shiny, stipitate, tubular, dorsally flattened, 2.5–3.5 mm, exceeding style apex, apex truncate, glabrous, internal appendage falcate with acicular tip, exserted, sharply inflexed over style apex, glabrous;
style apex shallowly depressed, green.
ovate, 6–7 × 4–5 mm, margin winged, faces minutely rugulose;
coma 2.5–3 cm.
oval, 18–20 × 9–10 mm, margin winged, faces minutely rugulose;
coma 4–4.5 cm.
erect on straight pedicels, fusiform, 9–13 × 1–1.2 cm, apex long-acuminate, smooth, pilosulous.
pendulous on spreading pedicels, ellipsoid, 5–7 × 1.7–3 cm, apex acuminate, softly muricate, densely to thinly tomentose.
Asclepias stenophylla
Asclepias welshii
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.)
With respect to the number of populations, Asclepias welshii is the most endangered Asclepias in the flora area and is federally listed as threatened in the United States. Most of the populations are remote and relatively secure; however, the most accessible population in a Utah park has been subject to the impacts of off-road vehicle recreation. Leaves and stems of A. welshii emerge densely tomentose, but the lower portions of the plants become sand blasted and smooth as the season progresses. The species is strongly rhizomatous, as befits its shifting substrate. Seedlings and sprouts from rhizomes often bear narrowly linear leaves that differ so strongly from the foliage of more robust stems that they are not easily attributed to this species. Similar heterophylly is found in A. arenaria, A. erosa, and other milkweeds of sandy substrates.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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