Asclepias stenophylla
Asclepias viridula
narrow-leaf milkweed, slimleaf milkweed
green milkweed, southern 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), erect, unbranched, 25–75 cm, minutely puberulent in a line with curved trichomes to glabrate, not glaucous, rhizomes absent.
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, sessile, with 1 or 2 stipular colleters on each side of leaf base;
blade linear to filiform, 4.5–9 × 0.15–0.25 cm, membranous, base cuneate, margins revolute, apex acute, mucronate, venation obscure, surfaces glabrous, margins sparsely ciliate to glabrate, 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, 4–15-flowered;
peduncle 0.8–2 cm, minutely puberulent in a line with curved trichomes, with 1 caducous bract at the base of each pedicel.
5–12 mm, puberulent with curved trichomes to pilosulous.
7–13 mm, minutely puberulent in a line 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, 1.5–2.5 mm, apex acute, glabrous;
corolla green, tinged brown, lobes reflexed with spreading tips, elliptic, 3–5 mm, apex acute to obtuse, sometimes emarginate, glabrous;
gynostegial column 0.8–1 mm;
fused anthers green and brown, cylindric, 1.5–2 mm, wings right-triangular, closed, apical appendages ovate;
corona segments cream, tinged brown or green, stipitate, conduplicate and dorsally rounded, 3–4 mm, slightly exceeding style apex, apex acute, spreading, with a proximal tooth on each side, glabrous, internal appendage falcate, exserted, arching towards style apex, glabrous;
style apex shallowly depressed, green.
ovate, 6–7 × 4–5 mm, margin winged, faces minutely rugulose;
coma 2.5–3 cm.
ovate, 8–9 × 5–6 mm, margin winged, faces minutely and sparsely rugulose;
coma 2.5–3 cm.
erect on straight pedicels, fusiform, 9–13 × 1–1.2 cm, apex long-acuminate, smooth, pilosulous.
erect on straight pedicels, fusiform, 9–12.5 × 0.6–0.9 cm, apex long-acuminate, smooth, glabrous.
Asclepias stenophylla
Asclepias viridula
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.)
Similarities among Asclepias cinerea, A. feayi, and A. viridula are discussed under those species; all three are slender, cryptic when not in flower, and appear to emerge and flower in response to precipitation and fire events. Asclepias viridula is perhaps the most cryptic of the three, by virtue of its green corollas, and it is the most limited in range. It is typically found in wetter sites than co-occurring A. cinerea. Asclepias viridula is found disjunctly in northeastern Florida and the Florida Panhandle. Its range barely crosses into Alabama, where it is known from a single site in Houston County. Reports from Georgia are probably based on misidentifications—no specimens are known, and further searches for A. viridula in Georgia are warranted. It is considered to be of conservation concern throughout its range. Although not listed as a threatened or endangered species under the ESA in the United States, the number of populations is low and merits further study of population persistence and viability.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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