Asclepias stenophylla
Asclepias curtissii
narrow-leaf milkweed, slimleaf milkweed
Curtiss' milkweed
1 or 2 (rarely more), erect to spreading, rarely branched, 15–85 cm, puberulent with curved trichomes, not glaucous, rhizomes absent.
1, erect, often purplish, sometimes branched in inflorescence, 15–100 cm, puberulent with curved trichomes, 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, petiolate, with 1 or 2 stipular colleters on each side of petiole;
petiole 4–7 mm, puberulent with curved trichomes;
blade oblong, elliptic, or oval to obovate, oblanceolate, ovate, or lanceolate, 1.8–5 × 0.5–2.5 cm, chartaceous, base obtuse to cuneate or rounded, margins entire, apex truncate to emarginate or acute, mucronate, venation brochidodromous to eucamptodromous, surfaces sparsely puberulent on midvein with curved trichomes, margins inconspicuously ciliate to glabrate, 2–6 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.
terminal and extra-axillary at upper nodes, sessile or pedunculate, 15–45-flowered;
peduncle 0–4 cm, puberulent on 1 side with curved trichomes, with 1 caducous bract at the base of each pedicel.
5–12 mm, puberulent with curved trichomes to pilosulous.
10–14 mm, sparsely 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;
calyx lobes lanceolate, 2–3 mm, apex acute, glabrous;
corolla green with bronze or purplish tinge, lobes reflexed with spreading tips, elliptic, 4–5 mm, apex acute, glabrous;
gynostegial column 0.5–1 mm;
fused anthers green, obconic, 1–1.5 mm, wings broadly triangular, widest at middle, closed, apical appendages ovate;
corona segments white with a green and/or purple dorsal midline, sessile, conduplicate and dorsally flattened, 5–6 mm, greatly exceeding style apex, apex attenuate, glabrous, internal appendage falcate, exserted, sharply inflexed over style apex, glabrous;
style apex shallowly depressed, green to cream.
ovate, 6–7 × 4–5 mm, margin winged, faces minutely rugulose;
coma 2.5–3 cm.
ovate, 8–9 × 5–6 mm, margin winged, erose, faces sparsely papillose;
coma 3.5–4 cm.
erect on straight pedicels, fusiform, 9–13 × 1–1.2 cm, apex long-acuminate, smooth, pilosulous.
erect on upcurved pedicels, fusiform, 8–10.5 × 0.8–1.5 cm, apex long-acuminate, smooth, puberulent with curved trichomes.
Asclepias stenophylla
Asclepias curtissii
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 curtissii is endemic to white-sand substrates at interior and coastal sites on the Florida peninsula. Although the species is not considered to face imminent threat of extirpation, the scrub habitats in which it is found have been, and continue to be, heavily impacted by development. Asclepias curtissii cannot be mistaken for any other milkweed in its range when in flower; however, it occurs in the same habitats as A. tomentosa, and these species overlap considerably in vegetative features. The leaves of A. curtissii can be distinguished from those of A. tomentosa by trichomes limited to the midvein (versus sparsely to densely puberulent or tomentulose throughout).
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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