Asclepias stenophylla
Asclepias erosa
narrow-leaf milkweed, slimleaf milkweed
desert milkweed
1 or 2 (rarely more), erect to spreading, rarely branched, 15–85 cm, puberulent with curved trichomes, not glaucous, rhizomes absent.
1–12, erect to decumbent, unbranched, 40–250 cm, thinly 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.
opposite, sessile or petiolate, with 0 or 1 stipular colleter on each side of petiole;
petiole 0–6 mm, thinly tomentose to glabrate;
blade ovate to lanceolate, 7.5–25 × 2.5–15 cm, succulent, base truncate to cordate, margins minutely erose, apex attenuate to acuminate, venation eucamptodromous, surfaces tomentose to glabrate, margins ciliate, minutely erose, 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.
terminal and extra-axillary, sometimes branched, pedunculate, 12–50-flowered;
peduncle 2–10 cm, tomentose, with 1 caducous bract at the base of each pedicel.
5–12 mm, puberulent with curved trichomes to pilosulous.
20–45 mm, densely tomentose to glabrate.
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 lanceolate, 4–5 mm, apex acute, tomentose to glabrate;
corolla green, lobes reflexed, tips sometimes spreading, oval, 6–9 mm, apex acute, tomentose towards tips abaxially, glabrous adaxially;
gynostegial column 1–1.5 mm;
fused anthers green, cylindric, 2.5–3 mm, wings right-triangular, closed, apical appendages ovate;
corona segments cream to ochroleucous, stipitate, conduplicate, dorsally rounded, 3–5.5 mm, slightly exceeding style apex, apex truncate with a proximal tooth on each side, glabrous, internal appendage falcate, exserted, sharply inflexed over style apex, glabrous;
style apex shallowly depressed, cream to greenish cream.
ovate, 6–7 × 4–5 mm, margin winged, faces minutely rugulose;
coma 2.5–3 cm.
ovate, 8–13 × 5–10 mm, margin narrowly winged, faces minutely rugulose, ridges papillose;
coma 2–2.5 cm.
erect on straight pedicels, fusiform, 9–13 × 1–1.2 cm, apex long-acuminate, smooth, pilosulous.
erect on upcurved pedicels, lance-ovoid to ovoid, 6.5–10 × 2–3.5 cm, apex acuminate to apiculate, smooth, thinly tomentose.
= 22.
Asclepias stenophylla
Asclepias erosa
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 erosa is one of the most remarkable milkweeds. It inhabits the driest regions in the flora area, yet it is one of the tallest and largest-leaved species of Asclepias. Its leaves attain a greater size than any other sympatric milkweed, and it may possess the largest leaves of any co-occurring vascular plant species in its range. It is found most commonly in desert arroyos, and it is assumed to be deep rooted and to access reliable sources of water, which would explain its anomalously large size. Like several other milkweeds inhabiting the American deserts, A. erosa has white coronas and is commonly visited by tarantula hawk wasps (Pompilidae, Pepsinae). It is considered rare in Utah, where it enters the state only in Washington County. Asclepias erosa is often confused with A. eriocarpa, another robust species with an overlapping range in southern California. Asclepias erosa has strictly opposite, sessile to shortly petiolate leaves with erose margins and corona segments that are level at the apex and only rarely pinkish, whereas A. eriocarpa has leaves that may be opposite, alternate, or whorled, with longer petioles, and entire margins, and corona segments with oblique apices and that are often pinkish.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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