Asclepias engelmanniana
Asclepias hypoleuca
Engelmann's milkweed
mahogany milkweed, talayote
1 or 2 (rarely more), erect, sometimes branched, 40–160 cm, glabrous, not glaucous, rhizomes absent.
1 (rarely 2 or 3), erect, unbranched, 25–100 cm, puberulent with curved trichomes to pilosulous, not glaucous, rhizomes absent.
alternate, sessile, drooping, with 1 stipular colleter on each side of leaf base;
blade linear, conduplicate, 5–19 × 0.15–0.3 cm, chartaceous, base cuneate, margins entire, apex acute, venation faintly brochidodromous to obscure, surfaces sparsely puberulent with curved trichomes to glabrate, margins ciliate, laminar colleters absent.
opposite, petiolate, with 1 stipular colleter on each side of petiole;
petiole 2–6 mm, puberulent with curved trichomes to pilosulous;
blade ovate or lanceolate to oblong, elliptic, or oval, 5.5–11.5 × 1–5 cm, chartaceous, base cuneate or obtuse to truncate, margins entire, apex obtuse to rounded, mucronate, venation eucamptodromous, surfaces tomentose abaxially, pilosulous or tomentulose to glabrate adaxially, margins ciliate, 8–12 laminar colleters.
extra-axillary, sessile or pedunculate, 14–23-flowered;
peduncle occasionally branched, 0–2 cm, pilosulous or puberulent with curved trichomes, with 1 caducous bract at the base of each pedicel.
terminal and extra-axillary, pedunculate, 12–35-flowered;
peduncle 3.5–10.5 cm, puberulent with curved trichomes to tomentulose, with 1 caducous bract at the base of each pedicel.
8–11 mm, pilose.
15–21 mm, densely puberulent with curved trichomes or pilosulous to tomentulose.
erect to spreading;
calyx lobes lanceolate, 3–4 mm, apex acute, pilosulous;
corollas tan to russet abaxially, pale green to greenish cream or ochroleucous to tan adaxially, lobes reflexed with ascending tips, elliptic, 4–5 mm, apex acute, glabrous;
gynostegial column 0.5–1.5 mm;
fused anthers brown, broadly barrel-shaped, 2–2.5 mm, wings crescent-shaped and narrowly open throughout, apical appendages narrowly pandurate, conduplicate, not obscuring corpuscula;
corona segments cream to tan or yellow, sessile, chute-shaped, 2–3 mm, equaling style apex, base saccate and auriculate, apex retuse to nearly truncate, glabrous, internal appendage absent or obscure, glabrous;
style apex depressed, green to yellowish green.
erect to pendent;
calyx lobes lanceolate, 3–4 mm, apex acute, pilose;
corolla green, sometimes red-tinged abaxially, deep maroon to greenish red or green adaxially, lobes reflexed, tips usually spreading, oblong to elliptic, 8–10 mm, apex acute, pilosulous abaxially, glabrous adaxially;
gynostegial column 1–1.5 mm;
fused anthers brown, broadly cylindric, 2–2.5 mm, wings right-triangular, open at tip, apical appendages ovate, erose;
corona segments deep maroon to greenish red or yellowish green, subsessile, conduplicate, 7–9 mm, greatly exceeding style apex, apex truncate, spreading and long-tapering with a proximal tooth on each side, glabrous, internal appendage absent or a low crest, glabrous;
style apex shallowly depressed, green.
ovate, 8–9 × 5–6 mm, margin winged, faces minutely papillose and rugulose;
coma 2–2.5 cm.
ovate, 6–7 × 4–5 mm, margin winged, faces minutely rugulose;
coma 3–3.5 cm.
erect on upcurved pedicels, lance-ovoid, 6–10 × 1.2–2 cm, apex long-acuminate, smooth, pilosulous.
erect on upcurved pedicels, fusiform, 9–11.5 × 1.2–1.4 cm, apex long-attenuate, smooth, sometimes faintly striate, pilosulous to tomentulose.
Asclepias engelmanniana
Asclepias hypoleuca
Asclepias engelmanniana is usually a tall herb with drooping leaves and spherical umbels of greenish yellow flowers rising above surrounding grassland plants. In spite of its distinctive appearance, it is quite similar to its close relatives, A. rusbyi and A. stenophylla. The yellowish green coronas, squat flowers, upcurved fruiting pedicels, and drooping leaves distinguish A. engelmanniana from A. stenophylla, which has more slender flowers with creamy coronas, straight pedicels in fruit, and spreading to ascending leaves. Despite ranges with only little overlap and few if any mixed populations, these two species are often confused, especially in the absence of flowers. Compared to its close relative, A. engelmanniana is distributed further west, in mixed- and short-grass prairies. A report of A. engelmanniana from South Dakota has not been confirmed, and reports from Iowa appear to have been based on misidentified specimens of A. stenophylla. Reports from Arkansas are unconfirmed and also very likely to be based on misidentifications. Asclepias engelmanniana is considered to be of conservation concern in Wyoming, where it has been recorded only from Goshen County. Asclepias rusbyi has been inconsistently distinguished from A. engelmanniana (for example, E. Sundell 1994), although the differences elucidated by R. E. Woodson Jr. (1954) are sound. These species are readily distinguished by the characters in the key and appear to have allopatric ranges. Reports of A. engelmanniana from Arizona, western New Mexico, Nevada, Utah, southwestern Colorado, and Sonora, Mexico, all pertain to A. rusbyi.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Asclepias hypoleuca grows at higher elevations in the sky-island mountain ranges than any other milkweed. It has been documented from the Chiricahua, Huachuca, Rincon, Santa Catalina, Santa Rita, and White mountain ranges in Arizona (Cochise, Pima, and Santa Cruz counties) and the Mogollon Mountains and Black Range in New Mexico (Catron and Grant counties). Because of its limited, high-elevation distribution, and the threats of changing climate, its conservation status in the flora area merits assessment. The bicolored leaves exhibit coloration similar to co-occurring silverleaf oak (Quercus hypoleucoides).
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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