Asclepias engelmanniana
Asclepias texana
Engelmann's milkweed
Texas milkweed
1 or 2 (rarely more), erect, sometimes branched, 40–160 cm, glabrous, not glaucous, rhizomes absent.
1–numerous, erect, sparsely to moderately branched in upper half, 25–90 cm, puberulent with curved trichomes in a line, 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 or 2 stipular colleters on each side of petiole on a ciliate interpetiolar ridge;
petiole 5–20 mm, puberulent with curved trichomes;
blade ovate to lanceolate or elliptic, 2–7 × 0.5–3 cm, chartaceous, base cuneate to obtuse, margins entire, apex acute to acuminate, venation eucamptodromous, surfaces puberulent with curved trichomes on veins abaxially, sparsely puberulent with curved trichomes on veins to glabrate adaxially, margins ciliate, laminar colleters absent.
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.
extra-axillary at upper nodes, some appearing terminal, pedunculate, 14–31-flowered;
peduncle 0.9–3 cm, puberulent with curved trichomes in a line, with 1 caducous bract at the base of each pedicel.
8–11 mm, pilose.
7–14 mm, puberulent with curved trichomes in a line.
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;
calyx lobes narrowly lanceolate, 1.5–2 mm, apex acute, puberulent with curved trichomes;
corolla white, sometimes tinged green, lobes reflexed with spreading tips, elliptic, 3.5–5 mm, apex acute to obtuse, glabrous abaxially, minutely papillose at base adaxially;
gynostegial column 1–1.5 mm;
fused anthers brown, columnar, 1.5–2 mm, wings narrowly right-triangular, closed, apical appendages deltoid;
corona segments white, sometimes tinged pink, stipitate, cupulate, dorsally rounded, 2–3 mm, equaling to slightly exceeding style apex, apex obtuse, glabrous, internal appendage acicular, exserted, arching over style apex, glabrous;
style apex shallowly depressed, white.
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 smooth or sparsely papillose;
coma 2–3 cm.
erect on upcurved pedicels, lance-ovoid, 6–10 × 1.2–2 cm, apex long-acuminate, smooth, pilosulous.
erect on straight pedicels, narrowly fusiform, 9–13 × 0.5–1 cm, apex long-acuminate to attenuate, smooth, glabrous.
Asclepias engelmanniana
Asclepias texana
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.)
In the flora area, Asclepias texana has a disjunct distribution on the Edwards Plateau and in the mountains of the Big Bend region. Although commonly occurring in canyons and riparian areas, A. texana is quite drought tolerant in cultivation. The tidy, bushy habit, long flowering stems topped by bright white spherical umbels, and rapid growth from seed make this a suitable candidate for horticultural use. It is known to be hardy to at least USDA Zone 7.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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