Asclepias tuberosa
Asclepias welshii
asclépiade tubéreuse, butterfly milkweed
Welsh's milkweed
1–numerous, erect to ascending, branched in inflorescence, 15–90 cm, densely hirsute, not glaucous, rhizomes absent.
1–20, erect to ascending, unbranched, 35–100 cm, densely tomentose to glabrate, not glaucous, rhizomatous.
alternate, petiolate, with 1 or 2 stipular colleters on each side of petiole;
petiole 1–4 mm, densely hirsute;
blade elliptic, oblong, or oblanceolate to lanceolate or linear, 2–12 × 0.3–3 cm, chartaceous, base cuneate or obtuse to rounded, truncate, hastate, or cordate, margins entire, apex acute to attenuate or obtuse to rounded, venation brochidodromous to eucamptodromous, surfaces hirsute, more densely so on midvein abaxially, margins ciliate, 0–4 laminar colleters.
subopposite to opposite (alternate), sessile or petiolate, stipular colleters absent;
petiole 0–8 mm, tomentose to glabrate;
blade oblong or obovate to oval or ovate, 4.5–14.5 × 2–8 cm, subsucculent, base rounded to cordate, margins entire, apex rounded or truncate to emarginate or acute, mucronate, venation brochidodromous, secondary veins orthogonal, surfaces tomentose to glabrate, margins eciliate, 0–10 laminar colleters.
corymbs of extra-axillary umbels on branches, sessile or pedunculate, 5–27-flowered;
peduncle 0–4 cm, sometimes branched, hirsute, with 1 caducous bract at the base of each pedicel.
extra-axillary from upper nodes, pedunculate, 22–80-flowered;
peduncle 2.5–6 cm, densely tomentose, with 1 caducous bract at the base of each pedicel.
9–24 mm, puberulent with curved trichomes to pilosulous.
10–15 mm, densely pilose to tomentose.
erect;
calyx lobes narrowly lanceolate, 2–3 mm, apex acute, hirsute to puberulent with curved trichomes;
corolla reddish orange (nearly red) to orange or yellow, lobes reflexed with spreading tips, narrowly elliptic, (5–)6–8 mm, apex acute, glabrous abaxially, minutely papillose at base adaxially;
gynostegial column 1.2–1.5 mm, fused anthers yellow to yellowish green, cylindric, 2–3 mm, wings right-triangular, closed, apical appendages ovate;
corona segments reddish orange (nearly red) to orange or yellow, substipitate, conduplicate, dorsally flattened, sulcate, 5.5–7 mm, greatly exceeding style apex, apex acute, glabrous, internal appendage subulate, exserted, arching above style apex, glabrous;
style apex shallowly depressed, yellow to yellowish green.
erect to pendent;
calyx lobes linear, 5–6 mm, apex acute, densely pilose to tomentose;
corolla ochroleucous or tan to red-violet, lobes reflexed with spreading tips, oval to oblong, 5.5–6.5 mm, apex acute, densely tomentose abaxially, glabrous adaxially;
gynostegial column 1–1.5 mm;
fused anthers green to tan, obconic, 1.5 mm, wings triangular, widest at middle, closed, apical appendages ovate;
corona segments cream to ochroleucous, sometimes green at base, shiny, stipitate, tubular, dorsally flattened, 2.5–3.5 mm, exceeding style apex, apex truncate, glabrous, internal appendage falcate with acicular tip, exserted, sharply inflexed over style apex, glabrous;
style apex shallowly depressed, green.
ovate, 8–9 × 4–5 mm, margin winged, faces minutely rugulose;
coma 3–5 cm.
oval, 18–20 × 9–10 mm, margin winged, faces minutely rugulose;
coma 4–4.5 cm.
erect on upcurved pedicels, fusiform, 7–14 × 1.2–2 cm, apex long-acuminate or attenuate, smooth, hirsute.
pendulous on spreading pedicels, ellipsoid, 5–7 × 1.7–3 cm, apex acuminate, softly muricate, densely to thinly tomentose.
= 22.
Asclepias tuberosa
Asclepias welshii
Subspecies 3 (3 in the flora).
Asclepias tuberosa is one of the most familiar and beloved North American milkweeds and is a favored element of pollinator gardens because of the cheery orange flowers that attract abundant insect visitors. The clear latex is unusual in the genus and is often commented upon by collectors. The subspecies of A. tuberosa are highly intergrading. It is often difficult to satisfactorily place a given specimen in a particular subspecies; however, the great majority are readily assigned. It appears that the conspicuous variation in leaf morphology across the subspecies corresponds to genetically structured population variation (R. E. Woodson Jr. 1947). However, it is unknown to what extent cultivation and other human activities have blurred the distinctions among the geographic variants. Future recognition of the subspecies should be supported by genetic study with modern techniques. Although yellow-flowered plants predominate in the western plains, color variation is often pronounced in single populations, and yellow flowers may be encountered anywhere in the range. Hybridization with A. syriaca is documented, but is exceedingly rare. Presumed hybrids can be recognized by intermediate floral and vegetative traits.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
With respect to the number of populations, Asclepias welshii is the most endangered Asclepias in the flora area and is federally listed as threatened in the United States. Most of the populations are remote and relatively secure; however, the most accessible population in a Utah park has been subject to the impacts of off-road vehicle recreation. Leaves and stems of A. welshii emerge densely tomentose, but the lower portions of the plants become sand blasted and smooth as the season progresses. The species is strongly rhizomatous, as befits its shifting substrate. Seedlings and sprouts from rhizomes often bear narrowly linear leaves that differ so strongly from the foliage of more robust stems that they are not easily attributed to this species. Similar heterophylly is found in A. arenaria, A. erosa, and other milkweeds of sandy substrates.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
1. Leaf bases hastate, blade margins crisped. | subsp. rolfsii |
1. Leaf bases cuneate, obtuse, rounded, truncate, or subcordate, blade margins planar. | → 2 |
2. Leaf bases cuneate to obtuse, rounded, or subcordate, apices rounded to acute, mostly east of the crest of the Appalachian Mountains. | subsp. tuberosa |
2. Leaf bases truncate or obtuse to cordate, apices acute to attenuate, mostly west of the crest of the Appalachian Mountains. | subsp. interior |
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