Asclepias tuberosa
Asclepias erosa
asclépiade tubéreuse, butterfly milkweed
desert milkweed
1–numerous, erect to ascending, branched in inflorescence, 15–90 cm, densely hirsute, not glaucous, rhizomes absent.
1–12, erect to decumbent, unbranched, 40–250 cm, thinly 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.
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.
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.
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.
9–24 mm, puberulent with curved trichomes to pilosulous.
20–45 mm, densely tomentose to glabrate.
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 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, 8–9 × 4–5 mm, margin winged, faces minutely rugulose;
coma 3–5 cm.
ovate, 8–13 × 5–10 mm, margin narrowly winged, faces minutely rugulose, ridges papillose;
coma 2–2.5 cm.
erect on upcurved pedicels, fusiform, 7–14 × 1.2–2 cm, apex long-acuminate or attenuate, smooth, hirsute.
erect on upcurved pedicels, lance-ovoid to ovoid, 6.5–10 × 2–3.5 cm, apex acuminate to apiculate, smooth, thinly tomentose.
= 22.
= 22.
Asclepias tuberosa
Asclepias erosa
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.)
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.)
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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