Asclepias tuberosa
Asclepias asperula
asclépiade tubéreuse, butterfly milkweed
Antelope horns, spider milkweed
1–numerous, erect to ascending, branched in inflorescence, 15–90 cm, densely hirsute, not glaucous, rhizomes absent.
1–40, erect to decumbent, unbranched or branched at base, 15–60 cm, puberulent with curved trichomes, not glaucous, rhizomes absent.
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.
alternate to subopposite, petiolate, with 1–3 stipular colleters on each side of petiole;
petiole 2–4 mm, puberulent with curved trichomes to pilosulous;
blade lanceolate to linear, 5–17 × 0.4–3.7 cm, chartaceous, base cuneate, margins entire, apex attenuate to acute, mucronate, venation eucamptodromous to brochidodromous, surfaces puberulent with curved trichomes to glabrate, more densely so on veins, margins ciliate, 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, sessile or pedunculate, 10–60-flowered;
peduncle 0–22.5 cm, puberulent with curved trichomes, with 1 caducous bract at the base of each pedicel.
9–24 mm, puberulent with curved trichomes to pilosulous.
16–30 mm, puberulent with curved trichomes to pilose.
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 spreading;
calyx lobes ovate to linear-lanceolate, 3–5 mm, apex acute, pilosulous to puberulent with curved trichomes;
corolla pale green, sometimes tinged red abaxially, campanulate, lobes ascending and exceeding corolla segments, ovate to oval, 7–10 mm, apex acute, puberulent with curved trichomes at apex abaxially, glabrous adaxially;
gynostegium sessile;
fused anthers brown and green, turbinate, 2–2.5 mm, wings trapezoidal, widest above middle, closed, apical appendages ovate, erose;
corona segments reddish purple and white, sessile, clavate-tubular, 4.5–7 mm, slightly exceeded by to equaling style apex, deflexed at base, margins connivent, apex incurved, rounded, upper margin and cavity papillose, internal appendage a low internal crest, papillose;
style apex depressed, green.
ovate, 8–9 × 4–5 mm, margin winged, faces minutely rugulose;
coma 3–5 cm.
ovate, 5–8 × 4–6 mm, margin winged, remotely erose, faces minutely rugulose-papillose, minutely hirtellous;
coma 2.5–4 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, 6–11.5 × 1–2.5 cm, apex short- to long-acuminate, weakly to strongly arcuate, shallowly rugose-ribbed, ribs sometimes muricate, striate, pilosulous.
= 22.
Asclepias tuberosa
Asclepias asperula
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.)
Subspecies 2 (2 in the flora).
With terminal umbels of large, green and purple flowers, Asclepias asperula is highly distinctive and in flower can be confused only with its sister species, A. viridis. Distinguishing characteristics and the existence of interspecific hybrids are discussed under A. viridis. Large bees, notably Bombus and Xylocopa, are commonly observed visiting the flowers of A. asperula.
The subspecies of Asclepias asperula are strongly differentiated away from their region of contact, which extends from south of the Texas Panhandle to the extreme tip of the Oklahoma Panhandle. In the region of contact, their distinguishing traits intermix. The common occurrence of intermediates and apparent introgressants argues against elevation of the subspecies to the specific rank. There is a surprising gap in the distribution of the species as a whole on the Llano Estacado in eastern New Mexico and the Texas Panhandle that may contribute to the differentiation of the subspecies.
(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 |
1. Corona segments reddish purple with white upper margin, 5–7 mm; follicle ribs inconspicuously muricate at apex or not at all. | subsp. asperula |
1. Corona segments white, dorsally reddish purple, 4.5–6 mm; follicle ribs sparsely to densely muricate for most of the length. | subsp. capricornu |
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