Asclepias tuberosa
Asclepias hirtella
asclépiade tubéreuse, butterfly milkweed
asclépiade hérissée, green milkweed, prairie milkweed, tall green milkweed
1–numerous, erect to ascending, branched in inflorescence, 15–90 cm, densely hirsute, not glaucous, rhizomes absent.
1–3+, erect to spreading, unbranched (rarely branched), 30–125 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.
opposite or alternate, sessile or petiolate, with 1–3 stipular colleters on each side of petiole, also in axil;
petiole 0–3 mm, puberulent with curved trichomes;
blade linear to linear-lanceolate, 4–16 × 0.2–1.5 cm, chartaceous, base cuneate, margins entire, apex acute to attenuate, mucronate, venation faintly brochidodromous, surfaces scabridulous to puberulent with curved trichomes, especially 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.
extra-axillary, sessile or pedunculate, 34–112-flowered;
peduncle 0–4 cm, hirtellous, with 1 caducous bract at the base of each pedicel.
9–24 mm, puberulent with curved trichomes to pilosulous.
11–23 mm, hirtellous.
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, 1.5–2 mm, apex acute, hirtellous;
corolla green to greenish cream, red-violet at tip, lobes reflexed, oblong, 3–5 mm, apex obtuse, glabrous;
gynostegial column 0.5–1 mm;
fused anthers green, cylindric, 1–1.5 mm, wings trapezoidal, closed, apical appendages oblate;
corona segments cream to greenish cream, rarely pinkish lavender or brown, often with a dorsal red-violet stripe or at base, sessile, laminar, strongly dorsally compressed, margins incurved, appressed to column, saccate, 1.5–2 mm, not exceeding point of anther wings, greatly exceeded by style apex, apex truncate, glabrous, internal appendage absent or obscure, glabrous;
style apex shallowly depressed, green.
ovate, 8–9 × 4–5 mm, margin winged, faces minutely rugulose;
coma 3–5 cm.
ovate, 10–11 × 7–8 mm, margin winged, faces smooth;
coma 3.5–4 cm.
erect on upcurved pedicels, fusiform, 7–14 × 1.2–2 cm, apex long-acuminate or attenuate, smooth, hirsute.
erect on upcurved pedicels, fusiform, 6–15 × 1–2 cm, apex attenuate, smooth, pilosulous to puberulent with curved trichomes.
= 22.
Asclepias tuberosa
Asclepias hirtella
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 hirtella has been included sometimes in a broadly circumscribed A. longifolia. The species are parapatric and usually are readily distinguished by the hirtellous pedicels of A. hirtella and puberulent pedicels with curved trichomes of A. longifolia. Asclepias hirtella is also typically much taller with more numerous umbels compared to A. longifolia. However, populations along the Gulf Coastal Plain from eastern Texas to the Mississippi River and from southern Mississippi to central Georgia have proved challenging because they include plants with the growth form of A. longifolia and the pedicel vestiture of A. hirtella. Consequently, these populations have not been consistently assigned to one species or the other. However, the previously overlooked difference in corona segment length correlates perfectly with the pedicel vestiture and with geography. Thus, Gulf Coast populations west of the Mississippi River and north of the immediate coastline are here interpreted to represent short-statured A. hirtella. This hypothesis warrants investigation with population genetic data—it is quite possible that populations of A. hirtella in southern Arkansas, Louisiana, and Texas merit recognition as a subspecies. Asclepias hirtella is rare at the margins of its range and is considered to be of conservation concern in Georgia, Kentucky, Louisiana, Michigan, Minnesota (Mower County), Mississippi, Tennessee, West Virginia (Jackson, Mason, and Putnam counties), and Ontario (Essex County).
(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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