Herbs, subshrubs, or shrubs, rhizomatous or not; latex milky (clear). Stems not twining, unarmed, pubescent or glabrous. Leaves persistent (caducous), opposite, subopposite, alternate, or whorled, petiolate or sessile; stipular colleters interpetiolar (absent); laminar colleters present or absent. Inflorescences extra-axillary or terminal, umbelliform, pedunculate or sessile. Flowers: calycine colleters present or absent; corolla green, white, cream, pink, red, maroon, purple, orange, or yellow, rotate with lobes usually reflexed to spreading (campanulate, tubu­lar), aestivation valvate; coralline corona absent; androecium and gynoecium united into a gynostegium adnate to corolla tube; gynostegial corona of 1 whorl of 5 thick, cupulate, laminar, or clavate segments, often bearing an adaxial falcate, subulate, acicular, lingulate, or crestlike appendage; anthers adnate to style, locules 2; pollen in each theca massed into a rigid, vertically oriented pollinium, pollinia lachrimiform, joined from adjacent anthers by translators to a common corpusculum and together forming a pollinarium. Fruits follicles, usually solitary, erect (spreading) on upcurved or straight pedicels or pendulous, green to brown, fusiform, lance-ovoid, ovoid, or ellipsoid, smooth (ornamented), pubescent or glabrous. Seeds ovate or lanceolate to nearly orbiculate, flattened (naviculate in A. albicans, A. linaria; flat to somewhat naviculate in A. subulata), usually winged, not beaked, comose (coma absent in A. perennis), not arillate; comas white. x = 11.

Distribution

North America, Mexico, Central America, South America, West Indies, Africa, introduced in Europe and pan-tropical areas.

Discussion

Species ca. 400 (77 in the flora).

In its broad sense, Asclepias is the largest genus of Apocynaceae. Attempts have been made to segregate the African species into some 20 genera, but these attempts have left a residuum of African species in Asclepias that are more closely related to segregate African genera than to the type-bearing American species (D. J. Goyder et al. 2007; Goyder 2009; D. Chuba et al. 2017). Hence, the broader circumscription is adopted here, although the American species as a whole (ca. 130 species) are monophyletic, and circumscription does not affect the nomenclature of the species in the flora.

Species of Asclepias are ecologically diverse, with habitats ranging from marshes and swamps to some of the driest regions in the North American and African deserts. They occur from sea level to over 3000 m elevation. However, the genus has diversified most dramatically in grasslands, savannas, woodlands, and forests, in seasonally dry, temperate and subtropical climates, and it is in these ecoregions that milkweeds are most conspicuous. All species are perennial, and the great majority are seasonally dormant herbs. Most species occur in environments subject to one or more of the following disturbances: extended freezing, drought, fire, or mammalian grazing. Nonetheless, the species are diverse in growth form and phenology—habit ranges from diminutive herbs less than 5 cm tall to herbs and shrubs exceeding 2 m in height, and flowering ranges from constrained two-week periods to year-round.

Two aspects of the ecology of Asclepias have received extensive study—reproductive and defensive strategies. As with Asclepiadoideae as a whole, Asclepias species have elaborate and precise insect pollination involving pollinia and coronas that mediate interactions with pollinators. Asclepias has been the most intensively studied genus in the subfamily with regard to the diversity of the pollinating fauna and forces that have shaped the evolution of floral traits. Whereas pollinator interactions in Asclepiadoideae genera vary widely from highly specialized to highly generalized (J. Ollerton et al. 2019), those species of Asclepias that have been studied quantitatively are moderately to highly generalized, including such species as A. tuberosa, commonly known as butterflyweed (M. Fishbein and D. L. Venable 1996). However, such studies are limited to fewer than a dozen species, and the existence of more specialized pollination seems likely for more rare and poorly known species. Species of Asclepias have also served as a model system for testing the hypothesis that the characteristically low fruit set in the genus is a consequence of sexual selection to increase flower number through competition among plants as pollen donors (M. F. Willson and B. J. Rathcke 1974; S. B. Broyles and R. Wyatt 1990; Fishbein and Venable 1996b).

The chemical ecology of Asclepias also has been widely studied because a class of secondary metabolites, cardiac glycosides (cardenolides), which is found in many Apocynaceae species, is sequestered by several specialist Asclepias-feeding insect herbivores and used as their own defense compounds (A. A. Agrawal et al. 2012). The interaction between Asclepias species and the monarch butterfly, Danaus plexippus, has been most intensively studied (L. P. Brower et al. 1967). Subsequently, the diversity of defensive traits of milkweeds (cardenolides, latex, trichomes, waxes, etc.) has been analyzed in an evolutionary context, considering also the functions of some of these traits as adaptations for drought resistance (Agrawal et al. 2009; M. Fishbein et al. 2018).

Species of Asclepias have been investigated for development of commercial products. Pro­duction of natural rubber from latex has not proved economical, even during the time of World War II shortages. There have been repeated efforts to develop an industry around harvesting seed comas as floss for stuffing and insulation. Mechanization has proved challenging, but a small industry is developing, based largely on A. syriaca because of its rapid growth and high seed production. Asclepias species have been used as a source of medicine by most Native American tribes for diverse ailments. One of the common names of A. tuberosa is pleurisy root, and the tuberous roots of this species have been included in herbal remedies for coughing and congestion. This seems to be the only commercialization of American milkweeds for medicinal purposes, and no clinical trials have taken place in North America involving milkweed-derived compounds. Despite toxicity, there is a tradition in native and immigrant American communi­ties, particularly in the Appalachian region, of using young shoots of A. syriaca as a vegetable, following boiling with two changes of water.

Species such as Asclepias curassavica, A. incarnata, and A. tuberosa are widely cultivated, and horticultural varietals are available. Recently, the popularity of diverse species of Asclepias as components of home and commercial gardens, as well as synthetic prairies, is skyrocketing, fueled in part by interest in growing food plants for monarch butterflies, or more generally as a nectar source for the conservation of pollinator communities. Surprisingly, A. subulata, found naturally on arid, sandy substrates, grows rapidly in standard potting media and is becoming ubiquitous in horticultural settings in Arizona. At least 25 species are available from nurseries across the flora area.

In the key and descriptions the following conventions are used. Leaf measurements pertain to midstem leaves, usually at the node below the first inflorescence. The lowest leaves and those in the flowering portion of the plant are commonly much smaller than midstem leaves and are less useful for identification. Vestiture of stems and leaves (unless otherwise specified) is described for mature structures below the inflorescences; many species possess trichomes on stems and leaves early in development that are lost with maturity but continue to be produced on new structures in the flowering portion of the plant. The gynostegial column refers to the cylindrical structure formed by the united filaments; its length is measured from its attachment to the corolla to the base of the anthers. The corona of Asclepias consists largely of five discrete segments that are usually cavitate and are often referred to as hoods. In some species, these segments are strongly compressed dorsally and the cavity is greatly reduced. In others, the margins of the segments are folded in, obscuring the cavity and creating a club-shaped form. The majority of species also have an adaxial appendage of each segment, often referred to as a horn, which arises from within the cavity and is often exserted. In some species, the appendage is merely a crest within the cavity, and it may be altogether absent. The diversity of corona forms is difficult to describe, but these features are highly useful for identification. They are ideally observed in fresh material, and photographs are invaluable.

A revised infrageneric classification of Asclepias is needed. The species in this treatment are arranged according to phylogenetic relationships obtained from analyses of plastid genomes (M. Fishbein et al. 2018) and a set of 768 nuclear loci (J. Boutte et al. 2019). These correspond largely to the major clades presented by Fishbein et al. (2011). Species 1–3 are early-diverging lineages, species 4–6 belong to the Sonoran Desert clade, species 7–16 belong to the Incarnatae clade, species 17–22 are not placed in a major clade, species 23–27 belong to the Mexican highland clade, and species 28–77 belong to the temperate North American clade. Within the temperate North American clade, several subclades are strongly supported: species 37 in the waxy clade, species 39–42 in the Podostemma clade, species 51–56 in the Colorado Plateau clade, species 57–60 in the dwarf clade, and species 61–77 in the eastern temperate clade.

Selected References

None.

Lower Taxa

Asclepias albicans, Asclepias amplexicaulis, Asclepias angustifolia, Asclepias arenaria, Asclepias asperula, Asclepias brachystephana, Asclepias californica, Asclepias cinerea, Asclepias connivens, Asclepias cordifolia, Asclepias cryptoceras, Asclepias curassavica, Asclepias curtissii, Asclepias cutleri, Asclepias eastwoodiana, Asclepias elata, Asclepias emoryi, Asclepias engelmanniana, Asclepias eriocarpa, Asclepias erosa, Asclepias exaltata, Asclepias fascicularis, Asclepias feayi, Asclepias hallii, Asclepias hirtella, Asclepias humistrata, Asclepias hypoleuca, Asclepias incarnata, Asclepias involucrata, Asclepias labriformis, Asclepias lanceolata, Asclepias lanuginosa, Asclepias latifolia, Asclepias lemmonii, Asclepias linaria, Asclepias linearis, Asclepias longifolia, Asclepias macrosperma, Asclepias macrotis, Asclepias meadii, Asclepias michauxii, Asclepias nummularia, Asclepias nyctaginifolia, Asclepias obovata, Asclepias oenotheroides, Asclepias ovalifolia, Asclepias pedicellata, Asclepias perennis, Asclepias prostrata, Asclepias pumila, Asclepias purpurascens, Asclepias quadrifolia, Asclepias quinquedentata, Asclepias rubra, Asclepias rusbyi, Asclepias ruthiae, Asclepias sanjuanensis, Asclepias scaposa, Asclepias solanoana, Asclepias speciosa, Asclepias sperryi, Asclepias stenophylla, Asclepias subulata, Asclepias subverticillata, Asclepias sullivantii, Asclepias syriaca, Asclepias texana, Asclepias tomentosa, Asclepias tuberosa, Asclepias uncialis, Asclepias variegata, Asclepias verticillata, Asclepias vestita, Asclepias viridiflora, Asclepias viridis, Asclepias viridula, Asclepias welshii

Key

1	Leaves alternate (sometimes a few subopposite).	> 2
2	Leaf blades filiform or needlelike to narrowly linear, 0.05–0.3 cm wide.	> 3
3	Stems 30–160 cm.	> 4
4	Stems woody, few–numerous; leaf blades needlelike, less than 4 cm; corona segments with rod-shaped, slightly exserted appendages.	Asclepias linaria
4	Stems herbaceous, 1–3 (rarely more); leaf blades not needlelike, greater than 4 cm; corona segment appendages absent or at most a low crest.	> 5
5	Stems glabrous; anthers 2–3 mm.	> 6
6	Gynostegial column 0.5–1.5 mm; fused anthers barrel-shaped, anther wings evenly crescent-shaped and separate throughout, apical anther appendages narrowly pandurate, conduplicate, not obscuring corpuscula.	Asclepias engelmanniana
6	Gynostegial column 0–0.5 mm; fused anthers obconic, anther wings connivent, except at the wider base, apical anther appendages deltoid, obscuring corpuscula.	Asclepias rusbyi
5	Stems puberulent with curved trichomes; anthers 1–2 mm.	> 7
7	Leaf blades conduplicate; corolla lobe apices minutely puberulent with curved trichomes abaxially; fused anthers obconic, 1.5–2 mm, anther wings narrowly crescent-shaped; corona segments 3–3.5 mm, appendage an internal crest, apex appearing 3-toothed; follicles erect on straight pedicels.	Asclepias stenophylla
7	Leaf blades flat; corolla lobes glabrous; fused anthers cylindric, 1–1.5 mm, anther wings trapezoidal to triangular; corona segments 1.5–2.5 mm, appendage absent or obscure, apex not appearing toothed; follicles erect on upcurved pedicels.	> 8
8	Corona segments 1.5–2 mm, not exceeding point of anther wings; pedicels and calyces hirtellous; follicles 1–2 cm wide; flowers 34–112 per umbel.	Asclepias hirtella
8	Corona segments 2–2.5 mm, exceeding point of anther wings; pedicels and calyces puberulent with curved trichomes; follicles 0.7–1.2 cm wide; flowers 13–28(–36) per umbel.	Asclepias longifolia
3	Stems 5–30 cm.	> 9
9	Corona segments strongly dorsally compressed and laminar or chute-shaped, appendage absent or merely a low crest, wholly included.	> 10
10	Leaf blades conduplicate; corolla lobe apices puberulent with minute, curved trichomes abaxially; fused anthers obconic, 1.5–2 mm, anther wings narrowly crescent-shaped; corona segments 3–3.5 mm, appendage an internal crest, apex appearing 3-toothed; follicles erect on straight pedicels.	Asclepias stenophylla
10	Leaf blades flat; corolla lobes glabrous; fused anthers cylindric, 1–1.5 mm, anther wings trapezoidal to triangular; corona segments 2–2.5 mm, appendage absent or obscure, apex not appearing toothed; follicles erect on upcurved pedicels.	Asclepias longifolia
9	Corona segments conduplicate, cupulate, or tubular, appendage lingulate, falcate or acicular, barely to well exserted.	> 11
11	Corolla lobes 4.5–6 mm; corona segments 3.5–4.5 mm; follicles 1.5–2 cm wide, rugose.	Asclepias involucrata
11	Corolla lobes 2.5–5 mm; corona segments 1–3.5 mm; follicles 0.5–1.5 cm wide, smooth.	> 12
12	Corollas red-violet; corona segment appendages lingulate, barely exserted; follicles pendulous on spreading to declined pedicels or erect on upcurved pedicels.	> 13
13	Stems erect to ascending; leaves sessile, blades strigose to glabrate; corona segments white; follicles pendulous on spreading to declined pedicels.	Asclepias cutleri
13	Stems decumbent; leaves petiolate, petioles 1–5 mm, blades puber­ulent with curved trichomes on midvein; corona segments red-violet at base, white to orange at apex, follicles erect on upcurved pedicels.	Asclepias uncialis
12	Corollas pink, cream, green, or tan; corona segment appendages falcate or acicular, well exserted; follicles erect on straight pedicels.	> 14
14	Leaf blades needlelike to narrowly linear, 0.05–0.1 cm wide; anthers 1–1.5 mm; corona segment appendages acicular, arching towards style apex, follicles 2.5–9.5 cm.	Asclepias pumila
14	Leaf blades narrowly linear, 0.1–0.4 cm wide; anthers 1.5–3 mm; corona segment appendages falcate, sharply inflexed over style apex; follicles 7.5–12.5 cm.	Asclepias michauxii
2	Leaf blades linear, elliptic, oblong, oval, lanceolate, ovate, oblanceolate, obovate, or falcate, 0.3–9 cm wide.	> 15
15	Corolla lobes ascending and exceeding corona segments, 7–15 mm; corona segments clavate-tubular, deflexed at base, ascending to incurved at apex, margins connivent.	> 16
16	Leaf blade bases cuneate, apices acute to attenuate; peduncles not branched; corolla lobes 7–10 mm, puberulent abaxially at apex with curved trichomes; corona segments 4.5–7 mm, margins and appendage papillose.	Asclepias asperula
16	Leaf blade bases rounded to subcordate, apices obtuse to rounded; peduncles usually branched; corolla lobes (9–)12–15 mm, glabrous; corona segments 3–5 mm, margins and appendage hirtellous.	Asclepias viridis
15	Corolla lobes reflexed, sometimes with spreading tips, not overtopping corona segments, 3–9 mm; corona segments cupulate, tubular, conduplicate, chute-shaped, or laminar, margins separate.	> 17
17	Corona segments strongly dorsally compressed and laminar or chute-shaped, appendage absent or merely a low crest.	> 18
18	Leaf blades conduplicate; corolla lobe apices puberulent abaxially with minute, curved trichomes; fused anthers obconic, anther wings narrowly crescent-shaped; corona segment appendages crestlike, segment apices appearing 3-toothed; follicles erect on straight pedicels.	Asclepias stenophylla
18	Leaf blades flat; corolla lobes glabrous; fused anthers cylindric, anther wings trapezoidal to triangular; corona segment appendages absent, segment apices untoothed; follicles erect on upcurved pedicels.	> 19
19	Stems 7–20 cm, hirtellous to pilose; leaf blades hirtellous; fused anthers 1.5–2 mm; follicles hirtellous to pilose; seeds 6–7 × 4–5 mm.	Asclepias lanuginosa
19	Stems 25–125 cm, puberulent with curved trichomes; leaf blades puberulent with curved trichomes or scabridulous; fused anthers 1–1.5 mm; follicles puberulent with curved trichomes to pilosulous; seeds 10–12 × 7–8 mm.	> 20
20	Flowers 34–112 per umbel; pedicels and calyces hirtellous; corona segments 1.5–2 mm, not exceeding point of anther wings; follicles 1–2 cm wide.	Asclepias hirtella
20	Flowers 13–28(–36) per umbel; pedicels and calyces puberulent with curved trichomes; corona segments 2–2.5 mm, exceeding point of anther wings; follicles 0.7–1.2 cm wide.	Asclepias longifolia
17	Corona segments cupulate, tubular, or conduplicate, at most slightly flattened dorsally, appendage lingulate, falcate, subulate, or conical, barely to well exserted from cavity.	> 21
21	Stems 15–100 cm; corolla lobes 4–9 mm.	> 22
22	Corona segments 5–7 mm.	> 23
23	Stems, leaves, and peduncles puberulent with curved trichomes, pilosulous, tomentulose, or glabrate; corollas pink, red, or green; coronas cream, segments with reddish or purplish dorsal stripes; anthers brown; follicles pilosulous; seeds 6–7 mm.	Asclepias hallii
23	Stems, leaves, and peduncles hirsute; corollas and coronas red, orange, or yel­low; anthers yellow to yellowish green; follicles hirsute; seeds 8–9 mm.	Asclepias tuberosa
22	Corona segments 2.5–3.5 mm.	> 24
24	Pedicels 9–12 mm; corolla lobes 4–5 mm; corona segments cupulate, apex obtuse; fruiting pedicels straight, follicles 0.5–0.7 cm wide; se United States.	Asclepias michauxii
24	Pedicels 15–50 mm; corolla lobes 6–9 mm; corona segments conduplicate, apex truncate; fruiting pedicels pendulous or upcurved, follicles 1.2–3 cm wide; California or Utah.	> 25
25	Leaf blades 2–8 cm wide, tomentose to densely puberulent with curved trichomes; anthers brown, 2.5–3 mm; follicles erect on upcurved pedicels, 5–10 cm; seeds 7–9 × 4–6 mm.	Asclepias eriocarpa
25	Leaf blades 0.5–2 cm wide, glabrous except for midvein; anthers green to yellowish green, 1.5–2 mm; follicles pendulous, 3.5–5.5 cm; seeds 12–13 × 8–9 mm.	Asclepias labriformis
21	Stems 5–15 cm; corolla lobes 3–6 mm.	> 26
26	Corollas green, sometimes tinged red or pink; corona segment appendages well exserted, sharply inflexed towards or over style apex; follicles rugose.	> 27
27	Leaf blades 0.2–0.8 cm wide; anthers 1.5–2 mm; corona segments cream, usually with a pink or red dorsal stripe, conduplicate, 3.5–4.5 mm; seeds 7–8 × 5–6 mm.	Asclepias involucrata
27	Leaf blades 0.5–2 cm wide; anthers 1–1.5 mm; corona segments yellow to ochroleucous, tubular, 2–3 mm; seeds 8–12 × 6–8 mm.	Asclepias macrosperma
26	Corollas red-violet; corona segment appendages equaling or barely exserted from segment; follicles smooth.	> 28
28	Corona segment appendages falcate, not or barely exserted; Colorado Plateau, ne Arizona, nw New Mexico, Utah.	> 29
29	Leaf blades persistently pilosulous, more densely so on veins; pedicels and calyces densely pilosulous to tomentulose; corona segment appendages barely exserted; Utah, n of Colorado River and w of Green River, from San Rafael Reef to Waterpocket Fold.	Asclepias ruthiae
29	Leaf blades sparsely pilosulous to glabrate, persistently hairy on veins; pedicels and calyces pilosulous; corona segment appendages included; Arizona, New Mexico, just entering Utah in Monument Valley.	Asclepias sanjuanensis
28	Corona segment appendages lingulate or conical, barely exserted; Arizona, Colorado, Nevada, New Mexico, Oklahoma, Texas, not on the Colorado Plateau.	> 30
30	Leaf blades lanceolate to ovate, 0.6–3 cm wide, strigulose or puberulent on midvein to glabrate; flowers 2–25 per umbel; anthers 1.5–2 mm; corona appendages conical; Nevada.	Asclepias eastwoodiana
30	Leaf blades linear to lanceolate, 0.2–1 cm wide, puberulent with curved trichomes on midvein; flowers 3–7 per umbel; anthers 1–1.5 mm; corona appendages lingulate; Arizona, Colorado, New Mexico, Oklahoma, Texas.	Asclepias uncialis
1	Leaves opposite, subopposite, or whorled (rarely alternate in A. eriocarpa).	> 31
31	One or more nodes with leaves whorled or (in A. quadrifolia) apparently whorled because of a reduced internode.	> 32
32	Shrubs, 50–400 cm; leaves ephemeral; stems often leafless or nearly so, glaucous; follicles spreading to pendulous; Sonoran Desert and se Mohave Desert, sw Arizona, se California, s Nevada.	> 33
33	Stems 5–40 (usually 1–few), 140–400 cm; pedicels 8–16 mm; corolla lobes 4.5–6 mm; corona segments 2–3 mm, conduplicate, not exceeding style apex.	Asclepias albicans
33	Stems 3–100 (usually few–numerous), 50–175 cm; pedicels 11–21 mm; corolla lobes 7–12 mm; corona segments 7–9 mm, tubular, greatly exceeding style apex.	Asclepias subulata
32	Herbs; stems 20–150 cm; leaves persistent; stems leafy, not glaucous; follicles erect on straight or upcurved pedicels; widespread, approaching but not entering the Sonoran Desert.	> 34
34	At least some leaves on main stem alternate or opposite, blades elliptic or lanceolate to ovate, 1–8 cm wide; corona lobes 2.5–4 mm.	> 35
35	Plants 1–10-stemmed; whorled leaves, when present, at upper nodes, leaf blades tomentose to densely puberulent with curved trichomes; pedicels and calyces densely tomentose; corolla lobes 7–9 mm; follicles erect on upcurved pedicels, lance-ovoid, 1.5–3 cm wide; California.	Asclepias eriocarpa
35	Plants single-stemmed; whorled leaves, when present, at a single, mid-stem node; leaf blades inconspicuously puberulent only on veins; pedicels puberlent on 1 side; calyces glabrous; corolla lobes 4–5 mm; follicles erect on straight pedicels, narrowly fusiform, 0.4–1 cm wide; e North America.	Asclepias quadrifolia
34	All leaves whorled (rarely opposite on vegetative branches), blades linear to linear-lanceolate or narrowly elliptic, 0.1–1.8 cm wide; corona lobes 1.5–2 mm.	> 36
36	Leaf blades 0.2–1.8 cm wide; pedicels 9–14 mm; corollas pink (rarely green with pink tinge); California, Idaho, Nevada, Oregon, Washington.	Asclepias fascicularis
36	Leaf blades 0.1–0.4 cm wide; pedicels 5–12 mm; corollas green to cream, sometimes tinged pink or tan; widespread, s or e of Idaho, Nevada.	> 37
37	Stems 1–8, vegetative branches often present in 1+ leaf axils; leaf blades 0.1–0.4 cm wide, glabrous; anthers 1.5–2 mm; corona segment margins entire; w United States, including w Kansas, w Oklahoma, w Texas.	Asclepias subverticillata
37	Stems solitary (rarely 2–3), vegetative branches absent; leaf blades 0.1–0.2 cm wide, puberulent with curved trichomes adaxially, especially on midvein to glabrate; anthers 1.2–1.5 mm; corona segment margins shallowly lobed to sharply toothed; e North America, e of Colorado, Idaho, New Mexico.	Asclepias verticillata
31	Leaves opposite or subopposite, not whorled.	> 38
38	Leaf blades filiform to narrowly linear, 0.05–0.3 cm wide.	> 39
39	Shrubs, 50–400 cm; leaves ephemeral; stems often leafless or nearly so, glaucous; follicles spreading to pendulous; Sonoran Desert and se Mohave Desert, sw Arizona, se California, s Nevada.	> 40
40	Stems 5–40 (usually 1–few), 140–400 cm; pedicels 8–16 mm; corolla lobes 4.5–6 mm; corona segments 2–3 mm, conduplicate, not exceeding style apex.	Asclepias albicans
40	Stems 3–100 (usually few–numerous), 50–175 cm; pedicels 11–21 mm; corolla lobes 7–12 mm; corona segments 7–9 mm, tubular, greatly exceeding style apex.	Asclepias subulata
39	Subshrubs or herbs; stems 4–125 cm; leaves persistent; stems leafy, not glaucous; follicles erect on straight or upcurved pedicels; approaching but not entering the Sonoran Desert.	> 41
41	Stems prostrate to decumbent.	> 42
42	Stems 4–18 cm; petioles 1–5 mm; umbels sessile; follicles ovoid, 3–5.5 cm; Arizona, Colorado, Kansas, New Mexico, Oklahoma, Texas, Wyoming.	> 43
43	Corollas green, sometimes tinged pink or red, lobes 4.5–6 mm; corona segments 3.5–4.5 mm; follicles 1.5–2 cm wide, rugose.	Asclepias involucrata
43	Corollas red-violet, lobes 3–5 mm; corona segments 1–2 mm; follicles 0.8–1.5 cm wide, smooth.	Asclepias uncialis
42	Stems 15–70 cm; petioles 0–2 mm; peduncles 0–6 cm; follicles narrowly fusiform, 7.5–13.5 cm; se United States.	> 44
44	Leaf blades scabridulous to puberulent with curved trichomes; corona segments laminar, 2–2.5 mm, appendage absent or obscure; fruiting pedicels upcurved; follicles 0.7–1.2 cm wide; seeds 11–12 × 7–8 mm.	Asclepias longifolia
44	Leaf blades glabrous; corona segments cupulate, 2.5–3.5 mm, appendage falcate, well exserted from cavity; fruiting pedicels straight; follicles 0.5–0.7 cm wide; seeds 7–8 × 5–6 mm.	Asclepias michauxii
41	Stems ascending to erect.	> 45
45	Subshrubs, usually persistently woody at base (sometimes herbaceous to base), cespitose; stems 5–20+, sparsely to densely branched, at least at base.	> 46
46	Stems 20–60 cm; petioles 2–4 mm; flowers 3–15 per umbel; corollas cream, sometimes tinged or striped pink; corona segments tubular, glabrous at apex, appendage acicular, arching over style apex, glabrous; fruiting pedicels straight.	Asclepias angustifolia
46	Stems 5–35 cm; leaves sessile; flowers 1–7 per umbel; corollas greenish cream or greenish yellow, sometimes tinged red or purple; corona segments conduplicate or chute-shaped, hirtellous at apex, appendage falcate, arching over style apex, or an included crest, hirtellous; fruiting pedicel upcurved.	> 47
47	Flowers 2–7 per umbel; anthers 1–1.5 mm; corona segments conduplicate, 4–5 mm, apex long-caudate, appendage falcate, exserted.	Asclepias macrotis
47	Flowers 1–2 per umbel; anthers 1.5–2 mm; corona segments chute-shaped, sharply inflexed, 2–3 mm, apex subcaudate, appendage an included crest.	Asclepias sperryi
45	Herbs, not woody at base; stems 1(–5), rarely branched (often branched in A. linearis).	> 48
48	Corona segments strongly dorsally compressed or chute-shaped, margins incurved, appendages absent or a low, inlcuded crest; follicles 1–2 cm wide.	> 49
49	Flowers 9-28 per umbel; pedicels puberulent with curved trichomes to pilosulous, straight in fruit; corona segments equaling style apex.	Asclepias stenophylla
49	Flowers 34-112 per umbel; pedicels hirtellous, upcurved in fruit; corona segments greatly exceeded by style apex.	Asclepias hirtella
48	Corona segments conduplicate or cupulate (may be slightly flattened dorsally, but not compressed), margins not incurved, appendages evident and exserted from cavity; follicles 0.3–1 cm wide.	> 50
50	Corona segment appendages acicular, subulate, or falcate, exserted and inflexed towards or over style apex.	> 51
51	Stems 60–125 cm; leaf blades 7–25 cm; corollas red, lobes 9–10 mm; corona segments yellow to reddish orange, 5–6 mm; fruiting pedicels upcurved.	Asclepias lanceolata
51	Stems 25–75 cm; leaf blades 3–9 cm; corollas green, tan, or pink, sometimes tinged brown or red, lobes 3–5 mm; coronas cream, sometimes tinged brown or green, segments 1.5–4 mm; fruiting pedicels straight.	> 52
52	Plants rhizomatous; interpetiolar ridges present; pedicels 6–8 mm; corona segments 1.5 mm; follicles 5–8.5 cm; Texas.	Asclepias linearis
52	Plants not rhizomatous; interpetiolar ridges absent; pedicels 7–13 mm; corona segments 3–4 mm; follicles 9–12.5 cm; Alabama, Florida.	Asclepias viridula
50	Corona segment appendages crestlike, included, or absent.	> 53
53	Corollas and coronas yellowish green to green; corolla lobes erect, concealing gynostegium and partially obscuring corona; corona appendages absent; follicles densely puberulent with curved trichomes.	Asclepias pedicellata
53	Corollas green, white, or cream, sometimes tinged gray, pink, red, lavender, or purple, lobes reflexed or spreading; coronas red, pink, violet, white, cream, gray, or a combination of these, appendage crestlike, included; follicles glabrous, glabrate, or pilosulous.	> 54
54	Corollas green, sometimes tinged red, lobes pilose abaxially; anthers tan to brown; coronas red, pink, purple, or red-violet, sometimes white at segment apices; fruiting pedicels upcurved, follicles pilosulous to glabrate; s Arizona, s New Mexico.	Asclepias quinquedentata
54	Corollas cream or white, tinged gray, pink, lavender, or purple, lobes glabrous; anthers green or lavender; corona segments cream or white, sometimes tinged gray, pink, or purple; fruiting pedicels straight, follicles glabrous; se United States.	> 55
55	Flowers erect; corollas white, sometimes tinged lavender, lobes spreading, 7–10 mm; anthers lavender; corona segments white, cupulate, apex edentate.	Asclepias feayi
55	Flowers spreading to pendent; corollas and coronas cream, tinged gray, pink, or purple; corolla lobes reflexed, 4–5 mm; anthers green; corona segments conduplicate, apex toothed laterally.	Asclepias cinerea
38	Leaf blades linear, elliptic, oblong, oval, lanceolate, ovate, deltate, oblanceolate, obovate, oblate, or orbiculate, 0.1–15 cm wide.	> 56
56	Both stems and leaf blades completely glabrous or glabrate, except for ciliate leaf margins in some species or abaxial midvein (A. cryptoceras).	> 57
57	Stems and/or leaf blades glaucous.	> 58
58	Corona segment appendages merely low, included crests or absent; w United States, w of Rocky Mountains plus Arizona, New Mexico, w Texas.	> 59
59	Corollas red-violet, lobes 6–7 mm; corona segments tubular, 2–3 mm; n California, extreme w Nevada, sw Oregon.	Asclepias cordifolia
59	Corollas green to yellowish green, sometimes tinged red abaxially, lobes 8–14 mm; corona segments conduplicate, 4–8 mm; Arizona, e California, w Colorado, Idaho, Nevada, s New Mexico, e Oregon, w Texas, Utah, e Washington, s Wyoming.	> 60
60	Stems prostrate to decumbent, 8–25 cm; umbels sessile; corolla lobe apices minutely hirtellous adaxially; anthers brown (sometimes with green apex), 1.8–3 mm; corona segments red-violet to pinkish purple, apex with recurved, papillose teeth, appendage absent; follicles 4.5–6 cm, glabrous, glaucous.	Asclepias cryptoceras
60	Stems erect, 30–80 cm; peduncles 5–20 cm; corolla lobes glabrous; anthers green, 3–3.5 mm; corona segments white, yellow to tan dorsally, apex edentate, appendage a low, included crest; follicles 8–13 cm, pilosulous, not glaucous.	Asclepias elata
58	Corona segment appendages exserted, falcate, subulate, or ensiform; e United States, w to Arizona, Colorado, Kansas, Nebraska, New Mexico, Texas, Utah.	> 61
61	Peduncles, pedicels, calyx lobes, and developing leaf blades tomentose, leaves becoming glabrate with age; w United States, plains e of Rocky Mountains to c Kansas, c Nebraska, c Oklahoma, w Texas.	Asclepias latifolia
61	Peduncles, pedicels, calyx lobes, and developing leaf blades glabrous, puberulent with curved trichomes, or pilosulous; e United States and Canada, w to e Kansas, e Nebraska, c Oklahoma, and e Texas.	> 62
62	Stems decumbent to erect; corolla lobes 5.5–6 mm; anthers brown; corona segments 3–3.5 mm, appendage ensiform, only slightly exserted.	Asclepias humistrata
62	Stems erect; corolla lobes 7–12 mm; anthers green; corona segments 4–7 mm, appendage falcate or subulate, well exserted.	> 63
63	Leaf blades 1.5–9 cm wide, apex obtuse, rounded, truncate, or emarginate (acute); pedicels 20–55 mm; anthers 2.5–3.5 mm; corona segments tubular, apex broadly obtuse and oblique or truncate and erose; follicles 1–3 cm wide.	> 64
64	Peduncles 5–40 cm; flowers 18–53 per umbel; pedicels puberulent with curved trichomes; corollas green, often tinged reddish purple, or bronze; coronas reddish purple to cream, segments stipitate, apex truncate, erose; follicles smooth, 9–16 cm; seeds 9–10 × 6–7 mm.	Asclepias amplexicaulis
64	Peduncles 1–6 cm; flowers 9–29 per umbel; pedicels glabrous, glaucous; corollas dark pink, pale at base of lobes; coronas pale to dark pink, segments subsessile, apex broadly obtuse, oblique; follicles sparsely muricate, 7–11 cm; seeds 7–9 × 5–6 mm.	Asclepias sullivantii
63	Leaf blades 1–5 cm wide, apex acute to attenuate; pedicels 11–20 mm; anthers 2–2.5 mm; corona segments conduplicate, apex acute to obtuse and emarginate; follicles 0.9–1.6 cm wide.	> 65
65	Leaves sessile, blade base rounded to obtuse; flowers pendent; corollas green to greenish cream, sometimes tinged red; coronas green to yellowish green or greenish cream, segments sessile, apex obtuse, emarginate, appendage falcate, sharply inflexed towards style apex; seeds 6–7 × 4–5 mm; Illinois, Indiana, Iowa, Kansas, Missouri, Wisconsin.	Asclepias meadii
65	Petioles 1–2 mm; leaf blade bases cordate; flowers erect; corollas pink to reddish purple; coronas pink to lavender, segments stipitate, apex acute, appendage subulate, arching above style apex; seeds 7–9 × 5–7 mm; coastal plain, e United States to e Texas.	Asclepias rubra
57	Neither stems nor leaf blades glaucous.	> 66
66	Leaf blades 0.1–2 cm wide.	> 67
67	Stems 10–60 cm; corolla lobes 1.5–6 mm; corona segments 1.5–4 mm.	> 68
68	Leaf blades linear, 0.2–0.6 cm wide; flowers 4–10 per umbel; corollas green, sometimes tinged red, lobes pilose abaxially; corona segment apices truncate, appendage crestlike, barely exserted; fruiting pedicels upcurved; seeds 4–5 × 3–4 mm; s Arizona, s New Mexico.	Asclepias quinquedentata
68	Leaf blades linear to elliptic, oval, oblong, or ovate, 0.3–6 cm wide; flowers 7–31 per umbel; corollas white, cream, or pink, lobes glabrous abaxially; corona segment apices obtuse to acute, appendage acicular or subulate, well exserted; fruiting pedicels straight or declined; seeds 6–15 × 4–14 mm; e United States and Canada to e Texas.	> 69
69	Subshrubs or herbs; stems cespitose; interpetiolar ridges present; petioles 10–12 mm; pedicels 7–13 mm; flowers erect to spreading; corolla lobes 3–4 mm; corona segments tubular, rounded dorsally, 1.5–2.5 mm, appendage acicular, arching above style apex; fruiting pedicels declined, follicles pendulous, 4–7 × 1–2.5 cm; seeds 12–15 × 11–14 mm, coma absent.	Asclepias perennis
69	Herbs; stems solitary; interpetiolar ridges absent; petioles 2–7 mm; pedicels 17–28 mm; flowers spreading to pendent; corolla lobes 4–5 mm; corona segments conduplicate, flattened dorsally, 2.5–4 mm, appendage subulate, inflexed over style apex; fruiting pedicels straight, follicles erect, 8–16 × 0.4–1 cm; seeds 6–7 × 4–5 mm, coma present.	Asclepias quadrifolia
67	Stems 25–125 cm; corolla lobes 7–13 mm; corona segments 5–10 mm.	> 70
70	Leaf blades 2.5–8 cm; flowers spreading to pendent; corollas green, sometimes tinged brown at lobe tips, lobes 10–13 mm; anthers 3–3.5 mm; corona segments cream to pale green, 8–10 mm, clavate, incurved, appendage an included crest; follicles 11.5–15 cm.	Asclepias connivens
70	Leaf blades 5–25 cm; flowers erect; corollas red, pink, or reddish purple, lobes 7–10 mm; anthers 2–2.5 mm; corona segments yellow, reddish orange, pink, or lavender, 5–7 mm, tubular or conduplicate, appendage subulate, well exserted; follicles 5.5–12 cm.	> 71
71	Petioles 0–1 mm; leaf blades linear to linear-lanceolate, 0.2–1.7 cm wide, base cuneate; flowers 4–16 per umbel; corollas red, lobes 9–10 mm; corona segments yellow to reddish orange, tubular, apex obtuse, 5–6 mm, appendage sharply inflexed over style apex; follicles 0.8–1 cm wide.	Asclepias lanceolata
71	Petioles 1–2 mm; leaf blades narrowly lanceolate to ovate, 1–4.5 cm wide, base cordate; flowers 9–20 per umbel; corollas pink to reddish purple, lobes 7–9 mm; corona segments pink to lavender, conduplicate, apex acute, 6–7 mm, appendage arching above style apex; follicles 1–1.5 cm wide.	Asclepias rubra
66	Leaf blades 2–15 cm wide.	> 72
72	Leaf blade bases cuneate.	> 73
73	Petioles 0–1 mm; leaf blades linear to narrowly elliptic or oblanceolate, 0.3–2.5 cm wide; flowers 4–8 per umbel; pedicels 10–20 mm; corolla lobes 10–13 mm; corona segments 8–10 mm, clavate, appendage an included crest; coastal plain, s Alabama, Florida, s Georgia, se South Carolina.	Asclepias connivens
73	Petioles 2–15 mm; leaf blades ovate to elliptic or oblong, 1–11 cm wide; flowers 7–41 per umbel; pedicels 17–45 mm; corolla lobes 4–12 mm; corona segments 2.5–5 mm, tubular or conduplicate, appendage subulate or falcate, well exserted; inland e North America, including n Alabama, n Georgia, w South Carolina.	> 74
74	Stems 25–60 cm; petioles 2–7 mm; leaf blades 2.5–12 × 1–6 cm; pedicels 17–28 mm; corollas pink or cream, lobes 4–5 mm; anthers tan to brown, 1–1.5 mm; corona segments conduplicate, apex obtuse, appendage subulate; follicles 0.4–1 cm wide; seeds 6–7 mm.	Asclepias quadrifolia
74	Stems 65–150 cm; petioles 5–15 mm; leaf blades 10–24 × 2–11 cm; pedicels 25–45 mm; corollas green (rarely pink-tinged), lobes 6–12 mm; anthers green, 2.5–3.5 mm; corona segments tubular, apex truncate, appendage falcate; follicles 1.5–2 cm wide; seeds 8–10 mm.	Asclepias exaltata
72	Leaf blade bases truncate or rounded to cordate.	> 75
75	Peduncles and pedicels puberulent with curved trichomes in 1 line; flowers 9–20 per umbel; corollas pink to reddish purple; coronas pink to lavender, 6–7 mm, segment apices acute; follicles fusiform, 1–1.5 cm wide; coastal plain, e United States to e Texas.	Asclepias rubra
75	Peduncles tomentose, puberulent with curved trichomes, or pilosulous; pedicels tomentose or densely pilose to glabrate; flowers 12–80 per umbel; corollas green, ochroleucous, tan, or red-violet; coronas cream to ochroleucous, 2.5–5.5 mm, segment apices truncate; follicles lance-ovoid to ovoid or ellipsoid, 1.7–3.5 cm wide; w United States.	> 76
76	Stems 40–250 cm; leaf blades 7.5–25 cm, apex attenuate to acuminate, margins erose; umbels terminal and extra-axillary; corolla lobe apices tomentose abaxially; seeds 8–13 mm; Sonoran and Mohave deserts and surrounding areas, w Arizona, s California, s Nevada, sw Utah.	Asclepias erosa
76	Stems 25–100 cm; leaf blades 4.5–14.5 cm, apex truncate to rounded (sometimes acute), margins entire; umbels extra-axillary; corolla lobes densely tomentose throughout abaxially or glabrous; seeds 7–8 mm or 18–20 mm; e of the Sonoran and Mohave deserts, ne Arizona, s Utah to the Great Plains.	> 77
77	Peduncles 0–2.5 cm, puberulent with curved trichomes or pilosulous; pedicels 15–35 mm; corollas green, lobes 7–9 mm, glabrous abaxially; anthers 3–3.5 mm; corona segments 3–5.5 mm, apex papillose; fruiting pedicels upcurved; seeds 7–8 × 5–6 mm; ne Arizona, s Utah to the Great Plains, substrates various including sand.	Asclepias latifolia
77	Peduncles 2.5–6 cm, tomentose; pedicels 10–15 mm; corollas ochroleucous or tan to red-violet, lobes 5.5–6.5 mm, tomentose abaxially; anthers 1.5 mm; corona segments 2.5–3.5 mm, apex glabrous; fruiting pedicels spreading; seeds 18–20 × 9–10 mm; local and rare in n Arizona, s Utah on unstabilized sand dunes.	Asclepias welshii
56	Either stems and/or leaf blades bearing trichomes, sometimes restricted to veins.	> 78
78	Stems prostrate or decumbent.	> 79
79	Stems 5–15 cm.	> 80
80	Leaf blades 1.7–5 cm; corollas green, sometimes tinged red, pink, or brown; corona segments 2–10 mm; follicles 4.5–9.5 × 1.2–2.5 cm.	> 81
81	Leaf blades 1.2–6.5 cm wide, apex obtuse to rounded; umbels extra-axillary; corolla lobes 7–12 mm; anthers 2–2.5 mm; corona segments green, sometimes tinged bronze, apex white or cream, sinuous-tubular, 5–10 mm.	Asclepias oenotheroides
81	Leaf blades 0.2–2 cm wide, apex acute to attenuate; umbels terminal; corolla lobes 4.5–6.5 mm; anthers 1–2 mm; corona segments cream with pink or red dorsal stripe or yellow to ochroleucous, conduplicate or tubular, but not sinuous, 2–4.5 mm.	> 82
82	Leaf blades 0.2–0.8 cm wide; anthers 1.5–2 mm; corona segments cream, usually with a pink or red dorsal stripe, conduplicate, 3.5–4.5 mm; seeds 7–8 × 5–6 mm.	Asclepias involucrata
82	Leaf blades 0.5–2 cm wide; anthers 1–1.5 mm; corona segments yellow to ochroleucous, tubular, 2–3 mm; seeds 8–12 × 6–8 mm.	Asclepias macrosperma
80	Leaf blades 1.5–12 cm; corollas red-violet; corona segments 1–2 mm; follicles 3–6 × 0.5–1.5 cm.	> 83
83	Corona segment appendages falcate, not or barely exserted; Colorado Plateau, ne Arizona, nw New Mexico, Utah.	> 84
84	Leaf blades persistently pilosulous, more densely so on veins; pedicels and calyces densely pilosulous to tomentulose; corona segment appendages included; Utah, n of Colorado River and w of Green River, from San Rafael Reef to Waterpocket Fold.	Asclepias ruthiae
84	Leaf blades sparsely pilosulous to glabrate, persistently and densely hairy on veins and margins; pedicels and calyces pilosulous; corona segment appendages barely exserted; ne Arizona, nw New Mexico, just entering Utah in Monument Valley.	Asclepias sanjuanensis
83	Corona segment appendages lingulate or conical, barely exserted; Arizona, Colorado, Nevada, New Mexico, w Oklahoma, w Texas, not on the Colorado Plateau.	> 85
85	Leaf blades lanceolate to ovate, 0.6–3 cm wide, strigulose or pilosulous on midvein to glabrate; flowers 2–25 per umbel; anthers 1.5–2 mm; corona appendages conical; Nevada.	Asclepias eastwoodiana
85	Leaf blades linear to lanceolate, 0.2–1 cm wide, puberulent with curved trichomes on midvein; flowers 3–7 per umbel; anthers 1–1.5 mm; corona appendages lingulate; Arizona, Colorado, New Mexico, w Oklahoma, w Texas.	Asclepias uncialis
79	Stems 15–250 cm.	> 86
86	Leaf blades 0.1–1 cm wide.	> 87
87	Corolla lobes 8–11 mm, pilosulous abaxially; corona segments 5–7 mm, appendage papillose; follicles muricate ridged; s Texas.	Asclepias prostrata
87	Corolla lobes 3–6 mm, glabrous abaxially; corona segments 1.5–4.5 mm, appendage glabrous; follicles smooth or rugose; widespread, but not s Texas.	> 88
88	Stems 25–70 cm; corona segments laminar, margins incurved, append­age absent or obscure; follicles 8–13.5 × 0.7–1.2 cm.	Asclepias longifolia
88	Stems 5–30 cm; corona segments cupulate or conduplicate, margins not incurved, appendage conical or falcate, barely to well exserted; follicles 3.5–6 × 0.5–2 cm or 7.5–12.5 × 0.5–0.7 cm.	> 89
89	Corollas red-violet; corona segments 1.5–2 mm, appendage conical, barely exserted, follicles spreading to pendulous, strigose.	Asclepias eastwoodiana
89	Corollas green to tan, sometimes tinged pink or red; corona segments 2.5–4.5 mm, appendage falcate, well exserted, sharply inflexed over style apex; follicles erect, puberulent with curved trichomes to pilosulous.	> 90
90	Stems 5–18 cm; anthers brown; corona segments conduplicate, apex truncate, 3.5–4.5 mm; fruiting pedicels upcurved, fol­licles ovoid, rugose, 4.5–5.5 × 1.5–2 cm; c United States.	Asclepias involucrata
90	Stems 15–30 cm; anthers green; corona segments cupulate, apex obtuse, 2.5–3.5 mm; fruiting pedicels straight, follicles narrowly fusiform, smooth, 7.5–12.5 × 0.5–0.7 cm; se United States.	Asclepias michauxii
86	Leaf blades 1–15 cm wide.	> 91
91	Corona segments 5–11 mm, greatly exceeding style apex, tubular, sinuous-tubular, or conduplicate-tubular, appendage papillose (rarely glabrous or glabrate).	> 92
92	Stems, leaf blades, peduncles, and pedicels pilosulous to tomentulose; petioles 2–3 mm, leaf blades 1.8–5 × 0.4–1.8 cm; flowers 3–8 per umbel; pedicels 8–14 mm; gynostegial column 3–3.5 mm; follicles pendent, muricate-ridged, tomentulose.	Asclepias prostrata
92	Stems, leaf blades, peduncles, and pedicels puberulent with curved trichomes or hirtellous; petioles 2–25 mm; leaf blades 4–15 × 1.2–7.5 cm; flowers 5–32 per umbel; pedicels 10–30 mm; gynostegial column 0.3–1.5 mm; follicles erect, smooth, puberulent with curved trichomes or hirtellous.	> 93
93	Leaf blades ovate or lanceolate to oblong or elliptic, base cuneate to obtuse, apex obtuse to rounded, sometimes conduplicate; pedicels 10–20 mm; gynostegial column 1–1.5 mm; anthers 2–2.5 mm; corona segments relatively slender, green, sometimes tinged bronze, apex white or cream, deeply emarginate; se Arizona, sw Colorado, w Louisiana, New Mexico, Oklahoma, Texas.	Asclepias oenotheroides
93	Leaf blades ovate to lanceolate, base cuneate or obtuse to truncate or sub­cordate, apex obtuse to acute, rarely conduplicate; pedicels 17–30 mm; gyno­stegial column 0.3–0.5 mm; anthers 1.7–2 mm; corona segments relatively stout, cream or green with cream apex, apex truncate; Arizona, se California, s Nevada, sw New Mexico.	Asclepias nyctaginifolia
91	Corona segments 1.5–6 mm, shorter than to slightly exceeding style apex, cupulate or conduplicate, appendage glabrous.	> 94
94	Corolla lobes 4–6 mm; corona segments 1.5–3 mm.	> 95
95	Leaf blades ovate to nearly orbiculate, 3–4 cm wide, base obtuse to cordate; flowers 20–55 per umbel; corolla lobes 5–6 mm; corona segments conduplicate, 2–3 mm, appendage absent; follicles 6–10 × 2–3 cm.	Asclepias solanoana
95	Leaf blades narrowly lanceolate to ovate, 0.6–3 cm wide, base cuneate to obtuse; flowers 2–25 per umbel; corolla lobes 4–5 mm; corona segments cupulate, 1.5–2 mm, appendage conical, barely exserted; follicles 3.5–6 × 0.5–1 cm; Nevada.	Asclepias eastwoodiana
94	Corolla lobes 6–11 mm; corona segments 3–6 mm.	> 96
96	Flowers 5–21 per umbel; corolla lobes pilose at base adaxially; gynostegial column 2–3.5 mm; corona segments red-violet, margins connivent, papillose, appendage absent.	Asclepias californica
96	Flowers 12–50 per umbel; corolla lobes papillose or glabrous at base adaxially; gynostegial column 1–1.5 mm; corona segments cream to ochroleucous or dark pink, margins separate, glabrous, appendage falcate.	> 97
97	Petioles 3–10 mm; leaf blade margins entire; peduncles 0–3.5 cm; corolla lobes densely tomentose abaxially throughout; anthers dark brown, 1.5–2 mm; corona segments sessile, apex obtuse, oblique; follicles 5–6.5 cm.	Asclepias vestita
97	Petioles 0–6 mm; leaf blade margins erose; peduncles 2–10 cm; corolla lobes tomentose abaxially only at tips; anthers green, 2.5–3 mm; corona segments stipitate, apex truncate; follicles 6.5–10 cm.	Asclepias erosa
78	Stems spreading to erect.	> 98
98	Corolla lobes 3–5 mm.	> 99
99	Corona segments laminar or chute-shaped, margins incurved, tightly appressed to gynostegium; appendage absent or obscure.	> 100
100	Stems 7–20 cm; leaf blades oblong or lanceolate to narrowly lanceolate, 4–8 cm, hirtellous; umbels terminal; anthers 1.5–2 mm; seeds 6–7 × 4–5 mm.	Asclepias lanuginosa
100	Stems 25–70 cm; leaf blades linear to linear-lanceolate, 4–18 cm, scabrid­ulous to puberulent with curved trichomes; umbels terminal and/or extra-axillary; anthers 1–1.5 mm; seeds 10–12 × 7–8 mm.	> 101
101	Flowers 34–112 per umbel; pedicels and calyces hirtellous; corona segments 1.5–2 mm, not exceeding point of anther wings; follicles 1–2 cm wide.	Asclepias hirtella
101	Flowers 13–28(–36) per umbel; pedicels and calyces puberulent with curved trichomes; corona segments 2–2.5 mm, exceeding point of anther wings; follicles 0.7–1.2 cm wide.	Asclepias longifolia
99	Corona segments conduplicate, cupulate, or tubular, margins not incurved, not appressed to gynostegium; appendage falcate, acicular, subulate, conical, lingulate, or crestlike, barely to well exserted.	> 102
102	Stems, leaf blades, and peduncles tomentose, at least initially.	> 103
103	Stems 6–15 cm; leaf blades orbiculate to obovate or oblate, 1.5–3.2 × 1.7–4 cm; peduncles 2.7–6 cm; corolla lobes glabrous abaxially; anthers 1–1.5 mm; corona segments conduplicate, appendage fal­cate; follicles smooth.	Asclepias nummularia
103	Stems 20–40 cm; leaf blades linear-lanceolate, 5–15 × 0.3–1.3 cm; peduncles 0.2–1.5 cm; corolla lobes minutely pilosulous abaxially; anthers 2–2.5 mm; corona segments tubular, appendage lingulate; follicles ribbed.	Asclepias brachystephana
102	Stems, leaf blades, peduncles, pedicels, and calyces puberulent with curved trichomes, pilose, strigulose, hirtellous, or glabrous.	> 104
104	Leaf blades linear, 0.1–0.8 cm wide.	> 105
105	Flowers pendent to spreading; corolla lobes pilose abaxially; corona segments conduplicate, sessile, apex truncate, append­age crestlike, barely exserted; fruiting pedicels upcurved.	Asclepias quinquedentata
105	Flowers erect; corolla lobes glabrous abaxially; corona seg­ments tubular or cupulate, stipitate, apex obtuse, appendage falcate or acicular, well exserted; fruiting pedicels straight.	> 106
106	Stems few–numerous, 20–60 cm; leaf blades spreading, not secund, midvein puberulent with curved trichomes; umbels all extra-axillary; corollas cream, sometimes tinged or striped pink; anthers brown; corona segments tubular, appendage acicular, arching above style apex; follicles glabrous; se Arizona.	Asclepias angustifolia
106	Stems 1–4 (rarely more), 15–30 cm; leaf blades ascending, appearing secund, midvein glabrous; umbels terminal, some­times also extra-axillary; corollas green to tan, tinged pink or red; anthers green; corona segments cupu­late, appendage fal­cate, sharply inflexed over style apex; follicles minutely puber­ulent with curved trichomes; se United States.	Asclepias michauxii
104	Leaf blades narrowly elliptic, oval, oblong, lanceolate, ovate, oblanceolate, or obovate, 0.3–6 cm wide.	> 107
107	Corollas white, pink, or cream; corona segment appendages acicular or subulate.	> 108
108	Interpetiolar ridges absent; petioles 2–7 mm; leaf blades 1–6 cm wide; umbels terminal (sometimes also extra-axillary); flowers spreading to pendent; pedicels 17–28 mm; corona segments conduplicate, sessile, appendage subulate.	Asclepias quadrifolia
108	Interpetiolar ridges present; petioles 5–20 mm; leaf blades 0.3–3 cm wide; umbels extra-axillary (may appear terminal); flowers erect to spreading; pedicels 7–14 mm; corona segments cupulate or tubular, stipitate, appendage acicular.	> 109
109	Leaf blades narrowly elliptic to oval or oblong, 5–14 cm; corona segments tubular, 1.5–2.5 mm; follicles pendulous on deflexed pedicels, lance-ovoid, 4–7 × 1–2.5 cm; seeds 12–15 × 11–14 mm, coma absent; bottomlands and ditches, e United States to e Texas.	Asclepias perennis
109	Leaf blades ovate to lanceolate or elliptic, 2–7 cm; corona segments cupulate, 2–3 mm; follicles erect on straight pedicels, narrowly fusiform, 9–13 × 0.5–1 cm; seeds 6–7 × 4–5 mm, coma present; canyons and slopes; c, w Texas.	Asclepias texana
107	Corollas green, sometimes tinged red, purple, or bronze, or red-violet; corona segment appendages falcate or conical.	> 110
110	Corollas red-violet; corona segments cupulate, 1.5–2 mm, appendage conical, barely exserted; fruiting pedicels spreading, follicles spreading to pendulous, lance-ovoid, 3.5–6 cm, strigose; Nevada.	Asclepias eastwoodiana
110	Corollas green, tinged purple or bronze, rarely reddish purple; corona segments tubular or conduplicate, 2–6 mm, appendage falcate, well exserted; fruiting pedicels upcurved, follicles erect, fusiform, 5–10.5 cm, pilosulous or puberulent with curved trichomes; Florida or w Texas.	> 111
111	Stems 15–20 cm, pilosulous; leaf blades 6–8 cm, sparsely hirtellous, mar­gins crisped; umbels terminal; peduncles 7–17 cm; pedicels 15–20 mm, pilose; anthers brown, 1.5–2 mm; corona segments tubular, 2–3 mm, apex truncate; w Texas.	Asclepias scaposa
111	Stems 15–100 cm, puberulent with curved trichomes; leaf blades 1.8–5 cm, sparsely puberulent with curved trichomes on midvein only, margins entire; umbels terminal and extra-axillary; peduncles 0–4 cm; pedicels 10–14 mm, sparsely puberulent with curved trichomes; anthers green, 1–1.5 mm; corona segments conduplicate, 5–6 mm, apex attenuate; Florida.	Asclepias curtissii
98	Corolla lobes 5–15 mm (rarely 4.5 mm in A. incarnata, with corolla and corona pink to white, corona segments 2–2.5 mm, and fruiting pedicels straight).	> 112
112	Stems, leaf blades, peduncles, or pedicels persistently tomentose or tomentulose.	> 113
113	Corona segments 7–15 mm.	> 114
114	Petioles 2–6 mm; leaf blade surfaces adaxially pilosulous or tomentulose to glabrate, abaxially tomentose; corolla lobes deep maroon to green adaxially, glabrous adaxially; anthers 2–2.5 mm; coronas deep maroon to yellowish green, segments 7–9 mm, appendage absent or merely a low crest; follicles fusiform, 1.2–1.4 cm wide; extreme se Arizona and sw New Mexico.	Asclepias hypoleuca
114	Petioles 4–12 mm; leaf blade surfaces uniformly green, tomentose to pilose; corolla lobes dark pink (rarely pale), hirtellous at base adaxially; anthers 2.5–3 mm; coronas pale pink to nearly cream, segments 9–15 mm, appendage subulate, sharply inflexed over style apex; follicles lance-ovoid, 2–3 cm wide; widespread in w United States and Canada, but not in se Arizona and sw New Mexico.	Asclepias speciosa
113	Corona segments 2–7 mm.	> 115
115	Corolla lobes glabrous abaxially.	> 116
116	Corollas pink, red green, or pinkish green, or reddish green; anthers brown; corona segments cream with red, pink, or purple dorsal stripe, apex acute, appendage glabrous; Rocky Mountains and associated alluvial plains, 1700–3000 m.	Asclepias hallii
116	Corollas green to yellowish green, sometimes tinged red or purple; anthers green; corona segments green with cream apex, sometimes tinged pink or purple, or cream, greenish cream, or ochroleucous, not striped red, pink, or purple, apex truncate to rounded, appendage papillose; se United States, Great Plains, or Colorado Plateau, to 2300 m.	> 117
117	Corona segments green with cream apex, sometimes tinged pink or purple; corona segment apices glabrous; follicles fusiform to narrowly lance-ovoid, 1–1.5 cm wide; se United States and e Texas.	Asclepias tomentosa
117	Corona segments cream to greenish cream or ochroleucous, apices papillose; follicles lance-ovoid to ovoid, 2–3 cm wide; Great Plains and Colorado Plateau to e Texas.	> 118
118	Petioles 7–17 mm; leaf blade bases rounded, truncate, or subcordate; anthers 2–2.5 mm; corona segment apices truncate to rounded, emarginate, not oblique, appendage apex upturned; seeds 9–12 × 6–8 mm.	Asclepias arenaria
118	Petioles 0–4 mm; leaf blade bases cordate; anthers 3–3.5 mm; corona segment apices truncate, not emarginate, oblique, appendage apex not upturned; seeds 7–8 × 5–6 mm.	Asclepias latifolia
115	Corolla lobes pilose, pilosulous, or tomentose abaxially, at least at tips.	> 119
119	Leaf blades linear-lanceolate, 0.3–1.3 cm wide; corolla lobes 4–6 mm; corona segments 1.5–2 mm; follicles conspicuously striate.	Asclepias brachystephana
119	Leaf blades ovate or oval to lanceolate, elliptic, oblong, or obovate, 1.5–15 cm wide; corolla lobes 5–11 mm; corona segments 2.5–6 mm; follicles not striate.	> 120
120	Corolla lobes pilose at base adaxially; gynostegial column 2–3.5 mm; corona segments red-violet, dorsally convex from base to apex, margins connivent, appendage absent.	Asclepias californica
120	Corolla lobes glabrous, minutely hirtellous, or papillose at base adaxially; gynostegial column 0.2–1.5 mm; corona segments cream or ochroleucous to dark pink (sometimes reddish purple in A. syriaca), dorsally straight or concave from base to apex, margins separate, appendage falcate or acicular.	> 121
121	Leaf blades 3–9 × 1.5–4.5 cm, pilosulous; flowers 5–21 per umbel; s Canada (Alberta to Ontario), n United States (Montana and Wyoming e to Illinois and Michigan).	Asclepias ovalifolia
121	Leaf blades 6–30 × 2–15 cm, tomentose or puberulent with curved trichomes to glabrate (sometimes pilosulous in A. syriaca); flowers 12–113 per umbel; Arizona, California, Nevada, and Utah, or widespread in e United States and Canada, w to Manitoba, North Dakota, and South Dakota (A. syriaca).	> 122
122	Pedicels 10–15 cm; corona appendages acicular; fruiting pedicels spreading, follicles pendulous; extreme n Arizona and s Utah.	Asclepias welshii
122	Pedicels 15–50 cm; corona appendages falcate; fruiting pedicels upcurved, follicles erect; w Arizona, California, s Nevada, and sw Utah, or e United States and Canada.	> 123
123	Pedicels pilose to puberulent with curved trichomes; corona segments slightly flattened dorsally, apex obtuse and usually flared, not oblique; follicles muricate (less commonly smooth); e United States and Canada.	Asclepias syriaca
123	Pedicels tomentose to glabrate; corona segments rounded dorsally, apex truncate or obtuse and oblique, not flared; follicles smooth or longitudinally ridged; Arizona, California, Nevada, and Utah.	> 124
124	Peduncles 0–3.5 cm; corolla lobes tomentose abaxially throughout; anthers 1.5–2 mm; corona segments sessile, apex obtuse; follicles ovoid, 5–6.5 cm, ridged.	Asclepias vestita
124	Peduncles 1–10 cm; corolla lobes tomentose abaxially only at tips; anthers 2.5–3 mm; corona segments stipitate, apex truncate; follicles lance-ovoid to ovoid, 5–10 cm, smooth.	> 125
125	Petioles 0–6 mm; leaf blade margins erose; corollas green; anthers green; coronas cream to ochroleucous, segment apices and appendages glabrous; follicles thinly tomentose; seeds 8–13 × 5–10 mm.	Asclepias erosa
125	Petioles 4–15 mm; leaf blade margins entire or undulate; corollas greenish cream, ochroleucous, pink, tan, or pinkish cream; anthers dark brown; coronas cream to dark pink, segment apices and appendages papillose; follicles densely tomentose, seeds 7–9 × 4–6 mm.	Asclepias eriocarpa
112	Stems, leaf blades, peduncles, and pedicels variously indumented, sometimes ephemerally tomentose or tomentulose.	> 126
126	Stems, leaf blades, peduncles, and pedicels hirsute or hirtellous.	> 127
127	Stems 100–150 cm; leaf blades 3–14 cm wide; umbels terminal and extra-axillary; peduncles 6–13 cm; flowers 21–53 per umbel; corollas cream to greenish cream or ochroleucous, sometimes tinged pink; corona segments conduplicate; se Arizona.	Asclepias lemmonii
127	Stems 6–70 cm (rarely to 200 cm in A. obovata); leaf blades 0.4–7.5 cm wide; umbels extra-axillary (sometimes appearing terminal); peduncles 0–2 cm; flowers 4–32 per umbel; corollas green, sometimes tinged red, purple, brown, or bronze; corona segments tubular; s United States, including se Arizona.	> 128
128	Stems 40–70(–200) cm; flowers erect to pendent; anthers 2.5–4 mm; corona segments bronze or yellow, often tinged red, sometimes paler or cream at apex, appendage falcate, glabrous; seeds 8–9 × 6–7 mm; s Canada (Alberta to Ontario), n United States (Montana and Wyoming e to Illinois and Michigan).	Asclepias obovata
128	Stems 6–50 cm; flowers erect to ascending; anthers 1–2.5 mm; corona segments cream or green with cream or white at apex, sometimes tinged bronze, appendage lingulate, papil­lose or glabrous; seeds 6–8 × 4.5–6.5 mm; California to w Louisiana, n to c Oklahoma.	> 129
129	Leaf blades 0.4–2.5 cm wide, base cuneate; flowers 4–8 per umbel; corolla lobes 5–7 mm; anthers green, 1–1.5 mm; corona segments 3.5–5.5 mm, appendage flush with apex and obscuring cavity.	Asclepias emoryi
129	Leaf blades 1.2–7.5 cm wide, base cuneate to subcordate; flowers 5–32 per umbel; corolla lobes 7–13 mm; anthers brown, sometimes green at base, 1.7–2.5 mm; corona segments (5–)7–11 mm, appendage exserted below apex, exposing cavity.	> 130
130	Leaf blades ovate or lanceolate to oblong or elliptic, base cuneate to obtuse, apex obtuse to rounded, sometimes conduplicate; pedicels 10–20 mm; gynostegial column 1–1.5 mm; anthers 2–2.5 mm; corona segments green, sometimes tinged bronze, white or cream at apex, relatively slender, apex deeply emarginate; se Arizona, sw Colorado, w Louisiana, New Mexico, Oklahoma, Texas.	Asclepias oenotheroides
130	Leaf blades ovate to lanceolate, base cuneate or obtuse to truncate or subcordate, apex obtuse to acute, rarely con­du­plicate; pedicels 17–30 mm; gynostegial column 0.3–0.5 mm; anthers 1.7–2 mm; corona segments cream or green with cream apex, relatively stout, apex truncate; Arizona, se California, s Nevada, sw New Mexico.	Asclepias nyctaginifolia
126	Stems, leaf blades, peduncles, and pedicels variously indumented, but trichomes not straight and spreading.	> 131
131	Corona appendages crestlike, included or barely exserted from segment, or absent.	> 132
132	Corolla lobes 10–13 mm; corona segments clavate, 8–10 mm, greatly exceeding style apex, segment apices sometimes connivent.	Asclepias connivens
132	Corolla lobes 4–10 mm; corona segments conduplicate or laminar with incurved margins, 1.5–4 mm, barely exceeding to greatly exceeded by style apex, segment apices separated by gynostegium.	> 133
133	Leaf blades linear to broadly oval or nearly orbiculate, 0.8–6 cm wide; flowers 22–60 per umbel; anthers 3–4 mm; corona segments laminar, appressed to gynostegium, margins incurved; follicles fusiform to lance-ovoid, 1.5–2 cm wide.	Asclepias viridiflora
133	Leaf blades linear to narrowly elliptic or narrowly lanceolate, 0.1–0.8 cm wide; flowers 2–10 per umbel; anthers 1–2 mm; corona conduplicate, margins not incurved, not appressed to gynostegium; follicles fusiform to lance-ovoid, 0.3–1 cm wide.	> 134
134	Leaf blades linear, 6–14 cm; peduncles 1.1–2.7 cm; flowers pendent to spreading; pedicels 14–21 mm; corolla lobes reflexed, exposing corona, 4–6 mm, pilose abaxially; anthers tan to brown; corona segments red, pink, red-violet, or purple at base, white at apex, 3–4 mm; appendage crestlike, barely exserted; fruiting pedicels upcurved, follicles 0.5–1 cm wide, pilosulous to glabrate.	Asclepias quinquedentata
134	Leaf blades linear to narrowly elliptic or narrowly lanceolate, 1.5–6 cm; peduncles 0.15–1 cm; flowers erect; pedicels 5–13 mm; corolla lobes erect, mostly concealing corona, 7–10 mm, glabrous abax­ially; anthers green; corona segments yellowish green to green, 1.5–2.5 mm, appendage absent; fruiting pedicels straight, follicles 0.3–0.5 cm wide, puberulent with curved trichomes.	Asclepias pedicellata
131	Corona appendages falcate or acicular, well exserted.	> 135
135	Corona segment apices truncate.	> 136
136	Petioles 10–25 mm; corollas white with red-violet at base of lobes; anthers brown, 1.5–2 mm; seeds 5–7 × 3–5 mm.	Asclepias variegata
136	Petioles 0–15 mm; corollas green, sometimes tinged pink, red, or purple; anthers green, 2.5–4 mm; seeds 6.5–13 × 4–10 mm.	> 137
137	Petioles 5–15 mm; leaf blades membranous, base cuneate; corona segments tubular, appendage arching above style apex; ne United States to n Alabama, n Georgia, w North Carolina, w South Carolina.	Asclepias exaltata
137	Petioles 0–9 mm; leaf blades chartaceous, coriaceous, or succulent, base obtuse to cordate; corona segments conduplicate, appendage sharply inflexed over or towards style apex; w United States or s Alabama, Florida, s Georgia, e North Carolina, e South Carolina, and e Texas.	> 138
138	Leaf blade apices attenuate to acuminate, margins erose; umbels terminal and extra-axillary; peduncles 2–10 cm; pedicels 20–45 mm; corolla lobes tomentose abaxially at tips; anthers 2.5–3 mm; corona segment appendages glabrous; seeds 8–13 × 5–10 mm; n Arizona, s California, s Nevada, sw Utah.	Asclepias erosa
138	Leaf blade apices acute to truncate, sometimes emargin­ate, margins entire or crisped; umbels only extra-axillary; peduncles 0–2.5 cm; pedicels 12–35 mm; corolla lobes gla­brous abaxially; anthers 3–4 mm; corona seg­ment appendages papillose; seeds 6.5–8 × 4.5–6 mm; se United States or c United States to n Arizona, s Utah.	> 139
139	Petioles 2–9 mm; leaf blades 1–5 cm wide; peduncles 0–0.3 cm; pedicels 12–19 mm; corona segments green with cream apex, often tinged pink or purple; follicles fusiform to narrowly lance-ovoid, 9–18 × 1–1.5 cm; se United States and e Texas.	Asclepias tomentosa
139	Petioles 0–4 mm; leaf blades 3–14 cm wide; peduncles 0–2.5 cm; ped­icels 15–35 mm; corona segments cream, sometimes yellow dorsally; follicles ovoid, 6.5–9.5 × 2–3 cm; c United States.	Asclepias latifolia
135	Corona segment apices acute to obtuse.	> 140
140	Stems often branched, at least in inflorescence; interpetiolar ridges present; corona segment appendages arching above style apex; fruiting pedicels straight.	> 141
141	Corollas red, sometimes yellow in throat (may be uniformly yellow or orange in cultivars), lobes 6–9 mm; gynostegial column 2–2.5 mm; corona segments yellow to orange, 3.5–4 mm, appendage falcate; follicles glabrous; seeds 6–7 × 4–5 mm; California, Florida, Louisiana, Texas.	Asclepias curassavica
141	Corollas pink to white, lobes 5–6 mm; gynostegial column 1.2–1.5 mm; corona segments pink to white, 2–2.5 mm, appendage acicular; follicles sparsely puberulent with curved trichomes to pilose or pilosulous; seeds 8–9 × 5–6 mm; widespread in United States and Canada.	Asclepias incarnata
140	Stems unbranched (rarely branched in A. syriaca, branched only in the inflorescence in A. hallii, A. purpurascens); interpetiolar ridges absent; corona segment appendages sharply inflexed over style apex (sometimes ascending in A. ovalifolia); fruiting ped­icels upcurved.	> 142
142	Stems 30–70 cm; flowers 5–29 per umbel; gynostegial column 0.2–1 mm; follicles 0.7–2 cm wide; seeds 5–7 mm.	> 143
143	Petioles 10–20 mm; corollas pale pink to red or green, tinged pink or red, lobes 7–8 mm, glabrous abaxially; anthers brown, 2–2.5 mm; corona seg­ments cream with red, pink, or purple dorsal stripe, 5–6.5 mm; follicles 8–12 cm; Rocky Mountains and associated alluvial plains.	Asclepias hallii
143	Petioles 3–10 mm; corollas cream to yellowish, lobes 5–7 mm, pilosulous abaxially at apices; anthers green, 1.5–2 mm; corona segments cream to yellowish, 3–4 mm; follicles 5–8 cm; s Canada (Alberta to Ontario), n United States (Montana and Wyoming e to Illinois and Michigan).	Asclepias ovalifolia
142	Stems 50–200 cm; flowers 17–113 per umbel; gynostegial column 1–1.5 mm; follicles 1.5–4 cm wide; seeds 7–8 mm.	> 144
144	Adaxial leaf blades sparsely pilosulous; umbels terminal and extra-axillary; flowers erect to spreading; corollas reddish purple, lobes glabrous abaxially; anthers brown; corona segments reddish purple, stipitate, conduplicate, apex acute, 5–6 mm; follicles 10–16 × 1.5–2.5 cm, smooth, puberulent with curved trichomes.	Asclepias purpurascens
144	Adaxial leaf blades tomentose to glabrate; umbels only extra-axillary; flowers erect to pendent; corollas dark to pale pink or green, lobes pilose abaxially; anthers green; corona segments reddish purple to cream, sessile, tubular, apex obtuse, flared, 4–5 mm; follicles 7–12 × 2–4 cm, muricate (sometimes smooth), tomentose.	Asclepias syriaca