Taxon	Illustrator ⠉
	Cynara cardunculus subsp. flavescens Carduus acanthoides subsp. acanthoides Cirsium discolor	John Myers John Myers John Myers
	Cirsium douglasii var. breweri Cirsium horridulum var. megacanthum Cirsium remotifolium var. odontolepis	John Myers John Myers John Myers
	Cirsium occidentale var. coulteri Cirsium occidentale var. californicum Cirsium occidentale var. candidissimum	John Myers John Myers John Myers
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Annuals, biennials, or perennials, 5–400 cm, spiny. Stems (1–several) erect, branched or simple, sometimes narrowly spiny-winged. Leaves basal and cauline; finely bristly-dentate to coarsely dentate or 1–3 times pinnately lobed, teeth and lobes bristly-tipped, faces green and glabrous or densely gray-canescent, usually eglandular. Heads discoid, borne singly, terminal and in distal axils, or in racemiform, spiciform, subcapitate, paniculiform, or corymbiform arrays. (Peduncles with ± reduced leaflike bracts.) Involucres cylindric to ovoid or spheric, (1–6 ×) 1–8 cm. Phyllaries many in 5–20 series, subequal or weakly to strongly, outer and middle with bases appressed and apices spreading to erect, usually spine-tipped, innermost usually with erect, flat, often twisted, entire or dentate, usually spineless apices (distal portion of phyllary midveins in many species with elongate, glutinous resin-gland, usually milky in fresh material but dark-brown to black when dry). Receptacles flat to convex, epaleate, covered with tawny to white bristles or setiform scales. Florets 25–200+; corollas white to pink, red, yellow or purple, ± bilateral, tubes long, slender, distally bent, throats short, abruptly expanded, cylindric, lobes linear; (filaments distinct) anther bases sharply short-tailed, apical appendages linear-oblong; style tips elongate (as measured in descriptions including the slightly swollen nodes, long cylindric fused portions of style-branches and very short distinct portions). Cypselae ovoid, ± compressed, with apical rims, smooth, not ribbed, glabrous, basal attachment scars slightly angled; pappi persistent or falling in rings, in 3–5 series of many flattened, plumose bristles or plumose, setiform scales (longer bristles shorter than corollas except in C. foliosum and C. arvense). x = 17.

Distribution

North America, Eurasia, n Africa

Discussion

Species ca. 200 (62 in the flora).

Only three genera in Cynareae are represented by native species in the New World, and of these Cirsium is by far the most widely distributed and diverse. Native species of Cirsium range from sea level to alpine and from boreal regions of Canada to the tropics of Central America. Members of the genus occur in a myriad of habitats including swamps, meadows, forests, prairies, sand dunes, and deserts.

Preliminary molecular phylogenetic studies by D. G. Kelch and B. G. Baldwin (2003) indicated that this diversity is the product of a rapid evolutionary diversification based upon a single initial introduction from Eurasia. Relationships among the North American species are apparently complex, and molecular studies have only begun to provide an outline of phylogeny for these plants. Although there has been a remarkable evolutionary and morphologic diversification in North American Cirsium, it has not been accompanied by very much divergence in the base sequences of genes commonly used to elucidate phylogenetic relationships. This suggests either that the diversification has been very rapid or that genetic markers in North American Cirsium mutate more slowly than in most other lineages.

Chromosomal diversification has accompanied the morphologic radiation of North American Cirsium. Many New World Cirsium species share the chromosomal base number of x = 17 that also predominates in most Eurasian species. Among the North American thistles, however, is a mostly descending dysploid series with chromosome numbers ranging from n = 18 to n = 10. Very few instances of polyploidy are known among New World Cirsium.

Cirsium species of remarkably different morphologies often are able to hybridize. Although in some hybrid combinations fertility is reduced, in others the formation of complex hybrid swarms indicates a lack of breeding barriers and the potential for emergence of novel character combinations. In the absence of adequate sampling and field observations, hybrids may go unrecognized—treated as distinct taxa or as variants of non-hybrid taxa, or left occupying the indeterminate folders of herbaria. In other cases hybridization has been invoked without much evidence as an explanation for Cirsium variants encountered in herbaria or in the field. Hybrid combinations are listed herein when evidence is convincing. Additional hybrids are likely to be found where the ranges of Cirsium species overlap. I have seen no documentation of hybridization between native American Cirsium species and introduced Eurasian taxa.

Much of the geographic range currently occupied by New World Cirsium species was greatly affected by the events of the Quaternary. Large areas were glaciated and other areas were vastly different during glacial episodes. The ancestors of thistles that currently occupy the high mountains of western North America were undoubtedly displaced elevationally and/or latitudinally during the recurrent glacial and interglacial episodes of the Pleistocene. Taxa that are currently isolated may have been in contact during glacial episodes with the opportunity for hybridization and genetic interchange. Episodes of prehistoric hybridization may have led to some of the character combinations found in modern American thistles, particularly in the western half of the continent. Current isolation and localized selection or genetic drift apparently have promoted differentiation of populations separated on mountaintop islands.

One of the most challenging aspects for a taxonomist studying New World Cirsium is the presence of species complexes that are apparently evolutionary works in progress. Some of the thistles, especially in the mountainous western part of North America, are frustratingly polymorphic with much overlapping variability and intergradation of characters. Early taxonomists, basing their work on a limited sampling of the morphologic diversity, named many of the forms as species, and the literature is rife with species names. The infilling that results from more collectors visiting more localities within the ranges of these complexes has blurred the boundaries between many of the proposed species and often added forms that do not “fit” the characteristics of named species. As I faced the challenges of preparing this treatment, I recognized that maintaining some of the named entities as species would, for consistency, require a further proliferation of species names. I have chosen to go the other way. Instead of proposing yet more ill-defined microspecies, I have chosen to recognize that the groups in question are rapidly evolving, only partially differentiated assemblages of races that have not reached the level of stability that is usually associated with the concept of species. Certainly much of such variation within the genus deserves a level of taxonomic recognition, or at least should be mentioned, but for those assemblages I think it much more prudent to recognize varieties—entities that may be expected to freely intergrade—rather than species.

Many problems remain to be worked out in North American Cirsium. Further investigation will undoubtedly reveal the need for refinement or major revision within some of the species groups. Studies that focus on variation within and among populations and on the biological basis for the variations are much needed. The field is open and the challenges are many.

Preparation of a workable key to Cirsium species has been frustratingly difficult. Extensive and overlapping ranges of variation in morphologic characteristics often require that a species be keyed two or more times. The resulting key is longer and more complex than I would prefer, and I have no doubt ignored, overlooked, or been completely unaware of variants that will not key out. Caveat clavitor!

The reputation of Cirsium has suffered greatly as a result of the introduction to North America of a few invasive weedy species from Eurasia. Cirsium vulgare (bull thistle) and C. arvense (Canada thistle—a misnomer) have long been despised as noxious weeds. In recent years C. palustre (European swamp thistle) has joined their ranks. Additionally, weedy Eurasian species of Carduus, Onopordum, Centaurea, etc., add to the public perception that all thistles are bad. Most North American native Cirsium are not at all weedy, and many are strikingly attractive plants. All are spiny plants that command respect, but they deserve a better reputation as one of North America’s evolutionary success stories.

Native Cirsium species have come under threat from biocontrol programs instituted to suppress populations of weedy introduced thistles. Beginning in 1968 the seedhead weevil Rhinocyllus conicus has been widely introduced in various areas of the United States and Canada, primarily to control weedy species of Carduus. S. M. Louda et al. (1997) reported that R. conicus has crossed over to several native species of Cirsium. They observed that the number of viable cypselae in infested heads was greatly reduced; e.g., heads of C. canescens infested by R. conicus produced 14.1 percent of the number of viable cypselae as in uninfested heads. Not all taxa are impacted as much as C. canescens, particularly those with later flowering phenology (Louda 1998). R. W. Pemberton (2000) reported that 22 Cirsium taxa in North America are known hosts of R. conicus. I suspect that the number is higher. During my field work I have observed that the heads of many Cirsium species are heavily parasitized, although I have not determined which of these are infested by R. conicus and which by native seedhead parasites. The long-term impacts of R. conicus and other biocontrol agents on native thistles, particularly rare taxa, remain to be determined.

Excluded species:

Cirsium canum (Linnaeus) Allioni was reported in 1924 from Massachusetts by C. H. Knowlton and W. Deane (Rhodora 26: 84) based on an 1899 collection from Weston. It has not been included in subsequent local or regional floras and apparently is not established in the flora area.

Cirsium scabrum (Poiret) Bonnet & Barratte was reported [as Cnicus giganteus (Desfontaines) Willdenow] in 1900 by A. Eastwood (Zoë 5: 59) based on a 1900 collection from Santa Cruz County, California. J. T. Howell (1959, 1960b) noted that Cirsium scabrum had not become naturalized. It apparently is not established in the flora area.

Selected References

Hsi, Y.-T. 1960. Taxonomy, Distribution and Relationships of the Species of Cirsium Belonging to the Series Undulata. Ph.D. dissertation. University of Minnesota.

Kelch, D. G. and B. G. Baldwin. 2003. Phylogeny and ecological radiation of New World thistles (Cirsium, Cardueae–Compositae) based on ITS and ETS rDNA sequence data. Molec. Ecol. 12: 141–151.

Moore, R. J. and C. Frankton. 1969. Cytotaxonomy of some Cirsium species of the eastern United States, with a key to eastern species. Canad. J. Bot. 47: 1257–1275.

Petrak, F. 1917. Die nordamerikanischen Arten der Gattung Cirsium. Beih. Bot. Centralbl. 35(2): 223–567.

Lower Taxa

Cirsium altissimum, Cirsium andersonii, Cirsium andrewsii, Cirsium arizonicum, Cirsium arvense, Cirsium barnebyi, Cirsium brevifolium, Cirsium brevistylum, Cirsium canescens, Cirsium carolinianum, Cirsium ciliolatum, Cirsium clavatum, Cirsium crassicaule, Cirsium cymosum, Cirsium discolor, Cirsium douglasii, Cirsium drummondii, Cirsium eatonii, Cirsium edule, Cirsium engelmannii, Cirsium flodmanii, Cirsium foliosum, Cirsium fontinale, Cirsium grahamii, Cirsium helenioides, Cirsium hookerianum, Cirsium horridulum, Cirsium hydrophilum, Cirsium inamoenum, Cirsium joannae, Cirsium kamtschaticum, Cirsium lecontei, Cirsium longistylum, Cirsium mohavense, Cirsium muticum, Cirsium neomexicanum, Cirsium nuttallii, Cirsium occidentale, Cirsium ochrocentrum, Cirsium ownbeyi, Cirsium palustre, Cirsium parryi, Cirsium perplexans, Cirsium pitcheri, Cirsium praeteriens, Cirsium pulcherrimum, Cirsium pumilum, Cirsium quercetorum, Cirsium remotifolium, Cirsium repandum, Cirsium rhothophilum, Cirsium rydbergii, Cirsium scariosum, Cirsium texanum, Cirsium tracyi, Cirsium turneri, Cirsium undulatum, Cirsium vinaceum, Cirsium virginianum, Cirsium vulgare, Cirsium wheeleri, Cirsium wrightii

Keys

Key to Groups of Cirsium Species

1	Plants of Great Plains, e North America, and Greenland	Group 1
1	Plants of nw Canada, Pacific Coast, Intermountain Region, sw Deserts, and Rocky Mountains	> 2
2	Involucres 3–5 cm	Group 2
2	Involucres 1–3 cm	Group 3

Group 1: Cirsium species of Great Plains, eastern North America, and Greenland

1	Involucres usually 1–2.5 cm	> 2
1	Involucres 2.5–5 cm	> 19
2	Plants dioecious or nearly so; common invasive weed	Cirsium arvense
2	Plants hermaphroditic, with bisexual florets	> 3
3	Bases of mid cauline leaves long-decurrent as spiny wings	> 4
3	Bases of mid cauline leaves not decurrent or sometimes short-decurrent, forming spiny wings to 3 cm	> 5
4	Heads crowded at stem tips; peduncles 0–1 cm; invasive weed in northern forests, wetlands	Cirsium palustre
4	Heads in open paniculiform arrays, borne singly at tips of slender peduncles 1–15 cm; New Mexico	Cirsium wrightii
5	Longer pappus bristles exceeding corollas by 1–8 mm; corollas very slender, throat scarcely wider than tube; abaxial faces of outer and middle phyllaries without glutinous ridge; heads sessile in dense mass at tip of thick fleshy stem, overtopped by crowded distal cauline leaves; w Canada	Cirsium foliosum
5	Longer pappus bristles shorter than corollas; corolla throat noticeably wider than tube; abaxial faces of outer and middle phyllaries with elongate glutinous ridge (milky when fresh, dark when dry, sometimes very narrow); heads evidently pedunculate, not overtopped by crowded distal cauline leaves	> 6
6	Apices of outer and middle phyllaries erect, ± appressed, tipped by erect or ascending spines or cusps	> 7
6	Apices of at least outer phyllaries widely spreading, tipped by spreading spines or short cusps	> 10
7	Abaxial leaf face densely white tomentose with non-septate trichomes	> 8
7	Abaxial leaf face thinly tomentose or glabrate and villous with septate trichomes	> 9
8	Leaves 4–8 cm wide, main spines 1–2 mm; Greenland	Cirsium helenioides
8	Leaves 0.5–4 cm wide, main spines usually 3–5 mm; se United States	Cirsium virginianum
9	Leaves deeply pinnatifid; widespread in e North America	Cirsium muticum
9	Leaves unlobed to shallowly pinnatifid; Virginia to Georgia	Cirsium repandum
10	Phyllary spines 0–1(–3) mm	> 11
10	Phyllary spines (1–)2–9 mm	> 13
11	Leaves thick, ± rigid, abaxially white-tomentose; New Jersey to Florida	Cirsium virginianum
11	Leaves thin, flexible, abaxially thinly tomentose, ± glabrate	> 12
12	Peduncles leafy-bracted; widespread, e North America	Cirsium muticum
12	Peduncles essentially naked; Virginia to Florida, Louisiana	Cirsium nuttallii
13	Leaf bases decurrent	> 14
13	Leaf bases not decurrent (except sometimes in C. pitcheri or C. texanum); phyllary spines slender	> 15
14	Adaxial leaf faces green, glabrous or thinly arachnoid; phyllary spines 2–7 mm, stout, broad-based; w Great Plains	Cirsium pulcherrimum
14	Adaxial leaf faces gray-tomentose; phyllary spines 1–3 mm, slender; sandy shores and dunes around Great Lakes…	Cirsium pitcheri
15	Involucres 1.2–2 cm	> 16
15	Involucres 2–3.5(–4) cm	> 17
16	Mid and distal cauline leaves linear to narrowly elliptic, bases tapered, cuneate, not decurrent; widespread, se United States	Cirsium carolinianum
16	Mid and distal cauline leaves ovate, broadly sessile, sometimes auriculate-clasping or short-decurrent; Louisiana, Oklahoma, Texas, adventive in Missouri	Cirsium texanum
17	Leaves gray- or white-tomentose on both faces, divided nearly to midvein into linear lobes; corollas usually white; beaches and dunes around Great Lakes	Cirsium pitcheri
17	Leaves adaxially green, ± glabrate, abaxially white-tomentose, undivided or shallowly to deeply pinnatifid; corollas lavender to purple (rarely white); widespread	> 18
18	Stems villous with septate trichomes or distally white-tomentose; wide- spread, e North America	Cirsium discolor
18	Stems gray- or white-tomentose throughout; n Great Plains, occasion- ally eastward	Cirsium flodmanii
19	Adaxial leaf faces covered with short, ± appressed, bristlelike spines; common weed.	Cirsium vulgare
19	Adaxial leaf faces without bristlelike spine	> 20
20	Corollas red; w Texas	Cirsium turneri
20	Corollas white to yellow, lavender, or purple	> 21
21	Each head closely subtended by involucre-like ring of spiny-margined bracts about as long as involucre; Maine to Florida, w to Texas	Cirsium horridulum
21	Heads not individually subtended by involucre-like ring of spiny-margined bracts, sometimes cluster of several heads collectively surrounded by crowded distal cauline leaves that overtop them	> 22
22	Abaxial face of outer and middle phyllaries without glutinous ridge; heads surrounded and overtopped by distal cauline leaves; widespread, w North America, disjunct on Mingan Archipelago, Quebec	Cirsium scariosum
22	Abaxial faces of outer and middle phyllaries with elongate glutinous ridge (milky when fresh, dark when dry); heads in most species elevated above cauline leaves	> 23
23	Spines (or sharp, cusplike tips) of outer and middle phyllaries erect, ± appressed	> 24
23	Spines (or short, cusplike tips) of outer and middle phyllaries sometimes erect, sometimes spreading	> 28
24	Phyllary spines 0–0.5 mm	Cirsium muticum
24	Phyllary spines 0.5–0.6 mm	> 25
25	Apices of innermost phyllaries usually scarious, often expanded, ± erose-dentate	> 26
25	Apices of innermost phyllaries linear-attenuate	> 27
26	Heads evidently pedunculate above distal cauline leaves; leaves usually less than 5 times longer than wide; midwest, e United States, s Canada	Cirsium pumilum
26	Heads ± crowded, surrounded and overtopped by distal cauline leaves; leaves usually 5+ times longer than wide; Canada, Black Hills of South Dakota, Wyoming	Cirsium drummondii
27	Stems usually branched, distal 1/2 usually leafy; adaxial leaf faces shaggy villous with septate trichomes, abaxial faces villous with septate trichomes, loosely arachnoid when young; Virginia to Georgia	Cirsium repandum
27	Stems usually simple, distal 1/2 nearly naked with only few bractlike leaves; adaxial leaf faces glabrous or sparingly villous with coarse, multicellular trichomes, abaxial faces loosely arachnoid when young, often ± glabrate; coastal, North Carolina to Louisiana	Cirsium lecontei
28	Phyllary spines 0–0.5 mm; widespread in e North America.	Cirsium muticum
28	Phyllary spines 2–12 mm	> 29
29	Stems thinly tomentose when young, ± glabrate or tomentum persisting distally; adaxial leaf faces green, glabrate	> 30
29	Stems uniformly and persistently gray- or white-tomentose; adaxial leaf faces thinly to densely tomentose when young, sometimes green and glabrate in age	> 32
30	Peduncles with much reduced bracts; usually some roots with tuberlike enlargements; Louisiana, Oklahoma, Texas.	Cirsium engelmannii
30	Peduncles leafy-bracted; roots without tuberlike enlargements	> 31
31	Cauline leaves usually unlobed or shallowly pinnatifid, or when more deeply lobed, lobes relatively wide; margins flat; apices of innermost phyllaries usually dilated and ± erose or serrulate; widespread, ne United States	Cirsium altissimum
31	Cauline leaves deeply pinnatifid into narrow, linear-lanceolate lobes; margins + revolute; apices of innermost phyllaries attenuate, not dilated; widespread in e North America	Cirsium discolor
32	Cauline leaves not decurrent or with decurrent wing 0–1 cm	> 33
32	Cauline leaves decurrent 1–5+ cm	> 34
33	Cauline leaves elliptic to oblanceolate, shallowly lobed to pinnatifid; cypselae 3–5 mm, apical collar stramineous; root sprouts arising from horizontal runner roots; n Great Plains, occasionally eastward	Cirsium flodmanii
33	Cauline leaves ovate to lanceolate, subentire to coarsely toothed or shallowly lobed; cypselae 6–7 mm, apical collar colored like body; root sprouts arising from deep taproots; Great Plains, occasionally east- ward	Cirsium undulatum
34	Spines of phyllaries 5–12 mm; corollas pale lavender or rarely white; Great Plains	Cirsium ochrocentrum
34	Spines of phyllaries usually 2–4 mm; corollas usually dull white, rarely lavender or purple	> 35
35	Leaves less deeply divided, lobes linear or oblong; phyllary spines 2–4 mm; Great Plains to c Rocky Mountains	Cirsium canescens
35	Leaves divided nearly to midvein into linear lobes; phyllary spines 1–2(–3) mm; beaches and dunes around Great Lakes	Cirsium pitcheri

Group 2: Large-headed Cirsium species of Pacific Coast, Intermountain Region, southwestern Deserts, and Rocky Mountains

1	Adaxial leaf faces with slender ± appressed bristlelike spines; common weed	Cirsium vulgare
1	Adaxial leaf faces without bristlelike spines	> 2
2	Abaxial faces of outer and middle phyllaries with elongate glutinous ridge (milky when fresh, dark when dry, sometimes very narrow)	> 3
2	Abaxial faces of outer and middle phyllaries without elongate glutinous ridge (sometimes present in C. arizonicum).	> 13
3	Corolla lobes 1.5+ times as long as tube; style 1.5–4 mm; se California to s Colo- rado, Arizona, New Mexico	Cirsium arizonicum
3	Corolla lobes shorter than to equaling tube; style 2–8 mm	> 4
4	Herbage glabrous or ± villous or tomentose with septate trichomes; fine, non-septate trichomes usually 0	> 5
4	Herbage ± densely tomentose with fine, non-septate trichomes; septate trichomes 0 (except sometimes in C. hookerianum)	> 7
5	Phyllaries strongly imbricate, outer and middle ovate or lanceolate, appressed, ascending to erect, spines 2–3 mm; Canada, Black Hills of South Dakota, Wyoming, Colorado	Cirsium drummondii
5	Phyllaries subequal or imbricate, outer and middle with short, appressed bases and long, linear, stiffly spreading to ascending apices; spines 3–35 mm	> 6
6	Outer phyllaries entire; spines 3–5 mm; n Rocky Mountains	Cirsium hookerianum
6	Outer phyllaries usually pinnately spiny; spines 7–35 mm; Rocky Mountains and high peaks of intermountain region	Cirsium eatonii
7	Glutinous ridges on phyllaries narrow, inconspicuous, sometimes absent; n Rocky Mountains	Cirsium hookerianum
7	Glutinous ridges on phyllaries well developed	> 8
8	Mid and distal cauline leaves evidently decurrent, spiny wings to 5 cm	> 9
8	Mid and distal cauline leaves not or scarcely decurrent	> 10
9	Phyllary spines usually 2–4 mm; corollas ochroleucous (rarely lavender-tinged)	Cirsium canescens
9	Phyllary spines usually 5–12 mm; corollas pale lavender	Cirsium ochrocentrum
10	Corollas red, pink, or reddish purple; Arizona, New Mexico	Cirsium ochrocentrum
10	Corollas white to lavender or purple	> 11
11	Corollas creamy white, rarely lavender-tinged; e Oregon, Washington, w Idaho	Cirsium brevifolium
11	Corollas lavender to purple; widespread	> 12
12	Cauline leaves elliptic to oblanceolate, shallowly lobed to pinnatifid; cypselae 3–5 mm, apical collar stramineous; root sprouts arising from horizontal runner roots; se Brit- ish Columbia to n Colorado, Great Plains	Cirsium flodmanii
12	Cauline leaves ovate to lanceolate, subentire to coarsely toothed or shallowly lobed; cypselae 6–7 mm, bodies and apical collars concolorous; root sprouts arising from deep taproots; widespread	Cirsium undulatum
13	Corollas pink, red, or reddish purple	> 14
13	Corollas white to lavender or purple	> 17
14	Corolla lobes shorter than throat; n California, sw Idaho, nw Nevada	Cirsium andersonii
14	Corolla lobes longer than throat	> 15
15	Corolla tube 3.5–5 mm; w Texas	Cirsium turneri
15	Corolla tube 7–18 mm	> 16
16	Biennials from taproots; stems usually solitary; style tips 4–5 mm; s Oregon, California, Nevada	Cirsium occidentale
16	Perennials from taprooted caudices or runner roots; stems 1–several; style tips 1.5–4.5 mm; se California to s Colorado, Arizona, New Mexico.	Cirsium arizonicum
17	Outer and middle phyllaries lanceolate to ovate, appressed, spines 1–12 mm	> 18
17	Outer and middle phyllaries with short appressed bases and stiffly spreading to ascending, lanceolate to linear-acicular apices; spines 3–35 mm	> 19
18	Biennials or monocarpic perennials from taproots; usually from moist sites; widespread	Cirsium scariosum
18	Perennials from runner roots; usually from dry sites; coastal c, n California.	Cirsium quercetorum
19	Outer phyllaries subequal, rigidly spreading, with spines 5–10 mm; middle and inner phyllaries imbricate, with bodies appressed and apices spreading, spine- less; Santa Clara County, California	Cirsium praeteriens
19	Outer and middle phyllaries subequal or imbricate, with spreading to incurved-ascending, lanceolate to linear-acicular apices; middle and inner phyllaries with bodies appressed or not, and at least the middle with apices deflexed to spreading or ascending, spine-tipped	> 20
20	Adaxial leaf faces thinly arachnoid-tomentose to densely felty with fine, non-septate trichomes, sometimes glabrate	> 21
20	Adaxial leaf faces glabrous or villous along midveins with septate trichomes	> 23
21	Phyllaries spiny-ciliate; coastal dunes, s, c California	Cirsium rhothophilum
21	Phyllaries usually entire	> 22
22	Heads borne singly or in corymbiform arrays; trichomes usually all fine, non- septate; California	Cirsium occidentale
22	Heads usually crowded in racemiform arrays; some trichomes on stems and leaves often septate; nw United States, w Canada	Cirsium hookerianum
23	Corolla lobes filiform with knoblike tips; style included or exserted only 1–2 mm beyond corolla lobes; British Columbia to coastal s California, Montana	Cirsium brevistylum
23	Corolla lobes linear but not filiform, not knobbed at tip; style exserted well beyond corolla lobes	> 24
24	Leaves and stems glabrous	> 25
24	Leaves and/or stems villous and/or tomentose	> 26
25	Basal and proximal cauline leaves 2–5 cm wide, strongly undulate; Rocky Mountains and high peaks of intermountain region	Cirsium eatonii
25	Basal and proximal cauline leaves 6–12 cm wide, not strongly undulate; plants of hanging gardens in sw Utah	Cirsium joannae
26	Basal and proximal cauline leaves plane to moderately undulate and shallowly to ± deeply divided into 5–10 pairs of usually well separated, linear to broadly triangular lobes; British Columbia to w Oregon	Cirsium edule
26	Basal and proximal cauline leaves strongly undulate and deeply divided into 10–20 pairs of closely spaced, usually narrow lobes; Rocky Mountains and high peaks of intermountain region	Cirsium eatonii

Group 3: Small-headed Cirsium species of Pacific Coast, Intermountain Region, southwestern deserts, and Rocky Mountains

1	Plants dioecious or nearly so; common invasive weed	Cirsium arvense
1	Plants with bisexual florets	> 2
2	Bases of mid cauline leaves long-decurrent as spiny wings	> 3
2	Bases of mid cauline leaves not decurrent or decurrent as spiny wings to 5 cm	> 4
3	Heads crowded at stem tips, peduncles 0–1 cm; invasive weed in wetlands, north- ern forests, British Columbia	Cirsium palustre
3	Heads in open paniculifrom arrays, borne singly on slender peduncles 1–15 cm; se Arizona, New Mexico	Cirsium wrightii
4	Heads nodding; adaxial leaf faces glandular or not	> 5
4	Heads usually erect; adaxial leaf faces usually not glandular	> 9
5	Adaxial leaf faces densely puberulent with mixture of short, multicellular, ± glandular trichomes and finer, arachnoid, non-septate trichomes; serpentine wetlands, coast ranges, c California	Cirsium fontinale
5	Adaxial leaf faces glabrous	> 6
6	Involucres densely tomentose; Rocky Mountains and high peaks of inter- mountain region	Cirsium eatonii
6	Involucres glabrous	> 7
7	Phyllaries maroon, drying dark brown or blackish; corollas rich rose- purple; s New Mexico	Cirsium vinaceum
7	Phyllaries green, drying green or light brown; corollas dull white to pink or purple; n Arizona, s Utah	> 8
8	Involucres 1.4–2 cm (excluding recurved outer phyllary apices that extend below involucre base); phyllary apices lance-ovate, rather abruptly contracted into recurved spines 3–25 mm, margins spar- ingly tomentose or glabrate; ne Arizona, se Utah	Cirsium rydbergii
8	Involucres 2.5–4 cm (including spreading to curved-ascending phyllary apices); phyllary spices linear, spines 5–12 mm, margins scabridulous-ciliolate; ne Arizona, se Utah	Cirsium joannae
9	Margins of outer phyllaries hispidulous-ciliolate, spiny-fringed, pinnately spiny, or with expanded, scarious appendages	> 10
9	Margins of outer phyllaries usually entire (bracts subtending head usually spiny).	> 27
10	Heads usually not closely subtended by clustered leafy bracts, often each subtended by 1 leaf or borne on peduncles with much reduced bracts	> 11
10	Heads usually closely subtended by clustered ± leafy bracts	> 18
11	Midribs of outer and middle phyllaries forming an elongate glutinous ridge (milky when fresh, dark when dry); margins of outer phyllaries minutely spiny-ciliate	> 12
11	Midribs of phyllaries not glandular or with narrow, inconspicuous glutinous ridge; margins of outer phyllaries usually ± conspicuously fringed or spiny-ciliate	> 13
12	Corollas deep purple; Arizona, New Mexico	Cirsium grahamii
12	Corollas pale rose-purple; coastal c California	Cirsium hydrophilum
13	Phyllaries long-acicular; involucres usually ± densely arachnoid with fine, non-septate trichomes; heads often ± sessile in tight clusters at tips of main stem and branches; British Columbia to w Oregon	Cirsium edule
13	Phyllaries not long-acicular; involucres glabrous or loosely floccose, or arachnoid with coarse, septate trichomes; heads borne singly or in various arrays	> 14
14	Arrays racemiform, spiciform, or subcapitate, or plants forming low, rounded mounds; plants ± fleshy; usually ± wet habitats, widespread.	Cirsium scariosum
14	Arrays ± openly branched; plants usually not fleshy	> 15
15	Plants stout, usually 100–300 cm; stems 2–10 cm diam. near base, hollow; middle and inner phyllaries entire; San Joaquin Valley, California	Cirsium crassicaule
15	Plants slender, usually 10–110 cm; stems usually less than 2 cm diam. at base, not hollow; middle and inner phyllaries often with fringed appendages	> 16
16	Biennials; barren stony habitats, w Colorado	Cirsium perplexans
16	Perennials, monocarpic or polycarpic; various habitats	> 17
17	Corollas 16–20 mm; Colorado, Utah, Wyoming	Cirsium clavatum
17	Corollas 18–28 mm; Washington to n California	Cirsium remotifolium
18	Leaf faces closely gray-white felty-tomentose, margins very prominently undu- late; dunes, headlands, s, c California	Cirsium rhothophilum
18	Leaf faces glabrous to arachnoid-tomentose adaxially, sometimes white- or gray-tomentose abaxially, margins flat to undulate	> 19
19	Corollas pale creamy yellow to bright yellow	> 20
19	Corollas white to purple	> 21
20	Biennials; basal and proximal cauline leaves absent at flowering; leaves unlobed or pinnatifid with lobes well separated, not overlapping; heads loosely to densely clustered at tips of main stem and branches, often also in distal leaf axils; involucres ± arachnoid, but not obscured by dense, woolly pubescence; corollas ± pale yellow; montane meadows, Colorado, Arizona, New Mexico	Cirsium parryi
20	Perennials; basal and proximal cauline leaves present at flowering; leaves regularly pinnatifid with closely spaced, ± overlapping lobes; heads borne in massive, heavy, commonly nodding terminal clusters or spikes; involucres ± concealed by dense, woolly pubescence; corollas bright yellow; subalpine and alpine, Colorado	Cirsium eatonii
21	Involucres ± glabrous or very thinly arachnoid	> 22
21	Involucres conspicuously arachnoid-tomentose	> 24
22	Capitulescence open, many-headed, corymbiform or paniculiform; inner phyllaries erect or twisted, not expanded; San Joaquin Valley, California	Cirsium crassicaule
22	Capitulescence racemiform, spiciform, or subcapitate, or plants forming low, rounded mounds; inner phyllaries often with expanded, erose, scarious tips	> 23
23	Outer phyllaries erose to lacerate or spiny-fringed; mountains of w, c Montana	Cirsium longistylum
23	Outer phyllaries usually not erose to lacerate; usually ± wet habitats, widespread	Cirsium scariosum
24	Heads sessile or subsessile, crowded in dense subcapitate to spiciform arrays; Rocky Mountains and highpeaks of inter- mountain region	Cirsium eatonii
24	Heads sessile to evidently pedunculate, not crowded in dense subcapitate to spiciform arrays	> 25
25	Distal leaves ± stiff, wickedly spiny, spines stout, often 10–15 mm; most outer phyllaries spiny-margined; coastal n, c California	Cirsium andrewsii
25	Distal leaves thin, ± weakly spiny, spines slender, usually less than 8 mm; few outer phyllaries spiny-margined	> 26
26	Style tips conspicuously exserted beyond corolla lobes; British Columbia to w Oregon	Cirsium edule
26	Style tips included or exserted only 1–2 mm beyond corolla lobes; British Columbia to coastal s California, Montana	Cirsium brevistylum
27	Corolla lobes twice as long as corolla throat or longer; se California to s Colorado, Arizona, New Mexico	Cirsium arizonicum
27	Corolla lobes shorter than to slightly exceeding corolla throat	> 28
28	Corollas bright pink, red, or rich purple	> 29
28	Corollas white to pale pink, lavender, or purple	> 30
29	Heads usually crowded at branch tips; peduncles usually 0–4 cm; corollas abruptly expanded from tube to throat; usually wetland sites; California, Nevada, Oregon	Cirsium douglasii
29	Heads usually borne singly; peduncles 1–30 cm; corollas gradually expanded from tube to throat; usually dry sites; California, Nevada, Oregon.	Cirsium occidentale
30	Middle and outer phyllaries usually spreading to reflexed	> 31
30	Middle and usually outer phyllaries appressed or stiffly ascending (sometimes spines abruptly spreading)	> 44
31	Plants lush, lax; herbage essentially glabrous or glabrescent throughout; heads in paniculiform arrays; hanging gardens and canyon bottoms; n Arizona, s Utah	> 32
31	Plants usually ± erect; herbage usually ± pubescent; heads in various arrays; widespread	> 33
32	Involucres 1.4–2 cm (excluding recurved outer phyllary apices that extend below involucre base); phyllary apices lance-ovate, rather abruptly contracted into recurved spines 3–25 mm, margins sparingly tomentose or glabrate; ne Arizona, se Utah	Cirsium rydbergii
32	Involucres 2.5–4 cm (including spreading to curved-ascending phyllary apices); phyllary apices linear, spines 5–12 mm, margins scabridulous-ciliolate; ne Arizona, se Utah	Cirsium joannae
33	Leaves deeply 2–3–pinnately divided, lobes linear to linear-lanceolate; abaxial leaf faces glabrous to thinly tomentose and villous along major veins; e Utah, w Colorado	Cirsium ownbeyi
33	Leaves shallowly to deeply pinnatifid; abaxial leaf faces usually thinly to densely tomentose	> 34
34	Spines of principal phyllaries usually 7–20+ mm	> 35
34	Spines of principal phyllaries usually 1–6 mm	> 38
35	Perennials	> 36
35	Biennials or short-lived monocarpic perennials	> 37
36	Phyllaries thinly arachnoid or glabrate; e Idaho and ne Utah to Wyoming and nw Nebraska	Cirsium pulcherrimum
36	Phyllaries densely and persistently arachnoid with slender trichomes connecting adjacent phyllaries; coastal bluffs, dunes, c California.	Cirsium occidentale
37	Main spines of cauline leaves usually less than 8 mm; California.	Cirsium occidentale
37	Main spines of cauline leaves usually 8–15 mm; deserts of sw United States	Cirsium neomexicanum
38	At least some inner phyllaries usually fringed or spiny-toothed	> 39
38	Phyllaries all entire	> 40
39	Corollas 16–20 mm; Colorado, Utah, Wyoming	Cirsium clavatum
39	Corollas 18–28 mm; Washington to n California	Cirsium remotifolium
40	Biennials; heads usually in open corymbiform arrays	> 41
40	Perennials; heads usually in ± congested flat-topped or racemiform arrays	> 42
41	Abaxial face of outer and middle phyllaries with elongate glutinous ridge (milky when fresh, dark when dry); cauline leaves with decurrent wings 1–3 cm; Oregon and e California to w Wyoming	Cirsium inamoenum
41	Abaxial face of outer and middle phyllaries without elongate glutinous ridge; cauline leaves not decurrent or spiny decurrent wings to 2 cm; California	Cirsium occidentale
42	Principal cauline leaves decurrent 1.5–3.5 cm; e Idaho and ne Utah to Wyoming and nw Nebraska	Cirsium pulcherrimum
42	Principal cauline leaves clasping or decurrent 1–10 mm	> 43
43	Abaxial leaf faces densely gray-white tomentose; stems and leaves tomentose with fine, non-septate trichomes, septate trichomes absent; s Oregon, n California	Cirsium ciliolatum
43	Abaxial leaf faces ± green, thinly tomentose; stems and leaves with mixture of fine, non-septate trichomes and coarser, septate trichomes, especially along stems and on midveins on abaxial leaf faces; widespread, n California to e Wyoming	Cirsium cymosum
44	Leaves all basal or crowded on very short, densely leafy stems; heads ± sessile, closely subtended by rosette leaves	> 45
44	Leaves basal and evidently cauline or all cauline; heads sessile or pedunculate, evidently raised above rosette leaves	> 46
45	Biennials or short-lived monocarpic perennials, taprooted; usually ± wet habitats, widespread	Cirsium scariosum
45	Perennials from creeping rootstocks; usually ± dry habitats; coastal c, n California	Cirsium quercetorum
46	Phyllaries spines 10–20+ mm	> 47
46	Phyllary spines usually less than 10 mm	> 48
47	Plants persistently tomentose; heads usually not closely subtended by well- developed leaves	Cirsium ochrocentrum
47	Plants sparsely arachnoid-tomentose, soon glabrescent; heads closely subtended by well-developed leaves; usually ± wet habitats, widespread	Cirsium scariosum
48	Corollas usually 25–50 mm	> 49
48	Corollas usually 16–25 mm	> 61
49	Biennials or short-lived monocarpic perennials, taprooted	> 50
49	Perennials, taprooted or from creeping runner roots	> 52
50	Cauline leaves evidently decurrent	Cirsium inamoenum
50	Cauline leaves not or only slightly decurrent. sometimes auriculate-clasping	> 51
51	Heads sessile or short-pedunculate, often crowded in spiciform or racemiform arrays; usually ± wet habitats, widespread.	Cirsium scariosum
51	Heads usually evidently pedunculate, usually in ± open, corymbiform arrays; usually dry sites, Oregon and California to w Wyoming	Cirsium cymosum
52	Corollas purple	> 53
52	Corollas white to pink or lavender	> 56
53	Phyllaries usually ovate; abaxial face of outer and middle phyllaries without elongate glutinous ridge; coastal n, c California.	Cirsium quercetorum
53	Phyllaries usually lanceolate; abaxial face of outer and middle phyllaries with elongate glutinous ridge (milky when fresh, dark when dry)	> 54
54	Cauline leaves elliptic to oblanceolate, shallowly lobed to pinnatifid; cypselae 3–5 mm, apical collar straw-colored; root sprouts arising from horizontal runner roots; se British Columbia to n Colorado, Great Plains	Cirsium flodmanii
54	Cauline leaves ovate to elliptic, oblong, or lanceolate, subentire to coarsely toothed or shallowly lobed; cypselae 6–7 mm, apical collar colored like body; root sprouts, if any, arising from deep taproots	> 55
55	Involucres of larger heads often exceeding 3 cm; corolla lobes 6.5–13 mm; widespread	Cirsium undulatum
55	Involucres of larger heads commonly less than 3 cm; corolla lobes 5.5–9.5 mm; w Colorado, ne New Mexico, se Utah	Cirsium tracyi
56	Larger leaves regularly pinnatifid with 8–15 pairs of lobes 0.5–2 cm; cauline leaves evidently decurrent	> 57
56	Larger leaves regularly or irregularly pinnatifid with 3–8 pairs of lobes, these often longer than 2 cm; cauline leaves not or only slightly decurrent	> 58
57	Principal leaf spines 5–20 mm; Wyoming to New Mexico, adventive in California	Cirsium ochrocentrum
57	Principal leaf spines 3–5 mm; Colo., Utah, Wyoming	Cirsium barnebyi
58	Abaxial faces of outer and middle phyllaries without elongate glutinous ridge; plants often growing as low, compact, rounded mounds; coastal c, n California	Cirsium quercetorum
58	Abaxial faces of outer and middle phyllaries with elongate glutinous ridge (milky when fresh, dark when dry, sometimes very narrow); plants usually erect	> 59
59	Cauline leaves elliptic to oblanceolate, shallowly lobed to pinnatifid; cypselae 3–5 mm, apical collar stramineous; root sprouts arising from horizontal runner roots; se British Columbia to n Colorado, Great Plains	Cirsium flodmanii
59	Cauline leaves ovate to lanceolate, subentire to coarsely toothed or shallowly lobed; cypselae 6–7 mm, apical collar colored like body; root sprouts arising from deep taproots;widespread	> 60
60	Involucres of larger heads often exceeding 3 cm; corolla lobes 6.5–13 mm; widespread	Cirsium undulatum
60	Involucres of larger heads commonly less than 3 cm; corolla lobes 5.5–9.5 mm; w Colorado, ne New Mexico, se Utah	Cirsium tracyi
61	Phyllaries usually without a glutinous ridge	> 62
61	Phyllaries, at least some, with a ± sticky-glandular ridge or raised translucent gland, this usually milky in fresh specimens and darkened in drying	> 68
62	Corollas 16–17 mm; leaves broadly elliptic to obovate, tapered to base, armed with slender, fine, flexible spines; Aleutian Islands	Cirsium kamtschaticum
62	Corollas 18+ mm; leaves linear or oblong to elliptic or oblanceolate, armed with stiff spines	> 63
63	Heads usually conspicuously long-pedunculate; Washington to n California	Cirsium remotifolium
63	Heads usually ± sessile or short-pedunculate	> 64
64	Phyllaries ascending to spreading, connected by long septate or non-septate trichomes	> 65
64	Phyllaries appressed, glabrous or ± tomentose	> 66
65	Style tips included or exserted only 1–2 mm beyond corolla lobes; Brit- ish Columbia to coastal s California, Montana	Cirsium brevistylum
65	Style tips conspicuously exserted beyond corolla lobes; Rocky Moun- tains and high peaks of intermountain region	Cirsium eatonii
66	Heads usually not closely subtended by crowded leafy bracts; stems not fleshy; dry soils, Colorado, Utah, Wyoming	Cirsium barnebyi
66	Heads usually closely subtended by crowded, leafy bracts; stems ± fleshy; usually wet soils	> 67
67	Longer pappus bristles shorter than corollas; corolla throat noticeably wider than tube; usually ± wet habitats, widespread.	Cirsium scariosum
67	Longer pappus bristles exceeding corollas by 1–10 mm; corollas very slender, throat scarcely wider than tube; Canada, n Rockies.	Cirsium foliosum
68	Phyllary bodies (at least outer) ascending or loosely appressed, spines usually erect or ascending; heads commonly in corymbiform or racemiform arrays	> 69
68	Phyllary bodies tightly appressed, spines ascending to abruptly spreading; heads borne singly or in paniculiform or corymbiform arrays, or in few-headed arrays at stem tips	> 71
69	Cauline leaves conspicuously decurrent; Oregon and e California to w Wyoming	Cirsium inamoenum
69	Cauline leaves auriculate-clasping, sometimes also short-decurrent	> 70
70	Pubescence all of fine, non-septate arachnoid trichomes, often dense and persistent on leaves; s Oregon, n California.	Cirsium ciliolatum
70	Pubescence usually mixture of fine, non-septate arachnoid trichomes and coarser, septate trichomes, usually ± loose and irregularly deciduous from leaves in age; Oregon, n Cali- fornia to w Wyoming	Cirsium cymosum
71	Leaves glabrous to thinly tomentose on both faces, ± glabrate adaxially	> 72
71	Leaves densely tomentose on abaxial faces, often on both faces	> 75
72	Pubescence usually mixture of fine, non-septate arachnoid trichomes and coarser, septate trichomes, usually ± loose and irregularly deciduous from leaves in age; Oregon and n California to w Wyoming	Cirsium cymosum
72	Pubescence, when present, of fine, non-septate trichomes; septate trichomes usually absent	> 73
73	Polycarpic perennials 20–100 cm; mountains of e Utah, w Colorado.	Cirsium clavatum
73	Biennials or monocarpic perennials	> 74
74	Plants 100–220 cm; phyllary spines 1–2 mm; wet soils, coastal c California	Cirsium hydrophilum
74	Plants 20–100 cm; phyllary spines 2–7 mm; dry slopes, Oregon, e California to w Wyoming	Cirsium inamoenum
75	Inner phyllaries usually dark purple to blackish near tips	> 76
75	Inner phyllaries greenish to brown or stramineous	> 77
76	Heads 1–6, borne singly or few at branch tips; Arizona, Colorado, New Mexico, Utah	Cirsium wheeleri
76	Heads 10–many, ± crowded at branch tips; moist soils, California, Nevada, Oregon	Cirsium douglasii
77	Basal leaves 8–20+ cm wide; e California to sw Utah, n Arizona	Cirsium mohavense
77	Basal leaves 0.7–7 cm wide	> 78
78	Cauline leaves not decurrent; Arizona, Colorado, New Mexico, Utah.	Cirsium wheeleri
78	Cauline leaves evidently decurrent	> 79
79	Larger leaves regularly pinnatifid with 8–15 pairs of closely spaced lobes; both leaf faces ± densely tomentose; Colorado, Utah, Wyoming.	Cirsium barnebyi
79	Larger leaves unlobed or with 5–8(–many) pairs of lobes, these usually separated by ± broad, U-shaped sinuses; adaxial leaf faces often glabrous or glabrate	> 80
80	Corollas dull white or faintly lavender-tinged (sometimes bright pink-purple in var. davisii); phyllary spines usually fine; Oregon and e California to w Wyoming	Cirsium inamoenum
80	Corollas pink to purple; phyllary spines usually stout, ± flattened; e Idaho and ne Utah to Wyoming and nw Nebraska.	Cirsium pulcherrimum

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