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13. Cirsium Miller, Gard. Dict. Abr. ed. 4. vol. 1. 1754.

Thistle, chardon [Greek kirsion, thistle]

David J. Keil

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.

Species ca. 200 (62 in the flora): North America, Eurasia, n Africa.

Only three genera in Cardueae 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.


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.

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   1 Cirsium vulgare
+ Adaxial leaf faces without bristlelike spines   (2)
2 (1) Abaxial faces of outer and middle phyllaries with elongate glutinous ridge (milky when fresh, dark when dry, sometimes very narrow)   (3)
+ Abaxial faces of outer and middle phyllaries without elongate glutinous ridge (sometimes present in C. arizonicum).   (13)
3 (2) Corolla lobes 1.5+ times as long as tube; style 1.5–4 mm; se California to s Colo- rado, Arizona, New Mexico   43 Cirsium arizonicum (in part)
+ Corolla lobes shorter than to equaling tube; style 2–8 mm   (4)
4 (3) Herbage glabrous or ± villous or tomentose with septate trichomes; fine, non-septate trichomes usually 0   (5)
+ Herbage ± densely tomentose with fine, non-septate trichomes; septate trichomes 0 (except sometimes in C. hookerianum)   (7)
5 (4) 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   53 Cirsium drummondii
+ 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 (5) Outer phyllaries entire; spines 3–5 mm; n Rocky Mountains   48 Cirsium hookerianum (in part)
+ Outer phyllaries usually pinnately spiny; spines 7–35 mm; Rocky Mountains and high peaks of intermountain region   51 Cirsium eatonii (in part)
7 (4) Glutinous ridges on phyllaries narrow, inconspicuous, sometimes absent; n Rocky Mountains   48 Cirsium hookerianum (in part)
+ Glutinous ridges on phyllaries well developed   (8)
8 (7) Mid and distal cauline leaves evidently decurrent, spiny wings to 5 cm   (9)
+ Mid and distal cauline leaves not or scarcely decurrent   (10)
9 (8) Phyllary spines usually 2–4 mm; corollas ochroleucous (rarely lavender-tinged)   22 Cirsium canescens
+ Phyllary spines usually 5–12 mm; corollas pale lavender   24 Cirsium ochrocentrum (in part)
10 (8) Corollas red, pink, or reddish purple; Arizona, New Mexico   24 Cirsium ochrocentrum (in part)
+ Corollas white to lavender or purple   (11)
11 (10) Corollas creamy white, rarely lavender-tinged; e Oregon, Washington, w Idaho   5 Cirsium brevifolium
+ Corollas lavender to purple; widespread   (12)
12 (11) 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   18 Cirsium flodmanii
+ 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   19 Cirsium undulatum
13 (2) Corollas pink, red, or reddish purple   (14)
+ Corollas white to lavender or purple   (17)
14 (13) Corolla lobes shorter than throat; n California, sw Idaho, nw Nevada   45 Cirsium andersonii
+ Corolla lobes longer than throat   (15)
15 (14) Corolla tube 3.5–5 mm; w Texas   44 Cirsium turneri
+ Corolla tube 7–18 mm   (16)
16 (15) Biennials from taproots; stems usually solitary; style tips 4–5 mm; s Oregon, California, Nevada   40 Cirsium occidentale (in part)
+ 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.   43 Cirsium arizonicum (in part)
17 (13) Outer and middle phyllaries lanceolate to ovate, appressed, spines 1–12 mm   (18)
+ Outer and middle phyllaries with short appressed bases and stiffly spreading to ascending, lanceolate to linear-acicular apices; spines 3–35 mm   (19)
18 (17) Biennials or monocarpic perennials from taproots; usually from moist sites; widespread   54 Cirsium scariosum
+ Perennials from runner roots; usually from dry sites; coastal c, n California.   56 Cirsium quercetorum
19 (17) 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   57 Cirsium praeteriens
+ 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 (19) Adaxial leaf faces thinly arachnoid-tomentose to densely felty with fine, non-septate trichomes, sometimes glabrate   (21)
+ Adaxial leaf faces glabrous or villous along midveins with septate trichomes   (23)
21 (20) Phyllaries spiny-ciliate; coastal dunes, s, c California   58 Cirsium rhothophilum
+ Phyllaries usually entire   (22)
22 (21) Heads borne singly or in corymbiform arrays; trichomes usually all fine, non- septate; California   40 Cirsium occidentale (in part)
+ Heads usually crowded in racemiform arrays; some trichomes on stems and leaves often septate; nw United States, w Canada   48 Cirsium hookerianum (in part)
23 (20) Corolla lobes filiform with knoblike tips; style included or exserted only 1–2 mm beyond corolla lobes; British Columbia to coastal s California, Montana   47 Cirsium brevistylum
+ Corolla lobes linear but not filiform, not knobbed at tip; style exserted well beyond corolla lobes   (24)
24 (23) Leaves and stems glabrous   (25)
+ Leaves and/or stems villous and/or tomentose   (26)
25 (24) Basal and proximal cauline leaves 2–5 cm wide, strongly undulate; Rocky Mountains and high peaks of intermountain region   51 Cirsium eatonii (in part)
+ Basal and proximal cauline leaves 6–12 cm wide, not strongly undulate; plants of hanging gardens in sw Utah   61 Cirsium joannae
26 (24) 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   46 Cirsium edule
+ 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   51 Cirsium eatonii(in part)

List of Keys

  • List of lower taxa


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