1a. Phyllaries distinctly spine-tipped (at least the outer and middle) , spine usually more than 2 mm long, but when very short, the larger involucres 2 cm or more in diameter...2
1b. Annuals with elongate taproots; heads smaller, mostly 9-12 (-14) mm long; leaf bases auriculate-clasping, the acute or rounded auricles large and conspicuous; achenes 3- to 5-nerved; terminal leaf lobe triangular...8
2a. Leaves scabrous-hispid or crisped-hispid and also sometimes silky-pubescent above, more or less cobwebby and sometimes crisped-hispid or tomentose beneath...3
2b. Leaves white-tomentose on both surfaces, often more thinly so above, totally lacking hispidity; dorsal glutinous ridge present on phyllaries...6
3a. Cauline leaves conspicuously decurrent, scabrous-hispid above, sparsely to densely cobwebby beneath; phyllaries herbaceous, spreading, gradually tapered into elongate spiny tips, lacking a dorsal glutinous ridge; common introduced weed...C. vulgare – bull thistle
3b. Cauline leaves not decurrent, crisped-hispid with multicellular hairs and also sometimes sparingly silky-pubescent above; phyllaries not herbaceous, appressed, with a dorsal glutinous ridge...4
4a. Leaves crisped-hispid on both surfaces, green; phyllaries with an erect apical spine; involucres 3-5 cm long; stem 3-5 dm tall, from persistent basal rosettes; dry or mesic prairies; rare...C. pumilum var. hillii (C. hillii) – Hill's thistle
4b. Leaves crisped-hispid above, white-tomentose beneath; phyllaries with an abruptly spreading apical spine; involucres 2.5-3.5 cm long; stem mostly 6-15 dm tall, basal rosettes not persistent...5.
5a. All leaves deeply lobed (except in juvenile forms) , the lobes linearacuminate, terminating in stout spines, the thickish margins involute; involucral spines 3-7 mm long; plants mostly of open places...C. discolor – prairie thistle
5b. Leaves shallowly lobed, irregularly dentate, serrate or entire, with small, weak spines, the margins thin, not involute, the lower leaves (and those of juvenile forms) sometimes deeply lobed, then the lobes wide, broadly acute; involucral spines 2.5-4.5 mm long; plants mostly of woods...C. altissimum – wood thistle
6a. Middle cauline leaves conspicuously decurrent, the narrowly linear to oblong lobes very distant, leaf blade divided nearly to midrib, the decurrent wing often similarly lobed; corollas cream-colored; plants not conspicuously spinescent; dunes of Lake Michigan...C. pitcheri – dune thistle
6b. Leaves not decurrent on stem or only very shortly so (to 1 cm) , the lobes lanceolate or deltoid; corollas purple or lavender, rarely white; rare introduced weeds...7
7a. Leaves narrowed to the base, rarely clasping; anthers 6.5-11.8 mm long, florets 2.1-3.6 cm long, achenes 3.5-5 mm long, yellowish brown with apical yellow band ca ½ mm wide; involucres 2.0-2.7 cm long, phyllaries narrow and slender; leaves lobed nearly to midrib, lobes narrowly triangular, usually less than 7 mm wide at base; plants strongly clonal by adventitious shoots on roots; rare adventive...C. flodmanii – Flodman's thistle
7b. Leaves broadest near the base, partially clasping; anthers 9.4-13.3 mm long, florets 2.7-4 cm long, achenes 5-7 mm long, brown, yellow apical band lacking or very narrow; involucres 3-3.5 cm long, phyllaries broad and stout; leaves shallowly lobed, lobes broadly triangular, usually more than 7 mm wide at base; plants weakly clonal by adventitious shoots on roots; rare adventive...C. undulatum – wavy-leaved thistle
8a. Perennials, strongly clonal by adventitious shoots on deep roots; imperfectly dioecious; heads numerous, crowded in 2's to 4's or short pedunculate; involucre 1-2 (-2.6) cm long, 0.8-1.1 cm wide at base when in flower, phyllaries usually glabrous and with a narrow dorsal glutinous ridge; very common weed...C. arvense – Canada thistle
8b. Biennials (at least monocarpic); florets perfect; plants of moist habitats...9
9a. Leaf bases not decurrent; heads solitary or several, pedunculate, not crowded; involucre 2.2-2.7 cm long, 1.2-1.9 cm wide at base when in flower, phyllaries cobwebby with prominent glutinous dorsal ridge; wet prairies and sedge meadows, common...C. muticum – swamp thistle
9b. Leaf bases strongly decurrent into prominent wings on stem; heads many, sessile or sub-sessile, crowded into a dense terminal inflorescence; involucre 0.9-1.2 cm long, phyllaries neither conspicuously glutinous nor cobwebby; rare, N Wis...C. palustre – European swamp thistle
Adapted from Keys to the Asteraceae of Wisconsin. Assembled and editied by Robert R. Kowal, 11 February 2007.
Etymology: Cirsium = from the Greek kirsion, "a kind of thistle;" also cirsos meaning "a swollen vein" for which thistle was once a remedy.
Characteristics: Spiny biennials or perennials. Leaves alternate, sessile, usually pinnatifid and spiny. Flowers all tubular, perfect, or (in C. arvense) dioecious, rose to purple, rarely white or cream. Involucral bracts imbricated in many rows, at least the outer ones tipped with a spine or mucro, often with a glutinous dorsal ridge. Pappus plumose.
Number of Species: 10 in Wisconsin (7 native/3 non-native); 62 in US & Canada.