Robert W. Freckmann Herbarium




Tribe 1. CARDUEAE (CYNAREAE) - thistle tribe


(Source: Johnson and Iltis 1963; Gleason and Cronquist, 1991; FNA, 2006;

edited by R. R. Kowal, 2006 July 24.)


1.   Neither plants nor heads prickly OR phyllaries spine-tipped and corollas yellow; achenes obliquely attached to the receptacle; marginal disk florets often enlarged and showy; phyllaries often with margins scarious and deeply cleft at tip (laciniate); pappus hairs mostly less than 3 mm long or lacking            

.........  CENTAUREA - star-thistle.


1.   Plants and/or heads prickly; corollas not yellow; achenes attached by the base to the receptacle; florets all alike; phyllaries not laciniate at tip; pappus hairs usually more than 5 mm long.


2.    Leaves unarmed, broadly rounded at base; tip of phyllary a hook

.........  ARCTIUM - burdock.


2.    Leaves prickly, lanceolate to ovate; tip of phyllary a straight spine or merely mucronate.


3.    Heads 1-flowered, aggregated into globose secondary heads

.........  ECHINOPS - globe-thistle.


3.    Heads many-flowered, only rarely sessile.


4.    Pappus plumose; phyllaries with needle-like spiny tips or merely mucronate, often with a glutinous ridge on back         

.........  CIRSIUM - thistle.


4.    Pappus barbellate to capillary; phyllaries not glutinous.


5.    Receptacle bristly; leaves and stem wings glabrous or nearly so; pappus capillary      

.........  CARDUUS - plumeless thistle.


5.    Receptacle alveolate (pitted) , not bristly; leaves and stem wings densely cottony-velutinous; pappus barbellate; extremely rare adventive        

.........  ONOPORDUM - Scotch thistle.



ARCTIUM - burdock


(s = 2x; x = 18)


1.   Heads 1‑1.6 cm long, (1.5‑) 2‑2.5 cm wide, subsessile or short pedunculate, in a racemose inflorescence; petioles of larger leaves hollow; common weed       

.........    A. minus - common burdock.



1.   Heads 1.5‑2.5 cm long, 1.5‑3.5 cm wide, long pedunculate, in a corymbose inflorescence; petioles solid or hollow; rare weeds.


2.    Heads 1.5‑1.7 cm long, 1.5‑2.1 cm wide, phyllaries densely cottony-pubescent; petioles of larger leaves hollow

.........  A. tomentosum - hairy burdock, cotton burdock.


2.    Heads ca 2.5 cm long, 3‑3.5 cm wide, phyllaries glabrous; petioles of larger leaves solid   

.........  A. Lappa - great burdock.



CARDUUS - plumeless thistle


1.   Phyllaries mostly 2 mm wide or more; involucres 2.8‑3 cm long, heads solitary, nodding (s = 2x; x = 8)            

.........  C. nutans - nodding thistle.


1.   Phyllaries rarely as much as 2 mm wide; involucres 1.4‑2 cm long, heads not nodding, clustered or solitary (s = 2x; x = 11)  

.........  C. acanthoides - plumeless thistle.



CENTAUREA - star-thistle, batchelor's button

including the genera PLECTOCEPHALUS and ACROPTILON


(Extremely rare adventive: C. diffusa - white knapweed.)


(x = 8-15)


1.   Corollas yellow; phyllaries usually spine-tipped; leaf bases more-or-less decurrent on the more or less winged stem; very rare adventives.


2.    Phyllaries spineless or tipped by weak spines 1-2 mm long

.........  C. macrocephala - big-head knapweed.


2.    Phyllaries tipped by long divergent spines 5-25 mm long.


3.    Heads sessile, each closely subtended and more-or-less concealed by involucre-like cluster of expanded, foliar bracts     

.........  C. (Cnicus) benedicta - blessed thistle.


3.    Heads pedunculate or, if sessile, not concealed by involucre-like cluster of expanded, foliar bracts.


4.    Central spines of phyllaries very slender, 4‑6 (‑9) mm long, with conspicuous secondary spines near the flattened base          

.........  C. melitensis - Maltese star-thistle.


4.    Central spines of phyllaries stout, 17‑20 mm long, with minute secondary spinules near the terete base

.........  C. solstitialis - yellow star-thistle, St. Barnaby's thistle.


1.   Corollas blue, purple, lavender, pink to white; phyllaries not spine-tipped; leaf bases not decurrent.


5.    Principal cauline leaves deeply pinnatifid into long narrow segments.


6.    Biennial with base lacking old leaf bases; involucres 1-1.4 cm long; phyllaries smooth, strongly ribbed, with black-brown pectinate tips 1-2 mm long; common weed

.........  C. Stoebe subsp. micranthos (C. maculosa, C. Biebersteinii) - spotted knapweed.


6.    Perennial with base conspicuously clothed with long, chaffy-fibrous old leaf-bases; involucres 2-2.5 cm long; phyllaries arachnoid, with dark pectinate tips 4-6 mm long; very rare adventive .........  C. Scabiosa - hard-heads.


5.    Principal cauline leaves simple or merely coarsely dentate or lyrate.


7.    Plants annual.


8.    Florets usually deep blue; involucres 1‑1.5 cm long, ovoid to cylindrical, on slender, mostly leafless peduncles; upper leaves linear, flocculose-pubescent, entire; pappus only 2‑3 mm long; common in cultivation

.........  C. Cyanus - bachelor's-button, cornflower.


8.    Florets rose-purple; involucre 1.5‑3 cm long, subglobose, on very leafy peduncles pronouncedly inflated at the top; leaves broadly lanceolate to oblong-lanceolate, scabrous-puberulent, subentire to entire; pappus well developed, 6‑10 mm long; very rare adventive      

.........  Plectocephalus (Centaurea) americanus - basketflower.


7.    Plants perennial.


9.    Involucre 0.9‑1.2 cm long, whitish green; outer phyllaries entire at tips; pappus 5‑11 mm long; very rare adventive        

..........  Acroptilon (Centaurea) repens - Russian knapweed.


9.    Involucre (1‑) 1.3‑1.8 cm long, brown to black, outer phyllaries laciniate to pectinate at tips; pappus less than 3 mm long or absent.


10.  Phyllary appendages decurrent along phyllary margins

.........  C. montana - mountain bluet.


10.  Phyllary appendages not or only slightly decurrent along phyllary margins. 


11.  Outermost phyllaries rounded to rounded ovate, the light brown scarious appendages entire or irregularly toothed to laciniate with very fine irregular cilia; pappus none

.........  C. Jacea - brown knapweed.


11.  Outermost phyllaries deltoid to deltoid-ovate, dark appendages deeply and regularly cut (pectinate); pappus very short (ca 1 mm) or none.


12.  Peripheral corollas of head not expanded and showy; pappus blackish, shorter than 1 mm; involucres appearing totally black, green parts of phyllaries obscured by black appendages; upper cauline leaves short pointed   

.........  C. nigra - black knapweed.


12.  Peripheral corollas of head enlarged, splayed outwards, and showy (heads radiant); pappus absent or rudimentary, when present usually not blackish; green parts of phyllaries sometimes evident or appendages light to dark brown; upper cauline leaves blunt.


13. Heads relatively narrow, pressed involucres usually longer than wide; involucres black and green, green parts of phyllaries usually visible            

.........  C. nigrescens - short-fringed knapweed.


13. Heads relatively broad, pressed involucres usually as wide or wider than long; involucres light to dark brown, green parts of phyllaries usually not visible; very rare adventive

.........  C. Monctonii (C. Debauxii) - meadow knapweed.



CIRSIUM - thistle


(x = 17; n = 18-10)


1.   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.    Leaves scabrous-hispid or crisped-hispid and also sometimes silky-pubescent above, more or less cobwebby and sometimes crisped-hispid or tomentose beneath.


3.    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 (s = 4x; x = 17)          

.........  C. vulgare - bull thistle.


3.    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.    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 (s = 2x; x = 15)

.........  C. pumilum var. Hillii (C. Hillii) - Hill's thistle.


4.    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.    All leaves deeply lobed (except in juvenile forms) , the lobes linear-acuminate, terminating in stout spines, the thickish margins involute; involucral spines 3‑7 mm long; plants mostly of open places (s = 2x, 2x-1, 2x-2; x = 11)

.........  C. discolor - prairie thistle.


5.    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 (s = 2x; x = 9)  

.........  C. altissimum - wood thistle.


2.    Leaves white-tomentose on both surfaces, often more thinly so above, totally lacking hispidity; dorsal glutinous ridge present on phyllaries.


6.    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 (s = 2x; x = 17)           

.........  C. Pitcheri - dune thistle.


6.    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.    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 2 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 (s = 2x; x = 11 & 12)            

.........  C. Flodmanii - Flodman's thistle.


7.    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 (s = 2x; x = 13)

.........  C. undulatum - wavy-leaved thistle.


1.   Outer and middle phyllaries with at most a short spine or mucro, this up to 1 mm long (and then involucre about 1 cm in diameter).


8.    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 (s = 2x; x = 17)        

.........  C. arvense - Canada thistle.


8.    Biennials (at least monocarpic); florets perfect; plants of moist habitats.


9.    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 (s = 2x [20, 21, 22, 23, 30], x = 10 & 11)          

.........  C. muticum - swamp thistle.


9.    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. (s = 2x; x = 17)   

.........  C. palustre - European swamp thistle.



ECHINOPS - globe-thistle


(s = 2x; x = 15 & 16)


.........  E. sphaerocephalus - great globe-thistle.


ONOPORDUM - cotton or Scotch thistle


(s = 2x; x = 17)


.........  O. Acanthium - Scotch thistle.


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