Thistles

Thistles (Cirsium)

.....
small fl heads	Group I
larger heads:
none of the invol bracts spine-tipped	Group II
at least outer bracts spine-tipped:
stem conspicuously winged by decurrent lf bases or not winged, but heads conspicuously & closely invested by a series or narrow, strongly spiny-toothed leaves (yellow flowers)	Group III
stem merely crisp-hirsute to subglabrate; upper sufraces of the lvs glabrous or hirsuite, but not tomentose	Group IV
stem densely white tomentose; upper lf surfaces thinly white-tomentose or eventually glabrate	Group V

Group I

Canada thistle (16. arvense) 1-3' high; fields & waste places; Eurasian, now widely introduced

Group II

.....	stem not spiny-winged	stem spiny-winged
invol 1-2cm; fruiting pass ca 1cm or less	.....	marsh thistle (14. palustre) 3-9' high; often invading woods; Eurasian, widely introduced; NF to DE & in mts to NC
invol 2-3.5cm; fruiting pappus 1.5-2.5cm	swamp thistle (13.muticum) swamps & bogs, wet meadows, & moist woods & thickets	.....

Group III

.....	stem not or scarcely winged	stem conspicuously winged
yellow fls	yellow thistle (1. horridulum) open places, esp in sandy soil or along salt or fresh marshes; coastal state & PA; ME to FL	.....
purple fls	.....	bull thistle (15. vulgare) very prickly; 3-6' high; pastures, fields, roadsides & waste places; Eurasian, widely estab in N Am

Group IV

.....	lvs merely arachnoid-tomentose beneath, often eventually glabrate	lvs closely & persistently white-tomentose beneath
invol 1.5-2.5cm; achenes 3-4mm	.....	Virginia thistle (7. virginianum) savannas, bogs, & wet pinelands; s NJ to FL
invol mostly 2.5-3.5cm; achenes 4-6mm	.....	field thistle (5.discolor) lvs cdeeply pinnatifid; 3-7' high; fields, open woods, river bottoms & waste pls; QE & ON to NC tall thistle (6. altissimum) lvs toothed or shallow lobed; 3-10' high; fields, waste places, river bottoms & open woods: MA to FL
invol 3.5-5cm; achenese 3-4mm	pasture thistle (4. pumilum) 1-3' high; pastures, old fields, & open woods; s ME to PA in in mts to NC	.....

Group V

.....	lvs coarsely toothed to pinnatifid	lvs pinnatifid
relatively slender plants of poorly drained sites, with a well developed tendency to spread by slender, short-lived creeping roots	.....	prairie thistle (9.flodmanii) swales & other moderately moist, poorly drained sites on the Great Plains, occ intro eastward
stouter plants of well-drained sites, creeping roots poorly or scarcely developed	wavy-leaved thistle (10.undulatum) hillsides, prairies & railroad tracks, & other open places, in well-drained soil; occ. eastward	.....