Comparison of spiny-leaved thistles: flowers pink, purple, or white, except where noted

  stem wings phyllary margins receptacle chaff pappus fruit attachment other

Carduus spp.

musk, giant plumeless, Italian, & slenderflower thistles

conspicuous

(to ~ 1 cm wide)

smooth dense bristles soft minutely barbed bristles, fused at base; deciduous basal, slightly angled Italian thistle leaves sometimes faintly mottled white

Cirsium spp.

bull, Canada, yellowspine, & wavyleaf thistles

conspicuous (to 1 cm wide; bull thistle), none (Canada thistle), or inconspicuous smooth dense bristles soft feathery bristles, fused at base; deciduous basal, slightly angled  

Onopordum spp.

Scotch, Illyrian, & Taurian thistles

conspicuous (mostly 1-4 cm wide) smooth membranous extensions around pits +/- firm, minutely barbed bristles, fused at base, deciduous basal receptacles deeply pitted & honeycomb-like; fruits transversely wrinkled

Cynara cardunculus

artichoke thistle

none smooth dense bristles long, stiff, feathery bristles, fused at base; deciduous basal basal leaves often compound & very large; receptacle fleshy

Silybum marianum

milk thistle

none spiny-toothed dense bristles soft, minutely barbed bristles, fused at base; deciduous basal, slightly angled leaves bright green & mottled white

Carthamus spp.

whitestem, smooth, & woolly distaff thistles

none outer leaf-like, spiny pinnate-lobed narrow scales 0 or many unequal scales in several series; persistent lateral notch near base smooth & woolly distaff thistles flowers yellow; fruits +/- 4-angled

Scolymus hispanicus

golden thistle

conspicuous (mostly to ~ 1 cm wide) smooth long scales enclose fruits 2-4 stiff, minutely barbed bristles; +/- deciduous basal flowers yellow, ligulate; sap milky

Cnicus benedictus

blessed thistle

none or inconspicuous outer smooth; inner with long, pinnate- branched spines at tips dense bristles ~ 20 stiff bristles in 2 series, outer long, inner short; persistent lateral notch near base flowers yellow; heads leafy; fruits cylindrical, with ~ 20 prominent ribs & pale crown-like teeth on apical rim