Cirsium undulatum (Nutt.) Spreng.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Species Description - Stout, short-lived perennial from a taproot, this sometimes branching below-ground and producing more than one stem, but without the well developed creeping roots of no. 9 [Cirsium flodmanii (Rydb.) Arthur]; stem 3–12 dm, usually branched above, densely, closely and persistently white-tomentose; lower lf-surfaces similarly tomentose, the upper surfaces more thinly so and sometimes eventually glabrate; lvs coarsely toothed to pinnatifid, the lobes ovate, deltoid, or occasionally oblong, seldom under 7 mm wide; heads mostly several or many, tending to be broadly and shallowly invaginated at the base; invol 2.5–4(–5) cm, its bracts with a glandular-glutinous dorsal ridge and commonly a little marginal tomentum, well imbricate, the inner with attenuate and often crisped tips, the others with spine-tip 3–5 mm; fls pink-purple, sometimes rather pale; achenes 4–7 mm, inconspicuously or not at all banded across the top, becoming mucilaginous when wet (unlike all related spp.); 2n=26. Hillsides, prairies, railroad-tracks, and other open places, in well drained soil; B.C. to Ariz., e. to s. Man., Minn., and Mo., and occasionally intr. eastward. June–Sept. (C. megacephalum)