Cirsium arvense (L.) Scop.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Species Description - Colonial perennial 3–15(–20) dm from deep-seated creeping roots, subglabrous, or the lvs ± white-tomentose beneath; heads ± numerous in an often flat- topped infl, unisexual or nearly so, the plants polygamo-dioecious; invol 1–2 cm high, its bracts all innocuous, or the outer with a weak spine-tip ca 1 mm; fls pink-purple (white); achenes 2.5–4 mm; pappus of the pistillate heads surpassing the corollas, that of the staminate heads surpassed by the cors; 2n=34. A noxious weed of fields and waste places; native of Eurasia, now widely intr. in n. U.S. and s. Can. July, Aug. The var. arvense has merely toothed or shallowly lobed, weakly spiny lvs. (C. setosum; C. a. var. mite) The more common var. horridum Wimmer & Graebner has more spiny, deeply pinnatifid lvs.