Stems 6-20 dm, speckled or sometimes more evenly purplish, seldom glaucous; lvs mostly in 4's or 5's, lance-elliptic to lanceolate or lance-ovate, relatively gradually narrowed to the short petiole (sometimes more abruptly contracted in ssp. bruneri), 6-20 נ2-9 cm, seldom evidently triplinerved, sharply serrate; infl or its divisions ±flat-topped in life; invol 6.5-9 mm, often purplish, its bracts well imbricate in several series, often 3-5-nerved, obtuse, essentially glabrous, or the outer often ±inconspicuously short-hairy; fls purple to rather pale lavender, (8)9-22 per head; 2n=20. Moist places, especially in calcareous soils; Nf. to B.C., s. to Md., W.Va., Ill., Neb., N.M., and Utah, and along the higher mts. to N.C. and Tenn. July-Sept. Two sspp., one with two vars.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.