Pleasantly aromatic rhizomatous perennial 5-12 dm, often branched; stems usually hairy at least above; lvs deltoid-lanceolate to lanceolate or rarely ovate, acuminate, ±serrate, rounded or truncate to broadly acute at base, thinly hairy or glabrous to conspicuously hairy above or canescent beneath, the larger 6-10 cm on petioles 1-1.5 cm; heads 1-3.5 cm wide (excluding the cors); bracteal lvs lanceolate or ovate; cal 7-10 mm, puberulent, its throat densely hirsute internally with erect white hairs a third to half as long as the cal-lobes, these subulate, 1-2 mm; cor bright (but rather pale) lavender, 2-3.5 cm, the somewhat arcuate upper lip densely villous at the tip; 2n=32, 34, 36. Upland woods, thickets, and prairies. Que. to Man. and B.C., s. to Ga., La., and Ariz. June-Sept. (M. mollis, the common form with the hairs of the stem decurved) Most of our plants are var. fistulosa, as principally described above. The western var. menthaefolia (Graham) Fernald, a shorter, less branched plant, often with a single head, the longest petioles seldom over 1 cm, enters our range in w. Minn. (M. menthaefolia)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.