Largely sterile hybrid of nos. 1 [Mentha arvensis L.] and 7 [Mentha spicata L.], resembling the smoother forms of no. 1, but usually with smaller floral lvs; lvs lanceolate to ovate, glabrous or nearly so, sharply toothed in the distal half; cal-tube glabrous, or sparsely pubescent in the upper half, the lobes pilose; 2n=54-120. European cultigen, locally escaped from cult. in our range. (M. cardiaca)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.