Much like no. 4 [Lycopus americanus Muhl.], and hybridizing freely with it in the Great Lakes-St. Lawrence region, but often with slender stolons as well as rhizomes, and averaging more hairy, the hairs along the veins of the lower lf-surface mostly 0.5-1.5 mm; lvs averaging wider and more bluntly toothed, commonly strigose above; bracts avg longer (5.5 mm); cal a little longer (3-4.5 mm) its lobes with long, subulate tip; anthers longer (0.5-0.7 mm); nutlets a little larger (1.3-1.7 נ 0.9-1.25 mm) and not so strongly tapering toward the base; 2n=22. European sp., casually intr. along the coast from Mass. to N.C., and now well established along the St. Lawrence R. and about lakes Erie and Ontario and w. Lake Huron. Hybridizes with no. 4 [Lycopus americanus Muhl.].
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.