A long history of cultivation of various taxa, including hybrids, has resulted in a shifting in application of names in this genus. Some hybrids are well established outside of cultivation, efficiently spreading vegetatively, and are here mapped and treated in full.

1. Flowers in axils of ordinary leaves (these, like the internodes, only gradually reduced upwards on the stem).

2. Uppermost bracteal leaves small but still ± leaf like, with a short petiole, and exceeding the flowers.

M. canadensis

2. Uppermost bracteal leaves reduced to inconspicuous bracts usually not extending beyond the flowers.

M. × gracilis

1. Flowers all or mostly crowded into terminal inflorescences, with most of their internodes obscured.

3. Leaves ± densely pubescent (even tomentose) beneath.

4. Leaf blades less than twice as long as broad, the tip and teeth obtuse or rounded, with many branched hairs beneath.

M. suaveolens

4. Leaf blades about twice or more as long as broad, acute, sharply toothed, with branched hairs few or none beneath.

M. × villosa

3. Leaves glabrous or nearly so.

5. Principal leaves sessile or with petioles less than 3 mm long; inflorescences (including corollas) ca. 7-9 (-10) mm thick, mostly 3.5-8 times as long.

M. spicata

5. Principal leaves with petioles (2.5-) 3-12 mm long; inflorescences (at least principal central one) ca. 12-15 mm thick and 2-3 times as long as broad or shorter.

6. Calyx strictly glabrous; leaves all or mostly obtuse to rounded at apex; foliage with lemon-like odor.

M. aquatica

6. Calyx at least slightly pubescent (on the lobes); leaves all clearly acute at apex; foliage with peppermint odor.

M. × piperita