1 Flowers in axillary verticils subtended by ordinary foliage leaves, and separated by internodes of ordinary length.

  2 Calyx glabrous throughout, or pubescent toward the tips only; calyx 2-3.5 mm long; plants usually sterile; fresh plant usually with spearmint odor or flavor

  2 Calyx pubescent throughout its length; calyx 1.5-2.5 mm long; plants usually fertile; fresh plant usually with a rather unpleasant odor or flavor.

    3 Leaves subtending the inflorescence mostly broadly rounded at the base; leaves of the inflorescence relatively narrow; [exotic]

    3 Leaves subtending the inflorescence mostly cuneate at the base; leaves of the inflorescence relatively broad; [native, though often in weedy situations]

1 Flowers in terminal spikes or heads, the subtending leaves absent or distinctly smaller than the foliage leaves.

      4 Inflorescence a terminal globose to ovoid head of 1-3 verticils.

        5 Pedicels, calyx, and leaves pubescent; plants usually fertile

        5 Pedicels and calyx glabrous, leaves glabrous or nearly so; plants usually sterile

      4 Inflorescence a spike of several to many verticils.

          6 Bracteal leaves much longer than the flowers, resembling the foliage leaves, but smaller or narrower

          6 Bracteal leaves linear to lanceolate, little surpassing the flowers.

             7 Calyx tube glabrous; leaves glabrous, or with scattered hairs on the lower surface.

               8 Petioles of the main leaves 4-15 mm long; spikes stout; plants sterile; fresh plant with peppermint odor or flavor

               8 Petioles of the main leaves 0-3 mm long; spikes slender; plants fertile; fresh plant with spearmint odor or flavor

             7 Calyx tube pubescent; leaves moderately to densely hairy on the lower surface.

                 9 Leaves lanceolate to oblong-lanceolate, > 3× as long as wide.

                   10 Hairs of the leaf undersurface unbranched; leaves widest near the middle, slightly rugose; fertile anthers 0.28-0.38 mm long; fresh plant with musty flavor or odor

                   10 Hairs of the leaf undersurface dendritic; leaves oblong lanceolate, widest toward the base, conspicuously rugose; fertile anthers 0.38-0.52 mm long; fresh plant with spearmint odor or flavor

                 9 Leaves oblong to ovate, 1-3× as long as wide.

                     11 Leaves generally 1-2× as long as wide, ovate-orbicular, broadly rounded to subcordate at the base, obtuse at the apex; leaf serrations rounded and often turned downward (thus appearing crenate); leaf surface strongly rugose, with scattered dendritic hairs below; fresh plant with sickly or sweet flavor or odor

                     11 Leaves generally 1-3× as long as wide, ovate to oblong, broadly cuneate to rounded at the base, acute at the apex; leaf serrations sharp; leaf surface moderately rugose; fresh plant with spearmint odor.

                       12 Leaves generally oblong, with nearly parallel sides and a broadly rounded base; flowers consistently with 4 fertile anthers