Spearmint,  Mentha spicata
Lamiaceae or Mint Family

Low plant, 1 to 3 feet tall.  Leaves opposite each other, stalkless or nearly so, egg-shaped, toothed, with strongly marked veins and a strong spearmint odor.  Flowers very small, pale pink or purple, in long interrupted spikes at the top of the stalk.  Individual flowers have 4 flaring lobes, the upper lobe larger.  Stems square in cross-section.  Blooms in summer.

Introduced exotic plant of wet places, frequently cultivated.  In Wildwood, occasional in damp places.

Square stems, opposite, strongly smelling leaves, and flowers in a terminal spike are characteristic of mints (Genus Mentha).  Peppermint (M. X piperita) is similar, but has a shorter, fatter flower head that is mostly not interrupted, and the larger leaves have distinct stalks.