Rhizomatous perennial; stems glabrous or sparsely hairy, to 60 cm or more tall, often submerged; aerial stems erect, often purple. Lvs very variable, petiolate; lamina 1-6 × 1-3.5 cm, lanceolate-elliptic, elliptic, oblong-ovate, ovate, or suborbicular, aerial lvs somewhat rugose, serrate, hairy or glabrous; base cuneate to rounded; apex sharply acute. Infl. of a terminal, ± rounded head and 1-3 distinct verticels of shortly pedicellate fls below, or an oblong spike, sometimes with 1-4 distinct verticels below, often glabrous except for calyx teeth. Calyx 3-4 mm long, tubular, purple, dotted with oil glands; teeth equal, ciliate, narrow-acuminate, much < tube at anthesis. Corolla 5-6 mm long, mauve or lilac, glabrous; tube exserted. Stamens small, included. Nutlets not formed.
A natural hybrid, Mentha aquatica x Mentha spicata, widespread as a garden escape and also naturalized, being found in moist soils in ditches, waste places etc.
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