Water Mint is a short- to medium-sized perennial. The flowers are dense and lilac-coloured; they grow at intervals high up on the stem, finishing with a group of flowers at the top. The stamens protrude; the calyx is long and hairy. The leaves are stalked, toothed and grow opposite each other. The stems are stiff.
Like many mints, Water Mint likes wet ground, so that it is found in marshes and lake shores throughout Ireland. The leaves give off strong minty scent when crushed. Water Mint sometimes hybridise with the ordinary garden mint, Spear Mint (Mentha spicata) to give peppermint, as well as with Corn Mint (Mentha arvensis).
The flowers appear in July, August and September. Water Mint grow in Europe, south-west Asia, North and South Africa and Madeira.