- Name also: Marsh Mint, Watermint
- Family: Mint Family – Lamiaceae (Labiatae)
- Growing form: Perennial herb. Rhizome creeping, with long runners.
- Height: 30–60 cm (12–25 in.). Stem erect–ascending, unbranched–branched, 4-edged, hairy–glabrous, often dark reddish.
- Flower: Corolla slightly zygomorphic, violet-red, approx. 5 mm (0.2 in.) long, 4-lobed. Uppermost lobe broader than the rest, with notched tip. Calyx narrowly campanulate, 5-lobed, grooved, clearly 13-veined. Lobes narrow. Stamens 4, almost of even length, longer than corolla. Gynoecium composed of 2 fused carpels. Inflorescence dense, hemispherical, in axillary whorls, sometimes also terminating branches.
- Leaves: Opposite, stalked. Leaf blade quite elliptic–narrowly elliptic, usually with blunt base, tips and teeth, almost glabrous.
- Fruit: 4-sectioned schizocarp. Mericarps glossy–granular, yellowish brown.
- Habitat: Shores, springs, stream banks, ditches, wet meadows, seashore meadows, inland ends of sea inlets where rushes thrive.
- Flowering time: July–August.
- Endangerment: Var. aquatica is vulnerable and var. litoralis is near threatened.
Water mint grows around water and is easy to recognise by its hemispherical or almost spherical inflorescence. Despite its name, the species is not limited to damp soil – sometimes it even grows on relatively dry patches beside fields and roads. The species’ habitat extends through Europe to northern Africa and western Asia, and it has travelled along with people to North and South America, Australia and certain Atlantic islands. It has been spread deliberately because fresh water mint stems make a nice herbal tea. Water mint is not impressed by Finland’s harsh winters and is only native to the Åland Isles. It later escaped from cultivation to colonise areas of mainland Finland along the coast of the Gulf of Finland, and it sometimes spreads outside the Åland Islands under its own steam.
(Mentha aquatica var. litoralis) is the variation of water mint that grows only beside the Baltic Sea. Earlier it was regarded as a separate species. It is virtually unbranched and glabrous, and additionally the leaves are narrower and the flowers lighter-coloured than water mint. It appears most commonly and distinctly on the Swedish coast and on nearby islands, and in Finland it grows on the Åland Islands and in distinct stands in Eurajoki in Satakunta province.