|Plant in flower|
It is a herbaceous rhizomatous perennial plant growing to 90 centimetres (35 in) tall. The stems are square in cross section, green or purple, and variably hairy to almost hairless. The rhizomes are wide-spreading, fleshy, and bear fibrous roots. The leaves are ovate to ovate-lanceolate, 2 to 6 centimetres (0.79 to 2.4 in) long and 1 to 4 centimetres (0.39 to 1.6 in) broad, green (sometimes purplish), opposite, toothed, and vary from hairy to nearly hairless. The flowers of the watermint are tiny, densely crowded, purple, tubular, pinkish to lilac in colour; flowering is from mid to late summer. Water mint is pollinated by insects, and also spreads by underground rhizomes, like other species of mint. All parts of the plant have a distinctly minty smell.
As the name suggests, water mint occurs in the shallow margins and channels of streams, rivers, pools, dikes, ditches, canals, wet meadows, marshes and fens. If the plant grows in the water itself, it rises above the surface of the water. It generally occurs on mildly acidic to calcareous (it is common on soft limestone) mineral or peaty soils. M. aquatica can occur in certain fen-meadow habitats such as the Juncus subnodulosus-Cirsium palustre plant association. It is a component of Purple moor grass and rush pastures - a type of Biodiversity Action Plan habitat in the UK
It hybridises with Mentha spicata (spearmint) to produce Mentha × piperita (peppermint), a sterile hybrid; with Mentha suaveolens (apple mint) to produce Mentha × suavis; with Mentha arvensis (corn mint) to produce Mentha × verticillata; and with both M. arvensis and M. spicata to give the tri-species hybrid Mentha × smithiana.
corolla mauve, leaves opposite 
|Wikimedia Commons has media related to Mentha aquatica.|
- Euro+Med Plantbase Project: Mentha aquatica
- Flora of NW Europe: Mentha aquatica
- Blamey, M. & Grey-Wilson, C. (1989). Flora of Britain and Northern Europe. ISBN 0-340-40170-2
- Huxley, A., ed. (1992). New RHS Dictionary of Gardening. Macmillan ISBN 0-333-47494-5.
- C. Michael Hogan. 2009. Marsh Thistle: Cirsium palustre, GlobalTwitcher.com, ed. N. Strömberg
- Rose, Francis (1981). The Wild Flower Key. Frederick Warne & Co. p. 310. ISBN 0-7232-2419-6.