Water mint is a
herbaceous rhizomatous perennial plant growing to 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, long and broad, green (sometimes purplish), opposite, toothed, and vary from hairy to nearly hairless. The
flowers of the watermint are tiny, densely crowded, purple (pinkish to lilac), tubular and form a terminal hemispherical inflorescence; flowering is from mid to late summer. Water mint is visited by many types of insects, and can be characterized by a generalized
pollination syndrome, but can also spread by underground rhizomes. All parts of the plant have a distinctly minty smell. Unbranched, hairless plants, with narrower leaves and paler flowers, native to areas of Sweden and Finland near the
Baltic Sea, have been called
Mentha aquatica var.
litoralis.
Mentha aquatica is a
polyploid, with 2
n = 8
x = 96
chromosomes. ==Taxonomy==