Stout, tall, grey hairy, tufted plant to 1.5 metres. Stems with 4, sometimes more, raised lines. Leaves opposite in whorls of three, the upper sometimes alternate. Flowers reddish purple, 10 to 15 mm in whorls forming long spikes, usually with 6 petals and 12 stamens.

Freshwater margins.

June to August.

Perennial.

Widespread in Britain, but scarce in Scotland except for the west coast.

Occasional in Leicestershire and Rutland. In the 1979 Flora survey of Leicestershire it was found in 41 of the 617 tetrads.