Some similarities to : species of Lavender Lavendula which have similarly narrow purple-blue flower spikes but the leaves look nothing like these and they don't grow in fresh water (or any body of water).

Uniquely identifiable characteristics

Distinguishing Feature : The spathe-like vertical leaves.

A cross section of the stems reveals a network of empty parallel channels which allow the free transport of oxygen to the roots and to prevent the anoxic conditions in submerged muds from becoming toxic with microorganism generated components such as sulfide, manganese or iron compounds.

It is a garden pond throw-out which can become naturalised at the edges of ponds and found in Northern England down to South Lancashire; also in West Cork, Ireland.

It grows in water up to 1m deep and can be up to 1.3m in length total, so for deeper bodies of water it usually occupies the periphery. The fruit (not shown) is a kidney-shaped capsule with 6 toothed edges. The seeds within the fruits are also kidney-shaped, 3.5 to 4.5mm long by 2 to 2.5mm wide. It is not frost hardy and dies off in winter. Presumably the seeds grow into new plants the following spring.

It was introduced into bodies of water in 1949 and so far mainly occupies the South of England.