Very tiny pinkish flowers in leaf axils.
Feathery green leaves. Sprawling reddish stems, submerged and rising above water level. Very rapid growth, roots in mud and to water depths of several metres.
Garden escape. Originally sold as a pond oxygenator and marginal plant, now spreading into streams, lakes. Can choke slow-moving waterways. Potentially invasive introduction.
Similar: Water-milfoils, Myriophyllum species. Native.
Designated as a significant invasive plant in Ireland:
Managing Invasive Alien species