Knotweeds are emersed plants, and there are many species. Knotweeds grow in marshes and swamps, wet forests and ditches.
A distinguishing characteristic of knotweed is its swollen nodes where the leaves meet the stems. Leaves are lance-shaped, narrow, and willow-like. The leaf bases form sheaths that surround the stem. Leaves are alternately attached. Knotweed flowers are small and pinkish or white, and grow in terminal spikes that typically are several inches long.
For brief control information, see Efficacy of Herbicide Active Ingredients Against Aquatic Weeds by K. Langeland, M. Netherland, and W. Haller.