[2][3][4] Waterwillow is found in swampland, in ditches, besides streams and in shallow water at the edges of ponds and lakes.

It often forms thickets and occurs in the United States from Maine to Florida and west to Minnesota, Tennessee and Louisiana, as well as in eastern Canada.

[5] Seeds of Decodon from the late Campanian (73.5 MA) of northern Mexico are the earliest fossil record of the genus.

[6] A "whole plant" description has been give for the Ypresian age Decodon allenbyensis described from the Eocene Okanagan Highlands Princeton Chert site.

A seed very similar to the modern American species has been found in sediments from Ipswichian in Ireland, and it is possible that the plant survived until the last interglacial in western Europe.