Decodon verticillatus, the sole species in the genus Decodon, is a flowering plant in the Lythraceae family. It is commonly known as waterwillow or swamp loosestrife. It is native to wetlands in the eastern half of the United States and Canada.
Waterwillow is a clump-forming shrubby perennial that grows in swamps or shallow water. The stems are arching, angular, smooth and woody near the base, and up to Template:Convert tall. They sometimes root at the tip when they bough over and touch the mud. The leaves are lanceolate, either in opposite pairs or in whorls of three or four. They are up to Template:Convert long and Template:Convert wide, smooth above and hairy beneath, on very short stalks. The rose-pink flowers grow in axillary clusters. The calyx is cup shaped, the corolla under Template:Convert wide with usually five petals narrowing at the base. The ten stamens are projecting with five longer than the rest. There is one pistil, one style and a superior ovary. The fruit is a spherical dark brown capsule with numerous reddish seeds. Flowering takes place in June and July.<ref>Homer D. House, Wild Flowers Of New York University of the State of New York, 1918</ref><ref>Water Willow (Decodon verticillatus). Northern Prairie Wildlife Research Center]</ref><ref>Biota of North America Program, 2013 county distribution map. Bonap.net (2014-12-15). Retrieved on 2016-01-22.</ref>
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.<ref>Swamp Loosestrife; Willow-Herb – Decodon Verticillatus. Chestofbooks.com. Retrieved on 2016-01-22.</ref>
Seeds of Decodon from the late Campanian (73.5 MA) of northern Mexico is the earliest fossil record of the genus.<ref>Fossil Records in the Lythraceae Graham, Shirley A. The Botanical Review79.1 (Mar 2013): 48-145.</ref> Seeds of the genus are known in Europe from Pliocene to lower Pleistocene. These seeds are assigned to an extinct species, D. globosus. 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.<ref>Template:Cite book</ref>
- The larva of the hydrangea sphinx, Darapsa versicolor, feeds on waterwillow.<ref>Oehlke, Bill. Darapsa versicolor (Harris, 1839) Choerocampa versicolor Hydrangea Sphinx. silkmoths.bizland.com</ref>
- The larva of the waterwillow stem borer moth, Papaipema sulphurata, feeds on waterwillow.<ref>Endangered Species/Biodiversity Conservation. Lloyd Center for the Environment</ref>