Decodon is a genus of flowering plants in the loosestrife family, Lythraceae (order Myrtales), represented today by a single living species, Decodon verticillatus — commonly known as waterwillow or swamp loosestrife. It is native to wetlands of the eastern United States and eastern Canada, ranging from Maine to Florida and west to Minnesota, Tennessee, and Louisiana.
Waterwillow is a clump-forming, shrubby perennial of swamps and shallow water. Its arching, angular stems are smooth and woody near the base and grow up to about 8 feet (2.4 m) tall; where a stem bows over and touches the mud it can root at the tip, helping the plant spread vegetatively into dense thickets. The lanceolate leaves are borne in opposite pairs or whorls of three or four. Rose-pink flowers appear in axillary clusters in June and July, each with a cup-shaped calyx, a corolla of usually five petals under an inch wide, and ten stamens of unequal length. The fruit is a spherical, dark brown capsule packed with numerous reddish seeds.
The genus name was published by Johann Friedrich Gmelin in 1791. Although only one species survives today, Decodon has a substantial fossil record: seeds from the late Cretaceous (Campanian, roughly 73.5 million years ago) of northern Mexico are its earliest known trace, and a well-preserved "whole plant" fossil, Decodon allenbyensis, has been described from the Eocene Princeton Chert site in British Columbia's Okanagan Highlands. European seed fossils from the Pliocene through early Pleistocene are assigned to an extinct species, D. globosus, and a seed closely resembling the living American species, found in Ipswichian-age (last interglacial) deposits in Ireland, hints that the genus may have persisted in western Europe far more recently than its current New World-only range suggests.
Waterwillow thickets in swamps, ditches, and along pond and lake margins also support wildlife: the larvae of the hydrangea sphinx moth and the waterwillow stem borer moth both feed on the plant.
Distribution
Native to wetlands of the eastern United States (Maine to Florida, west to Minnesota, Tennessee, and Louisiana) and eastern Canada. GBIF distribution records across numerous eastern North American regions corroborate this range.
Ecology
Grows in swamps, ditches, streamsides, and shallow water at the edges of ponds and lakes, often forming dense thickets. Larvae of the hydrangea sphinx moth (Darapsa versicolor) and the waterwillow stem borer moth (Papaipema sulphurata) feed on the plant.
Taxonomy Notes
The genus name Decodon was published by J. F. Gmelin in 1791. Only one species survives today (Decodon verticillatus), but the genus has a long fossil record: seeds from the late Campanian (~73.5 Ma) of northern Mexico are its earliest trace; a whole-plant fossil, D. allenbyensis, is described from the Eocene Princeton Chert (Okanagan Highlands); European Pliocene–early Pleistocene seeds are assigned to the extinct D. globosus; and a seed closely resembling the modern species from Ipswichian (last interglacial) deposits in Ireland suggests the genus may have persisted in western Europe much later than its present New World-only range.
