Any ornamental plant of the family Lythraceae, especially in the genera Lythrum and Decodon , and two genera of the primrose family ( Lysimachia and Steironema ).
Purple loosestrife ( Lythrum salicaria ), native to Eurasia, grows 2–6 ft (0.6–1.8 m) high on riverbanks and in ditches. Its branched stem bears whorls of narrow, pointed, stalkless leaves and ends in tall, tapering spikes of red-purple flowers. Introduced into North America early in the 19th century, it has become a noxious weed in many parts of the U.S. and Canada because its dense growth outcompetes native wetland vegetation that provides food and habitat for wildlife.