This rhizomatous perennial grows erect stems, often 6 to 8 feet (1.8-2.4 m) tall. It usually occupies moist or marshy sites, but once established tolerates intermittently dry areas.
The leaves are simple, entire, and grow opposite each other or are whorled along the stem.
Rose-purple flowers have 5 to 7 petals and are arranged in long vertical racemes at the end of the stems. They are very showy mid-summer to fall.
The small seeds, less than 1/16 inch (1 mm), are dark. This is an aggressive weed in moist sites because of its stout roots and its tendency to spread and propagate from seed and rhizomes.
Purple loosestrife is an introduced European ornamental. It has escaped to infest aquatic sites - streambanks, shorelines of shallow ponds and reservoirs, canals and ditches where it impedes water flow. Habitat for wildlife is greatly reduced. Many states have declared Purple loosestrife a noxious weed. The Nevada Division of Agriculture has requested nurseries stop selling it as an ornamental.
Showy rose-purple flowers bloom in long vertical racemes.