Purple loosestrife is a semi-aquatic herbaceous plant belonging to the loosestrife family, Lythraceae, native to the wetlands of Eurasia.
It is an herbaceous perennial plant, growing 1-2 m tall, forming colonies 1.5 m or more in width with numerous erect stems growing from a single woody root mass. The stems are reddish-purple and square in cross section. The leaves are lanceolate, 3-10 cm long and 1-2 cm broad, downy and sessile, and arranged opposite or in whorls of three. The flowers are reddish purple, 10-15 mm diameter, with six petals. Flowering lasts throughout the summer. When the seeds are mature, the leaves often turn bright red through dehydration in early autumn; the red color may last for almost two weeks. The dead stalks from previous growing seasons are brown.
Purple loosestrife has become an invasive species since its introduction into temperate New Zealand and North America where it is now considered a noxious weed. The seeds probably first arrived in the plant's non-native areas in muddy ballast water emptied from ships. The plants grow vigorously and spread very fast when removed from their natural controlling agents. Infestations result in dramatic disruption in water flow in rivers and canals, and a sharp decline in biological diversity. Native food and cover plant species, notably cattails, are completely crowded out, and the life cycles of organisms from waterfowl to amphibians to algae are affected.