Non-native Invasive Freshwater Plants

Purple Loosestrife (Lythrum salicaria)

Purple loosestrife (Lythrum salicaria) is a Eurasian native perennial wetland plant which is responsible for the degradation of a considerable amount of wetland habitat in the United States. Invasion of North American wetlands by purple loosestrife began in the early nineteenth century when the plant was introduced both as a contaminant of European ship ballast and as a valued medicinal herb for treatment of diarrhea, dysentery, and ulcers. By the 1830’s, purple loosestrife was well established along the New England seaboard. The continued expansion of the plant across the country coincided with an increase in national and regional transportation systems, commercial distribution of the plant for horticultural uses, and regional propagation of plant seed for growing bee forage.

Growth Habit

Purple loosestrife prefers to grow in marshes, ponds, stream banks, ditches, and lake shores; occasionally it can be found in upland areas. It can grow to two meters (about six feet) tall and has spikes of five-petaled reddish-purple flowers. The plant’s leaves occur opposite each other along a square stem. A single mature plant can produce more than 2.5 million seeds annually. The seeds are long-lived and easily dispersed by water and in mud adhering to aquatic wildlife, livestock, and people. A strong rootstock serves as a storage organ, providing resources for plant growth if the above ground stems are cut, burned, or killed by application of foliar herbicides. All of these characteristics make a very aggressive plant extremely difficult to control.

In Washington, purple loosestrife is classified by law as a noxious weed. This law requires a landowner to control the spread of the plant or prevent any seed production by the plant. Because purple loosestrife grows so aggressively, over time large stands of the plant take over an area and replace the native plant species. This results in eliminating the natural foods and cover essential to many wetland inhabitants including waterfowl. In addition, purple loosestrife can grow in and around irrigation systems and impede the flow of water. The Winchester Wasteway, an irrigation system in Grant County, Washington, used to have one of the largest stands of purple loosestrife in the country. Purple loosestrife is a cause for grave concern among water users and purveyors.


No native herbivores or pathogens in North America are known to suppress purple loosestrife. Current means of plant control include mechanical cutting, application of herbicides, or introduction of classical biological control insects. These insects are from the plant’s native range in Europe. Several types of insects including a root-mining weevil and a leaf-eating beetle have been introduced into Washington in an attempt to manage purple loosestrife. The root-mining weevil lays eggs into the stem of the plant and the developing larvae work their way down into the roots. There the larvae feed extensively on the root tissue, weakening the plant from within. The adult weevils also feed on the newly formed purple loosestrife leaves. The leaf-eating beetles affect the plant in two ways: the adults feed on newly formed leaf tissue and the larvae feed on bud, leaf and stem tissue.

The introduction of these insects on purple loosestrife on the Winchester Wasteway has worked. The insects (the leaf-eating beetle) has had significant impacts on purple loosestrife stands; defoliating the plants and preventing seed set. Observers reported that the affected plants appeared dead or dying and looked as if they had been sprayed with herbicides. Unfortunately, as purple loosestrife declined, another state-listed noxious weed, the non-native genotype of the common reed Phragmites autralis colonized the area once occupied by purple loosestrife. County weed board staff and others collected insects from the Wasteway and have dispersed them to purple loosestrife locations throughout Washington. It appears that in areas with fluctuating water levels (tidal Columbia River for example), leaf-eating insects do not successfully establish.


Purple loosestrife is sometimes confused with fireweed (Epilobium angustifolium) a pink-flowered herb that inhabits dry often disturbed sites such as clear-cut areas and roadsides. It is also confused with hardhack (Spiraea douglasii), a woody shrub also with pink to purple flowers. Neither fireweed or hardhack have square stems. Purple loosestrife has vivid purple-pink flowers and blooms in summer and early fall. The stems are generally square. The leaves are usually oppositely arranged and are lance-shaped.

Please report sightings of purple loosestrife to your county or district weed board.

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