Purple Loosestrife

Lythrum salicaria

Family: Lythraceae

Other common names: spiked loosestrife

Weed Class: B

Year Listed: 1988

Native to: Europe, Asia, Australia and Northern Africa

Toxic: not known to be

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Why is it a noxious weed?

Purple loosestrife is a vigorous competitor and can crowd out other vegetation including native species. It can quickly dominate a site and adapt to environmental changes. Loosestrife stands provide poor cover for waterfowl.

How would I identify it?

General Description: Purple loosestrife is an emergent aquatic plant. It can reach up to 10 feet tall and 5 feet wide and has a persistent, perennial tap root and spreading rootstock.

Flower Description: Flowers are densely clustered on a 4-16 inch terminal flowering spike. Flowers are showy and magenta with 5 to 7 petals.

Leaf Description: Leaves are alternate, opposite or in whorls of 3. They are 1.5 to 4 inches long, lance-shaped to narrowly oblong and sometimes are covered with fine hairs.

Stem Description: Stems are herbaceous and upright, branched or unbranched and somewhat square with 4 to 6 sides. Each plant may have 30 to 50 stems with flowers that form at the ends.

Fruit/Seed Description: Seeds are in capsules.

May be confused with: Native spirea (Spirea douglasii) and fireweed (Chamerion angustifolium, syn. Epilobium augustifolium) look similar to purple loosestrife. If you need help with plant identification, please contact your county noxious weed coordinator.

How does it reproduce?

Purple loosestrife reproduces by seed and vegetatively by stems fragments in favorable conditions. A mature plant can produce 2.7 million thin-walled, flat seeds.

Where does it grow?

Purple loosestrife occurs in freshwater and brackish wetlands as well as streams, marshes and other habitats with moist ground or standing water.Please click here to see a county level distribution map of purple loosestrife in Washington.

How do I control it?

Mechanical Control

Cutting alone is not a control option for purple loosestrife. Shoots and adventitious roots will develop.

Cultural Control

Biological Control

Loosestrife beetles, Galerucella calmariensis and Galerucella pusilla, adults and larvae impact plant growth and reproduction by feeding heavily on the plant's leaves, stems and buds. The loosestrife root weevil, Hylobius transversovittatus adults feed on plant foliage and the larvae feed within the roots.The loosestrife seed weevil, Nanophyes marmoratus, adults and larvae impact the plants by feeding on unopened flower buds. For more information about these biological control agents of purple loosestrife, please visit WSU Extension Integrated Weed Control Project.

Herbicide Control

Because herbicide availability and regulation differ between states, we recommend the Washington Department of Ecology website for information on aquatic weed management and herbicides, or contact your county noxious weed coordinator.