Purple Loosestrife
Lythrum salicaria
Washington State Classification
Class B — Control Required (Designated)
Purple loosestrife is a vigorous competitor and can crowd out other vegetation including native species.
Required control in Region 1 (all western Washington counties)
Quick Reference
Identification
Growth Habit
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.
Leaves
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.
Flowers
Flowers are densely clustered on a 4-16 inch terminal flowering spike. Flowers are showy and magenta with 5 to 7 petals.
Fruit & Seeds
Seeds are in capsules.
Impact
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.
Ecology & Spread
Habitat
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.
Spread Mechanisms
Reproduction
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.
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Control Methods
Mechanical
- Cutting alone is not a control option for purple loosestrife. Shoots and adventitious roots will develop. Digging up small infestations or individual plants can be effective. Make sure to remove the roots. Remove and properly dispose of plant parts (stems, roots, root crown) to avoid plants parts resprouting. Monitor the site for any resprouting plants or seedlings. Check with your county noxious weed control board about local disposal options.
Cultural / Prevention
- Careful digging is useful to manage weed populations. However, digging can carry undesirable weed seed to the surface and foster further germination. Cut the stems at the base to remove flower and seed heads prior to seed dispersal.
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.