WSU Extension

Hortsense

Weeds : Purple loosestrife (Purple lythrum) : Lythrum salicaria
(revision date: 6/9/2014)

Family: Lythraceae
Cycle: Perennial
Plant Type: Broadleaf

Use Integrated Pest Management (IPM) for successful weed management.

Biology
Purple loosestrife is an erect plant growing six to eight feet tall from a woody, rhizomatous root system. The original plant spreads to form a large, pure stand that is impenetrable by other species. However, spread from the roots is limited and purple loosestrife reproduces primarily by seed. Leaves are typically opposite (sometimes in whorls) and lack petioles. The leaves are attached directly to the four- or six-sided main stem or branches. The smooth-margined leaves are generally lance-shaped, tapering to a pointed tip. Leaves and stems are hairy. Flowers are borne in long spikes at the tips of stems and branches. The five to six petals are a distinctive purple to magenta color. Plants have a bright red fall color (when conditions permit). Brown stalks persist through winter. SPECIAL INFORMATION: Dense stands of purple loosestrife impact native wetland ecosystems, and adversely affect native species. In WASHINGTON, both purple loosestrife and wand loosestrife (Lythrum virgatum) are designated as Class 'B' noxious weeds. Both species are also on the Lythrum quarantine list, which prohibits sale, purchase, and transport of plants, seeds, and plant parts of these species and their hybrids, including horticultural varieties. In OREGON, purple loosestrife is a Class 'B' noxious weed. In addition, it is on the noxious weed quarantine list and is also designated a target (T) weed. Target weeds are considered a priority weed for statewide management. Control may be required in your county. Consult your local Noxious Weed Control Board for more information.
Habitat
Purple loosestrife is locally well-established on marshy sites, stream banks, and other wet areas. It can be found in both eastern and western Washington.

Management Options

Non-Chemical Management
  • 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.
Select non-chemical management options as your first choice!

Chemical Management

Applications should be made by those holding a current aquatic pest control license. In Washington, a special permit is required for use of herbicides in aquatic sites. Contact the Washington Department of Ecology or the Washington State Noxious Weed Control Board for more information.

Images
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Caption: Purple loosestrife in bloom
Photo by: N.R. Ness
Caption: Purple loosestrife flower
Photo by: D.G. Swan
Caption: Purple loosestrife invading wetland
Photo by: D.G. Swan
Caption: Purple loosestrife in bloom
Photo by: D.G. Swan