Purple loosestrife, a regulated Class B noxious weed, is a 6-10-foot-tall perennial that grows on lakes and waterways throughout King County. Simple, smooth-edged leaves grow opposite or whorled on stiff, 4-6-sided stems. In July-September, tall spikes of tiny, 5-7-petaled, magenta flowers appear at stem ends, eventually dispersing up to 2.5 million pepper-sized seeds. This weed also spreads via its woody taproot, rhizomes, and vegetative growth.

Habitat and impact

Purple loosestrife is typically found invading lakeshores, wetlands, ponds, and wet pastures and ditches. The flowers are magenta, and they are found on tall, narrow spikes from July to October. Purple loosestrife has square stems, which help to tell it apart from some of the look-alikes that grow in the same areas. In winter months, dead brown flower stalks remain with old seed capsules visible on the tips. Purple loosestrife's appearance is similar to fireweed and spirea and is sometimes found growing with garden loosestrife along King County's lakeshores and wetlands.

Purple loosestrife is a long-lived wetland perennial that can reach over 9 feet tall. Plants can produce over two million seeds the size of ground pepper. This plant spreads by seed and root fragmentation. Purple loosestrife is native to Europe and Asia, and was initially introduced to the northeastern seaboard of the United States in the ballast of ships in the 1800's. It has also been repeatedly and continually introduced as a garden plant. Because of the impacts to fresh and brackish wetlands across the nation, purple loosestrife is targeted for control and is found on many noxious weed lists throughout the country including Washington State.

En español

Purple loosestrife folleto informativo (información sobre identificación y control) (pdf 969 Kb)

Legal status in King County, Washington

Class B noxious weed. Control is required in King County. This species is also on the Washington quarantine list (known as the prohibited plants list) and it is prohibited to transport, buy, sell, offer for sale, or to distribute plants or plant parts of this species, into or within the state of Washington. It is further prohibited to intentionally transplant wild plants and/or plant parts of this species within the state of Washington. For more information on noxious weed regulations and definitions, see Noxious weed lists and laws.

Additional information on purple loosestrife

• Purple Loosestrife Weed Alert (144 kb, Acrobat file) 
• Washington State Noxious Weed Control Board (external link)

What to do if you find this plant in King County, Washington

Please notify us if you see purple loosestrife growing in King County. Our program staff can provide the property owner or appropriate public agency with site-specific advice on how best to remove it. We map all known locations of regulated noxious weeds such as purple loosestrife in order to help us and others locate new infestations in time to control them.  There is an active and effective biological control program in use against purple loosestrife in Washington State and in King County and we will be able to let you know if biological control agents have been released to manage specific populations in King County.

Purple loosestrife photos

Report purple loosestrife in King County, Washington

• Please notify us through our online infestation form

Locate purple loosestrife in King County, Washington

• Use our interactive noxious weed map and search for purple loosestrife

Related information

• Agriculture in King County, Washington
• Northwest yard and garden
• Animals, plants and habitat

Related agencies

• Dept. of Natural Resources and Parks
• Water and Land Resources Division

Program offices are located at 201 S. Jackson St., Suite 600, Seattle, WA 98104. To contact staff, see the Noxious Weed Control Program Directory, send an email, or call 206-477-WEED (206-477-9333).