This Class B noxious weed is most widespread in King County but is also found in Thurston, Whatcom and Stevens Counties. Control is required in King County.

Garden loosestrife was introduced to North America from Europe as an ornamental in the 1900’s and is now naturalized in wetlands and lakeshores in parts of the northwest, midwest and eastern United States and Canada. Garden loosestrife displaces native vegetation along streambanks, wetlands and shorelines and reduces habitat needed by waterfowl and fish, including several important salmon species.

Although garden loosestrife hasn’t been in our county as long as purple loosestrife, it is slowly building up steam in our part of the world and is even out-competing purple loosestrife in some areas. Once garden loosestrife establishes in an area, it spreads by creeping roots called rhizomes that seem to extend forever and create dense, underground mats that are extremely difficult to remove.

King County has more garden loosestrife than anywhere else in our region and most of it is growing around Lake Sammamish, Lake Washington and the Sammamish River and on Lake Burien. There are also populations of garden loosestrife in the Snoqualmie Valley including one large infestation around the edges of a oxbow slough near Fall City, scattered along the shores of the Snoqualmie River and its tributary the Raging River, on Lake Alice, and in a few other isolated wetlands and roadside locations.

Identifying garden loosestrife (also known as yellow loosestrife) can be confusing, especially by its name. First, although it shares habitat and invasive tendencies with purple loosestrife (Lythrum salicaria), it looks very different and is not even related to this other noxious wetland invader. Also, garden loosestrife has a closely related look-alike also known as garden or yellow loosestrife (Lysimachia punctata) that is often used as an ornamental in this area. Both have bright yellow 5-petaled primrose-like flowers on tall, softly hairy stems. However, the noxious garden loosestrife has branched flowering stems with flowers clustered near the top of the plant (not just in the leaf axils), less than ½ inch wide, with distinct orange margins along the edges of the sepals, and it is more likely to be found in wetlands than gardens. Garden loosestrife is easiest to identify when it flowers in July and August.

Garden loosestrife photos - click thumbnail for larger image

Check out this link from the Catholic University of Leuven, Belgium for some great closeups of garden loosestrife.

For information on garden loosestrife identification and control, please download our garden loosestrife weed alert (1.01 MB Acrobat file) or for comprehensive biology and control information, download the Garden Loosestrife Best Management Practices (355 KB Acrobat file).

For more information about garden loosestrife, please see the written findings of the WA State Noxious Weed Control Board.

If you find garden loosestrife in King County, please notify us through our online infestation form.

To find out where we have records of this weed in King County, use our interactive noxious weed map and search for garden loosestrife.