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Washington State Weeds

Purple Loosestrife

purple loosestrife

Lythrum salicaria L. • Class B

Family Name: Primulaceae  (Prim-yew-lay-see-ee)
Common: Primrose family
Genus: Lythrum (Ly-thrum) 
Meaning: From the Greed word for blood in a gory sense, from wounds etc.
Species: salicaria (sa-lih-KAR-ee-uh)
Meaning: Willow-like

Established plants grow to heights of 4 to 10 feet high and produce a showy display of magenta colored flower spikes throughout the summer. Short slender branches spread out to form a crown of up to 5 feet wide on established plants. Its somewhat square-ish stems are 4 to 6 sided, with evenly spaced nodes. Stems that are submerged in water develop the tissue characteristic of aquatic plants. Its leaves are lance shaped to heart shaped, its lower leaves are larger and smooth, while upper leaves are smaller and covered in fine hairs. 

 Why Is it a Noxious Plant?

Purple Loosestrife is highly invasive and crowds out native vegetation. It thrives on moist or saturated soils, and is a very hardy perennial which can rapidly degrade wetlands, diminishing their value for wildlife habitat by replacing native plants used for ground cover, food or nesting material. Purple loosestrife catches soil and debris that accumulate and increase the elevation of the land. Eventually, the soils dry, and the wetland is displaced. 

Where Does it Grow?

It is found in fresh water and brackish wetlands. It is a successful colonizer and potential invader of any wet disturbed site. 


Purple Loosestrife is a prolific seed producer and spreads primarily by floating seeds. One plant can produce more than 2 million seeds annually. Warm temperatures and open, moist soils are required for successful germination. 

Control Options:

Cutting, mowing or digging purple loosestrife is only partially effective. Though these methods can reduce or prevent seed production, the plants usually re-sprout. Also, the access issues involved with purple loosestrife make these techniques difficult. 

  • Flower heads should be cut off and bagged up to prevent seed production and spread. 

  • Biocontrol agents (beetles) have been released in Washington State. For information about the biological control of this or any other noxious weed, see the WSU Extension Integrated Weed Control Project .. . 

  • When purple loosestrife is found in aquatic areas, please note that aquatic herbicides are restricted for use in Washington State to licensed applicators only. 

  • Spot spraying with a product containing the active ingredient glyphosate (Used in Roundup Pro, etc.) is effective for controlling purple loosestrife in terrestrial (dry) environments.  
  • When using herbicides, always read and follow all label instructions and obey all label precautions. (Note: pesticide product registration is renewed annually and product name and formulations may vary from year to year.) 

More Information:

 Download our Flyer or visit Washington State Noxious Weed Control Board Here. Photo by Leo Michels


More Pictures:
Purple loosestrife Purple loosestrife

Purple loosestrife

 Purple loosestrife