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Portage Bay and Montlake Park Shoreline Aquatic Weed Treatments

What’s going on in Portage Bay?

Seattle Parks and Recreation and the King County Noxious Weeds Program are actively managing invasive aquatic weeds in South Portage Bay and the forested shoreline on the north side of Montlake Park.

Invasive, non-native weeds grow so densely that they displace native plants, degrade shoreline water and habitat quality, and decrease opportunities for open water recreation.

The non-native weeds we are controlling are fragrant water lily found in Portage Bay, plus yellow flag iris, purple loosestrife, and garden loosestrife, found on the Montlake Park shoreline.

All weeds will be treated twice with an aquatic-use herbicide containing active ingredient imazapyr (not glyphosate). The Washington State Department of Ecology has approved and permitted imazapyr for aquatic use. Each treatment duration will be a minimum of one day and no more than two days.

Treatment areas will be clearly marked and temporarily closed to public access while the work is being done. The closure allows for the work to be done quickly and all aspects of the project are selected to minimize risk to humans, animals, birds, and other wildlife in the area.

The work poses no hazards to nearby habitat areas, beaches, parks or other recreational sites. All areas will re-open the morning after a treatment is complete.

If you would like more information about this project, please email us:

Treatment Dates:

The first treatment for aquatic noxious weeds in South Portage Bay will be conducted by NW Aquatic Management on Tuesday, July 12. There will be a second treatment sometime before September 15. We will update this page when a second treatment date has been finalized.

Invasive weeds in Portage Bay:

Photos: King County Noxious Weed Program

Garden loosestrife
Purple loosestrife
Yellow flag iris
Fragrant water lily