Willamette Slough Habitat Restoration Continues

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​We’re working with Willamette Riverkeeper to stop the spread of an aggressive, invasive water plant that has gained a stranglehold on the Willamette Slough at Minto Brown Island Park, hurting recreation opportunities and wildlife alike. 

Uruguayan water primrose, also called Ludwigia (Ludwigia hexapetala), forms dense mats in slow-moving backwater channels, oxbow lakes, and sloughs. While this yellow-flowered plant may appear quite pretty, it has the potential to choke entire waterways, severely restricting recreational access, degrading water quality, and creating an environment that is unfriendly to native fish and wildlife.

Up and down the Willamette River, groups are working to stop the spread of this highly invasive plant. The City of Salem and Willamette Riverkeeper have teamed up to do the same in the Willamette Slough. This three-year project aims to control Ludwigia in the slough through the careful use of an aquatic-approved herbicide sprayed on the plants by state-licensed applicators. 

The goal is to reduce the plant’s population to such a degree that additional herbicide treatment will not be necessary once the project is complete. In the future, any new or remaining Ludwigia will be controlled by hand-pulling.  

The first of the three years of herbicide treatment occurred last summer with one spray event in July and a follow-up in September. The results of those treatments are already quite noticeable, with the extent of Ludwigia in the slough now substantially reduced from what it was last year before the first treatment.  

Building on last year’s successes, we are now preparing for another round of herbicide treatments, which we expect to occur from July 26  through August 6, 2021 (excluding the weekend), with a follow-up treatment occurring toward mid-September. The Willamette Slough will undergo its third and final year of treatment in the summer of 2022. 

Recreational users are advised to steer clear of the slough while treatment is in progress and for 24 hours after the last scheduled treatment day to limit potential exposure to the herbicides and to ensure maximum treatment effect. Treated plants will show signs of a blue-green dye, which is mixed with the herbicide so that applicators can see where it has been applied and reduce the amount of herbicide needed for control efforts. While activity in treated areas should be avoided until the day after the last scheduled treatment, the blue-green dye may be visible for up to two weeks after treatment. 

The Willamette Slough restoration efforts will run from the south end of the Willamette Slough to its mouth, where it meets the Willamette River. Notice of the treatment and any precautions that should be taken during treatment will be placed in parks and at boat ramps. 

Funding for this project was provided to Willamette Riverkeeper by Meyer Memorial Trust and Bonneville Power Administration. Learn more about Willamette River habitat restoration efforts and why projects like this matter with this storymap.   

More about Ludwigia

The Willamette Slough was identified by the Willamette Aquatic Invasive Network as having a large source population of Ludwigia. Ludwigia spreads long distances through plant fragments and by seeds. During high water events the plant is most likely to move downstream with the flow of the river and establish new populations at other riverside and off-channel habitats.  Due to its location, treating Ludwigia in the Willamette Slough will help prevent its downstream spread.

Invasive aquatic plants and animals can also be dispersed by watercraft so it is important to clean, drain, and dry your boat before transferring it to another waterbody.

Additional information about restoration efforts in the Minto Brown Island Park Conservation Area can be found on our website




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Jennifer Mongolo Natural Resources Planner
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