The City of Salem and Willamette Riverkeeper are gearing up for a third year of targeted treatment of an  aggressive,  invasive water plant that had gained a stranglehold on the Willamette Slough at Minto Brown Island Park in prior years, hurting recreation opportunities and wildlife alike.

Uruguayan water primrose, also called Ludwigia, 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 have been working to stop the spread of this highly invasive plant. Since 2020, the City of Salem and Willamette Riverkeeper have been teaming up to do the same in the Willamette Slough. This four-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. Treatments will continue through the summer of 2023 and will be followed by selective planting of native trees and shrubs along the slough’s riparian area with potential reintroduction of native aquatic plants in some areas of the slough.

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.

Two years of herbicide treatment have occurred, with one spray event in July and a follow-up in September of each year. The effect of those treatments is already quite noticeable; the extent of Ludwigia in the slough is now substantially reduced from what it was two years ago before the first treatment.

Building on the successes of the past two years, we are now preparing for another round of herbicide treatments, which we expect to occur from July 11 through July 22, 2022 (excluding the weekend), with a follow-up treatment occurring toward the end of August 2022. The Willamette Slough will undergo its fourth and final year of treatment in the summer of 2023. 

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.

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.