Aquatic Weed Management

In 2021, the South Washington Watershed District (SWWD) oversaw an aquatic plant survey on Colby, La, and Markgrafs Lakes. The results of the survey showed there are varying levels of the invasive aquatic plant species curly-leaf pondweed and Eurasian watermilfoil in the lakes.

Eurasian watermilfoil and curly-leaf pondweed form dense mats at the water’s surface, which inhibit water recreationists and demand additional staff resources to ensure mats do not obstruct lake outlets. They also overtake habitat, outcompete native aquatic plants and provide unsuitable shelter, food and nesting habitat for native animals.

In response to the aquatic plant survey findings, the watershed district and a contractor are applying treatments to both lakes. The treatments will improve the recreational, natural and aesthetic values of the lakes.

Colby and La Lake Treatment

An herbicide was applied to Colby Lake to treat Eurasian watermilfoil and curly-leaf pondweed in fall 2022 and supplemental treatments will continue in late October to early November 2023.

Herbicide application will begin in late October to early November 2023 for La Lake to treat curly-leaf pondweed. Exact timing for applications to the lakes will depend on water temperatures. There will be no restrictions to using the lakes associated with the type of herbicide applied and the Colby Lake fishing pier will remain open. 

For both lakes, signage will be clearly placed at lake access points during treatment notifying lake users of the recommended contact restrictions. Further treatments of the lakes may be necessary in the future to keep the invasive species under control.

Markgrafs Lake Treatment

Herbicide application began spring 2023 for Markgrafs Lake to treat curly-leaf pondweed. Markgrafs Lake responded well to its initial treatment so there are no additional applications planned at this time. 

All in-lake management is permitted by the Minnesota Department of Natural Resources and performed by licensed applicators.

For more specific information on the herbicide treatments in each of the lakes, please the South Washington Watershed District website.