Aquatic Weed Management

The South Washington Watershed District oversaw an aquatic plant survey on Colby and Markgrafs Lakes in 2021. 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 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 April to early May this year. There will be no restrictions to using the lake associated with the type of herbicide applied and the fishing pier will remain open. 

Markgrafs Lake Treatment

Herbicide application will begin in spring 2023 for Markgrafs Lake to treat curly-leaf pondweed. Exact timing for application to Markgrafs Lake will depend on water temperature, which should be around 50-60 degrees at the time of the treatment. The type of herbicide that will be applied does come with recommendations to limit consumption during and following treatment for domestic animals like dogs.

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. 

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.