Treatment Updates

May 1, 2025

The MN DNR has approved RPBCWD's application for chemical (herbicide) control of Curly-leaf Pondweed treatments on these lakes: Riley, Red Rock, and Mitchell. Lake Susan is pending DNR approval. The herbicide treatment is scheduled for the week of May 5th, weather permitting. See the list of lakes below for detailed information about individual lake treatment flyers with maps. The herbicide applicator will post signage on shoreline that is within 150 feet of the application area.

May 2, 2025

The DNR has now approved the permit for Lake Susan for Curly-leaf Pondweed treatments on the week of May 5th.

May 5, 2025

The contractor will be treating Red Rock Lake and Mitchell Lake TODAY. If time allows, they will also treat Lake Susan. Tomorrow (May 6th), the contractor will treat Lake Riley (and Lake Susan if not completed on May 5th).

May 6, 2025

Yesterday (May 5th), due to ideal conditions allowing efficient application, the contractor was able to complete CLP treatments on all four lakes: Red Rock, Mitchell, Susan, and Riley.

Aquatic Invasive Plant Control

The Riley Purgatory Bluff Creek Watershed District conducts aquatic plant surveys for multiple lakes on a rotational basis. The purpose of the surveys is to monitor native and non-native plant populations to inform decisions about aquatic plant management for lake health. RPBCWD controls invasive aquatic plants to benefit native aquatic plant populations, which are important for lake health, and not intentionally for recreation.

Curly-leaf Pondweed (Potamogeton crispus) and Eurasian Watermilfoil (Myriophyllum spicatum) are the most problematic invasive non-native aquatic plants in the District. The District uses herbicide treatments to control these Aquatic Invasive Species (AIS) to a level that allows native plant populations to increase. Treatments managed by RPBCWD occur at no cost to residents.

Treatment Decisions

RPBCWD staff make the decision to proceed with a treatment to control Curly-leaf Pondweed and/or Eurasian Watermilfoil based upon data from previous year's point intercept vegetation surveys, population trends of these invasive species in a lake, and delineation surveys for these species in the spring of a potential treatment year. RPBCWD must apply for a permit from the Minnesota DNR to control aquatic invasive species in a lake. If a permit is granted, RPBCWD will work with a contractor to apply aquatic herbicide to control aquatic invasive plants in that lake. 

The decision to treat a particular lake is based upon aquatic plant surveys that determine the distribution and density of target invasive species. RPBCWD also coordinates management efforts with city water resources staff.

If you're considering control of aquatic plants along your shoreline, you likely need a permit for aquatic plant management from the Minnesota DNR.

2025 Lake Treatment Information

Treatment dates are subject to change due to weather or other circumstances. Stay in the know: Subscibe to lake notifications by email.

Treatment Timing

RPBCWD staff only know the general timeframe of a treatment until weather predictions become more precise and lake water temperatures reach treatment standards. The general treatment timeline is selected about a week before treatment, however applicator availability, MN DNR permit approval, or abrupt weather changes may affect scheduling or not allow us to determine exact dates until shortly before treatment.

Treatments are scheduled during optimal weather conditions (low wind, water temperature near 60 degrees F). Dates of treatment may change due to changes in weather or other unforseen circumstances. 

Curlyleaf Pondweed (CLP)

All Curlyleaf Pondweed treatments will be limited to early spring and will not extend beyond four weeks following ice-off OR until treatments can occur when water temperatures are between 50°F and 60°F, whichever is later (per MN DNR regulations). The herbicides most commonly used to control CLP are detrimental to other native aquatic plants. This regulation is designed to protect native plative which typically grow later than CLP and treatment effects are lessened because of it.

Eurasian Watermilfoil (EWM)

Eurasian Watermilfoil treatments typically occur mid-June to mid-July when the plant is most actively growing.

Herbicide Information

Selection of herbicides is based upon a number of factors including target invasive species, previous treatments, lake characteristics, cost, or other variables. After herbicide treatment, the Minnesota DNR recommends not using the lake water within or near treatment areas for these activities for the listed number of days.