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The Weeders Digest - Articles > Minnetonka Milfoil - Triumph and Tribulation


Eurasian Watermilfoil (EWM), commonly known as milfoil, was first discovered in Lake Minnetonka in the fall of 1987 when a specimen of the exotic plant was collected in Excelsior Bay. Inspection of the shoreline the following year found that milfoil was widespread throughout Lake Minnetonka. This observation indicates that milfoil most likely was introduced into Lake Minnetonka probably years before it was noticed and reported.


Milfoil has not become abundant and caused problems throughout Lake Minnetonka. For example, milfoil has become abundant in some parts of the lake, such as Phelps Bay, but not in others, such as West Arm. Harvesting records confirm that many more acres have been historically harvested in Phelps Bay versus West Arm. For further details of historical acreage harvested on Lake Minnetonka, please refer to the attached table.

One of the main differences between these two parts of Lake Minnetonka is that the water clarity is higher in Phelps Bay than it is in West Arm. The lower water clarity in West Arm likely limits the growth of milfoil, which, like other aquatic plants, requires relatively clear water to survive. In Minnesota, milfoil has caused problems in lakes by producing extensive mats where water depths are less than 15', water clarity is high (mid-summer Secchi disk readings of six feet or more), and the fertility of the bottom ranges from moderate to high. In Lake Minnetonka, it is estimated that milfoil might grow, and perhaps become abundant, in up to 3,000 acres of this 14,000 plus acre lake.

It is important to note that milfoil may cause problems in an area of Lake Minnetonka one year, but not the next. This appears to be mainly due to the weather, which can cause variations from year to year in environmental conditions of lakes, especially clarity, temperature, and depth of water. These in turn can cause large variations in the abundance of aquatic plants, including milfoil. For example, the extent and severity of nuisances caused by milfoil were greater during the late 80's and early 90's when the weather was generally hot and dry, and the water level in the lake was low, than during the 90's when the weather was generally cool and wet, and the water level in the lake was high.


Through cooperative efforts with the MN DNR, the 14 Lake Minnetonka communities, Hennepin Parks, the University of Minnesota, and private citizens around the lake, the LMCD has been able to manage the growth of milfoil on Lake Minnetonka since 1989. Generous donations were received from private residents around the lake that allowed for the purchase of four harvesters and supplementary equipment in 1989. A fifth harvester was added to the fleet during the 2000 season. Additional cooperation with the MN DNR, the 14 Lake Minnetonka communities, Hennepin Parks, and the University of Minnesota have assisted in off-setting annual operating expenses.

The LMCD has two main management objectives in its annual Milfoil Harvesting Program. These objectives include: (1) providing access to usable water to riparian owners and the general public through areas of matted milfoil, and (2) preventing the unintended transport of milfoil from the lake on trailered watercraft to other bodies of water. Harvesting efforts are performed up to authorized dock use areas, with riparian lakeshore owners responsible to maintain within their authorized dock use area.

The milfoil harvesting season generally runs from about mid-June through August. In addition to the five harvesters owned and operated by the LMCD, Hennepin Parks owns a harvester and operates it part-time on Lake Minnetonka. The harvesters cut the milfoil approximately four feet below the water surface and then remove it from the lake unto the harvester. Milfoil removed from the lake is disposed of through various compost sites located in the area.

The LMCD compliments its harvesting efforts by applying 2, 4-D herbicide to selected public accesses around Lake Minnetonka in early summer. The focus of these chemical treatments is to reduce the growth of milfoil around the public accesses and to limit the amount of milfoil being taken out of the lake. Please refer to the diagram below on where to locate Eurasian watermilfoil and other aquatic plants when leaving Lake Minnetonka and other bodies of water.


Since 1991, the University of Minnesota has been conducting research on Lake Minnetonka and other Minnesota lakes to evaluate the potential to use biological control to manage Eurasian watermilfoil. These efforts have been primarily focused on an aquatic weevil, which is a type of beetle that can damage plants under experimental conditions. Unfortunately, it is not yet clear how these insects might be reliable used to control milfoil in lakes. This work is on-going.

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