- Our Work
- Invasive Species
A healthy and diverse population of native aquatic plants is essential to good water quality and a healthy lake. Responsible for protecting that population, the MN DNR carefully regulates activities that may threaten it. Generally speaking, control/removal of native emergent plants (bulrush and cattails) is not allowed; native submergent plants can be removed when interference with recreation or navigation can be shown. However, the District’s current “weed” problems center around Flowering Rush and Curly-leafed pondweed, exotic species to which more flexible treatments can be applied.
The district has been involved in aquatic plant management since its inception. Initially such activities were focused on lakes Melissa and Sallie, but with the introduction and rapid spread of the exotic Flowering Rush, management expanded to include Big and Little Detroit Lake and Curfman. Flowering Rush has moved down the Pelican chain and is now found in the Pelican River, in Muskrat Lake, and in Mill Pond. These waterbodies also contain the main Curly-leafed pondweed infestations.
Primary focus lakes Melissa, Sallie, Detroits (Big, Little, Curfman), Muskrat and Mill Pond
Weed problems and weed control are not new to District lakes. For example, in 1949 Lake Detroiters was founded, in part to deal with “algae and weed control” on Big and Little Detroit. In 1950 a Detroiters committee looked into the possibilities of “weed-spraying”, and 1952, another committee was formed “to study dredging as a means of weed and algae control”. In 1953 there was association interest in renting the City’s weed-cutting machine, and for three years (1954-56) Lake Detroiters sponsored applications of sodium arsenite to portions of the shoreline for purposes of weed control. In the late 1950’s copper sulfate was purchased by the Association and made available to members for control of weeds and algae.
Curly-leafed Pondweed is an exotic afflicting many Minnesota lakes and has the unusual (for aquatic plants) habit of dying in June, accumulating in mats in the middle of the lake, and blowing to shore in large masses. Found in Detroit Lake since at least 1968, Curly-leafed pondweed continues to be a major nuisance and is found in Big and Little Detroit, the Pelican River, Muskrat, Sallie and Melissa.
Curly-leafed pondweed on Big Detroit 1968 (left) to 2002 (right)
Flowering Rush was first identified in Deadshot Bay in the mid-1970’s and spread into the Big Detroit by the end of that decade. By the early 1980’s it was found in many places around Big and Little Detroit; and moved down the Pelican River to Muskrat, Sallie and Melissa. Its populations continue to spread and thicken.
Aquatic Plant Harvesting began in 1967, shortly after the District was established. A formal Watershed District Project was established at that time, and successor projects on Sallie and Melissa have been more or less continuous since. At the onset, mechanical removal of “weeds” was undertaken not only to improve boating and swimming, but also to remove nutrients. Subsequent research showed that the nutrient reduction component was relatively small compared to the available in-lake nutrients, so that purpose was given less attention.
Original Harvester, 1968 (named the "Blue Goose")
Detroit lake residents did not petition PRWD for a harvesting project until the late 1980’s. At about the same time there were some unsuccessful attempts to control Flowering Rush using herbicides. Though the Detroit PRWD Harvesting Project started in 1989 for general weed harvesting purposes, from 1995 onward, the main focus has been on Flowering Rush control based upon the strategy that repeated cutting sapped the strength of the plant. For several years Curly-leafed pondweed beds have been subjected to early summer harvesting (before the plant matures and floats to shore), but this effort has achieved only partial success.
A Change in Perspective
Though the DNR’s official Flowering Rush Management Plan still is tied formally to the harvesting strategy, it has become increasingly clear to DNR scientists and PRWD staff (and to residents, for that matter) that the harvesting approach has not dealt effectively with this nuisance plant; indeed, there is some evidence suggesting that harvesting may be contributing to its spread. Accordingly the District, through its Detroit Harvesting Project, and under the strict control and supervision of the DNR has been experimenting with herbicides that might offer some control of Flowering Rush.
In addition to encouraging continued herbicide testing in small plots, the DNR has given informal approval for the District to do some large scale field testing of certain chemicals near public use areas (the public beach area, public accesses, etc), and to consider applications of herbicide to known areas of curly-leafed pondweed concentrations (these were mapped in 2004).
The DNR also has signaled a change in policy regarding mechanical harvesting through PRWD projects. General lake harvesting patterns are to be reduced and strictly aimed at providing boating access to shorelines. This will be accomplished by shifting most harvesting tracks from parallel to perpendicular to the shoreline.
PRWD harvesting projects are in a state of transition. It is apparent that there will be less mechanical harvesting, and it seems likely that there will be more herbicide use. Both harvesting and chemically- treated areas will be limited to high density usage areas (public swimming beach, public access, and possibly the sand bar) or where boating is a general problem (for all boaters, not just specific lakeshore residents.
Likewise, the control of Curly-leafed Pondweed will probably center on harvest or herbicide use in a few, relatively small parts of those lakes where it has been found in homogeneous, heavily populated stands (see the Detroit case). Control efforts would extend over several years.
Private residential and commercial treatment efforts will continue to be permitted by DNR, and may expand as harvesting becomes more restricted. However, it is unlikely that control measures (including mechanical or chemical) will be permitted except under very specific conditions, as in the cases of “weed” interference with boat navigation or swimming, and then only if no native emergent plants are present.