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Pentwater Lake Aquatic Plant Control Program

A primary objective of the Pentwater Lake improvement program is to control the spread of invasive, exotic plants. In managing aquatic plants, it is important to recognize that aquatic plants provide several valuable ecological functions. Maintaining a diversity of native plants is as important as controlling nuisance and exotic species.

Pentwater Lake Plant Control Reports
Upcoming Treatments & Harvests

Mechanical Harvesting Map

Harvest to start July 29

​July 24 Herbicide Treatment Map
Survey date: July 15

June 12 Herbicide Treatment Map

Estimated Harvest Date: July 2024

2023 Treatments
2023 Treatment Map

2023 Mechanical Harvesting

2022 Mechanical Harvesting

Exotic plant species that are potentially a threat to Pentwater Lake include Eurasian milfoil, hybrid milfoil, European frogbit, and starry stonewort. Genetic testing done in 2016 showed that both Eurasian milfoil and hybrid milfoil exist in Pentwater Lake. Eurasian milfoil and hybrid milfoil are nearly identical, making it impossible to distinguish the difference with the naked eye. Early detection and rapid response are key to effective control of invasive aquatic plant species. Each year, biologists conduct multiple surveys of the lake to identify exotic plant locations and targeted herbicide treatments and/or mechanical removal are conducted to control nuisance plant growth. The herbicides treatments require a permit from the Michigan Department of Environment, Great Lakes and Energy and the mechanical removal operations require a permit from the U.S. Army Corps of Engineers. Click here to view 2022 plant control maps. Click here to view 2021 plant control maps.

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Progressive AE

Eurasian milfoil (Myriophyllum spicatum)

European Frogbit

Illustration by Bruce Kerr

Courtesy of EGLE

European frog-bit (Hydrocharis morsus-ranae)

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Progressive AE

Starry stonewort (Nitellopsis obtusa)

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European frog-bit (Hydrocharis morsus-ranae) on Pentwater Lake near Longbridge Road and Monroe Road

Q & A About the Pentwater Lake Aquatic Plant Control Program

Who oversees the plant control program?

Plant control activities are coordinated under the direction of the board’s environmental consultant, Progressive Companies. Beginning in May and continuing through August, biologists from Progressive Companies conduct GPS-guided surveys of the entire lake to identify problem areas, and detailed plant control maps are provided to our plant control contractor. Progressive then conducts follow-up surveys to evaluate contractor performance, and provides status reports to the board.

Who conducts the herbicide treatments and mechanical harvesting?

Herbicide treatments and mechanical harvesting are conducted by PLM Lake & Land Management Corp. The contracts are competitively bid and performance-based. The contractor is only compensated for work that is performed satisfactorily.

Pentwater Lake Aquatic Plant Survey Map

Pentwater Lake aquatic plant survey map with numbered GPS waypoints. 

Click on the map to download a copy.

Who determines when and where treatments and harvesting will occur?

The timing and scope of plant control work is based on where nuisance plants are found when biologists from Progressive Companies conduct their surveys. 

Why are there still plants in the lake following treatments or harvesting?

In managing aquatic plants, it is important to recognize that most plants are beneficial. Aquatic plants produce oxygen during photosynthesis, help stabilize shoreline and bottom sediments, and provide cover and habit for a variety of fish. The main objective of the plant control program on Pentwater Lake is to control nuisance, invasive species while maintaining beneficial plants. We do not want to remove all plants from the lake. This would negatively impact the fishery and cause many other problems such as algae blooms.

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Mechanical harvesting of starry stonewort.

Is there a permanent fix to the problem?

If conditions are favorable, aquatic plants will grow. However, there are steps property owners can take to help minimize plant growth in the lake such as limiting the use of lawn fertilizers and maintaining natural vegetation along the shoreline.

How about a pre-emptive strike?

To be effective, aquatic herbicides must be applied directly to the plant beds when the plants are actively growing. There are no approved pre-emergence aquatic herbicides like there are for agriculture.

Are herbicide treatments safe?

The aquatic herbicides that are permitted by the Department of Environment, Great Lakes, and Energy (EGLE) are registered by the United States Environmental Protection Agency and the Michigan Department of Agriculture and Rural Development. They also undergo toxicological review by the EGLE. In Michigan, aquatic herbicide use requires an EGLE permit. The permit lists herbicides approved for use in the lake, respective dose rates, and shows specific areas in the lake where treatments are allowed. If herbicides are applied according to label instructions and permit requirements, they should pose no danger to public health and the environment.

How do the treatments impact fish?

If applied properly, herbicides have no direct impacts on fish. In general, lakes with a variety of plants often support more productive fisheries. The plant control program in Pentwater Lake is designed to remove invasive plants while preserving plants that provide habitat and cover for fish.

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Why didn’t my property get treated?


Treatments and harvesting occur where the targeted invasive plants are found during lake surveys. Not every property gets treated or harvested every time; your property may have plants, but if it doesn’t contain the targeted invasive plants, it’s not treated or harvested.

How will I know about use restrictions?


All lake residents will receive a written notice regarding pending treatments. The written notice will list all herbicides that may be used and use restrictions. At the time of treatment, state regulations require that areas within 100 feet of treatment areas be posted with a sign that lists specific herbicides applied and the associated use restrictions. If there is no sign posted along your property, it means your area was not treated and there are no use restrictions. Mechanical harvesting has no use restrictions.

When is it safe to swim after a treatment?

All herbicides have a 24-hour swimming restriction that will be posted on signs along areas of the shore that have been treated. However, if you do not have a sign posted or the sign indicates that only algaecides were applied, there are no swimming restrictions.

When can I water my lawn following a treatment?

If you draw water from the lake for irrigation, be sure to read the sign posted along your shoreline at the time of treatment. Most irrigation restrictions do not apply to established lawns. However, it you water flowers or a garden, adhere to the irrigation restrictions posted on the sign.

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