Starry stonewort (Nitellopsis obtusa)

Who determines when and where treatments will occur?

When and where treatments are conducted is determined by the weather and where nuisance plants are found when biologists from Progressive AE conduct their surveys. Treatments generally begin early in the growing season when the plants are actively growing (May or early June) and continue through August. Click here for the most treatment map.

Why are there still plants in the lake following treatments?

Not all plants are treated. The goal of the program is to strike a balance by controlling invasive plant species and maintaining beneficial species. We do not want to remove all the plants in the lake. This would be bad for the fishery and cause a host of other problems, such as massive algae blooms.

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 Environmental Quality (DEQ) 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 DEQ. In Michigan, aquatic herbicide use requires a DEQ 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.

Cabomba (Cabomba caroliniana)

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.

Exotic plant species that are potentially a threat to Pentwater Lake include Eurasian milfoil, Cabomba, and starry stonewort. Early detection and rapid response is 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 are conducted to control nuisance plant growth.


Pentwater Lake aquatic plant survey map with numbered GPS waypoints. Map provided by Progressive AE.

Progressive AE

Progressive AE

Progressive AE

Why didn’t my property get treated?

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

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.

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.

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.

Eurasian milfoil (Myriophyllum spicatum)

In Michigan, a permit must be acquired from the Department of Environmental Quality before herbicides are applied to inland lakes. The permit lists the herbicides that are approved for use, respective dose rates, use restrictions, and indicates specific areas of the lake where treatments are allowed. In addition to a state permit, federal regulations require herbicide applicators to acquire a pesticide general permit and to prepare and submit a pesticide discharge management plan.

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 AE. Beginning in May and continuing through August, biologists from Progressive AE 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?

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