Aquatic plant control in Michigan ponds and lakes

Aquatic plants are an important part of the ecosystem but can become a nuisance when excessive growth occurs.

Aquatic plants growing along the shallow shoreline areas. | Michigan Sea Grant
Aquatic plants growing along the shallow shoreline areas. | Michigan Sea Grant

It is that time of year when Michigan Sea Grant and Michigan State University Extension answer questions from pond and lake owners on excessive aquatic plant growth and methods of control. Unfortunately, by the time pond and lake owners are looking at the option of aquatic plant control excessive nutrients have already entered their aquatic system driving plant growth.

Aquatic plants provide habitat for fish and aquatic insects, and oxygen to the water when the sun is out. When there is excessive aquatic plant growth they consume oxygen during the night and on cloudy days. This can drive oxygen levels down which is detrimental to fish. Over the winter months excessive aquatic plant growth can cause winterkill of the fish as these plants drive the oxygen levels down to dangerous levels. Once nutrients accumulate in a pond they are difficult to remove unless the pond is dredged.

The best way to control excessive aquatic plant growth in a pond is prevention. Usually phosphorus is the limiting nutrient that drives aquatic plant growth in freshwater environments. There should be a buffer strip around a pond, such as grass, that can trap nutrients before they enter the pond. Fertilizers that are used for lawn growth should not be used near a pond and livestock should be kept from drinking at the pond. Waterfowl use should be discouraged as they add excessive nutrients. Do not feed fish in a pond artificially formulated diets as these are rich in nutrients and promote aquatic plant growth. When ponds are constructed keep deciduous trees away from the pond as they lose their leaves ending up in the pond as a nutrient source promoting excessive plant growth.

Once excessive aquatic plant growth occurs in a pond the owner has to make a decision on what control methods to use. Many think that the use of an aquatic herbicide is an easy approach to this problem. There are specific aquatic herbicides that are effective on certain aquatic plants and most are expensive. Some aquatic herbicides require that you hire a certified applicator. The use of aquatic herbicides is only a temporary approach as they will kill the aquatic plants releasing nutrients to start the growth cycle over again. When one aquatic plant is controlled usually another one will take its place as the nutrients are in place to continue to drive aquatic plant growth.

Another method of aquatic plant control is mechanical. This is very labor intensive as it involves cutting, pulling, or raking the aquatic plants. If this method is used the aquatic plants should be removed and disposed of away from the pond. In some states grass carp are used to consume aquatic plants but in Michigan this is illegal.

The best practice when constructing a new pond is to limit the nutrient input. Ponds in Michigan should be at least 15 feet in depth to help prevent winterkill of fish. When ponds are first constructed they should have a slope of three feet for every one foot depth of water to reduce shallow areas that are more prone to aquatic plant growth. Aquatic plants should not occupy more than one fourth of the pond.

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