Aquatic plants provide habitat for fish and aquatic insects, and provide oxygen to the water when the sun is out. When there is excessive aquatic plant growth, problems arise as those additional plants consume oxygen during the night and on cloudy days. This can drive oxygen levels down, which could be detrimental to the fish. Additionally, 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 build up in a pond, they are difficult to remove unless the pond is dredged out. Phosphorus, once it gets into a pond, can drive aquatic plant growth for many years.

Michigan State University Extension and Michigan Sea Grant answer a variety of questions each year from Michigan pond and lake owners who have problems controlling excessive aquatic plant growth. Many times, they ask if they can use grass carp to solve their excessive aquatic plant growth problem. The answer is “no,” as they are an illegal species to possess in the state of Michigan, except under a Michigan Department of Natural Resources permit for education or research purposes. This is often confusing to Michigan residents as the neighboring states of Illinois, Indiana and Ohio allow the use of grass carp under certain conditions with the proper permit.

Instead of using grass carp, 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 use should be discouraged from drinking at the pond. Many times, when people have a pond, they encourage waterfowl use; and this should be avoided as waterfowl add excessive nutrients when on the pond. Do not feed the fish in your 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 because when they lose their leaves, many end up in the pond as another source of nutrients to promote excessive plant growth.

Once excessive aquatic plant growth occurs in a pond, the pond owner has to make a decision on what control methods to take to correct the situation. 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, for the most part, they are expensive. Some aquatic herbicides require that you hire a certified applicator to use a specific chemical. The use of aquatic herbicides is only a temporary approach, as they will kill the aquatic plant that will die and ultimately release its nutrients and 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 this aquatic plant growth.

Another method of aquatic plant control is mechanical. This can be very labor intensive as it involves cutting, pulling, or raking the aquatic plants. If this method is used the aquatic plants should be removed from the pond and disposed of a good distance from the pond.

The best practice when constructing a new pond is to limit the nutrients to it as much as possible. Ponds in Michigan should be at least fifteen 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.