Aquatic Weeds

Aquatic plant management: aquatic “weeds”

What are aquatic weeds?

“Aquatic weeds” or “lake weeds” can be catch-all terms for aquatic plants and algae, including both native and non-native plants, that are unwanted for one or more reasons. While most lake users accept a certain level of aquatic plant growth, these plants may become “weeds” once they grow past a certain amount or grow in high-recreation areas. An internet search for “weeds in my lake” returns many products and services designed  to remove and/or kill these weeds. However, more information is needed to determine if these aquatic plants should be controlled, and if so, how best to control them. 

Aquatic plants are important. Plants provide oxygen as a byproduct of photosynthesis, are a major food source for fish and other wildlife in the lake, help stabilize bottom sediments and help prevent shoreline erosion. Many are considered beneficial (native Chara, some pondweeds, naiads, wild celery) or neutral (native watermilfoil, coontail, waterweed), while others are a nuisance or invasive ( curly leaf pondweed, Eurasian watermilfoil, starry stonewort). For example, lily pads can be considered a nuisance – yet they provide habitat for small game fish such as northern pike. Understanding which specific plants are in the patch of “weeds” will help determine if action is necessary and which control strategies will actually be effective. 

Can they be controlled?

A plant survey (as part of a plant management plan) will enable people to know more about the aquatic plants in their lake. That is, where and what beneficial native plants are growing, where and what invasive plants and weedy native plants are growing, and where there is a mix of native and invasive plants. This, in turn, will allow appropriate control efforts that reduce non-target impacts to beneficial plants and  wildlife and maximize the treatment effectiveness, which leads to cost savings. 

Although control strategies have improved, current practices can only reduce populations to low(er) levels and often require annual maintenance and/or repeated treatments – very few are able to completely solve a weed problem. While herbicide treatment is the most common approach for nuisance plants, integrated control strategies (e.g., those that combine chemical, mechanical/physical, and/or biological methods) are often more effective than a single strategy. In addition, the weed problem may be due to external factors such as a leaking septic tank or other source of nutrient pollution. Completing an aquatic plant survey as part of a lake management plan will help identify these potential issues that, once addressed, may lead to a reduced weed problem. This is one reason it is important to take a broad look at the problem before spraying (or hiring someone to spray) herbicide. In addition, in many states the application of herbicides to lakes requires a permit from the state’s natural resource agency.

Where do I find out more?

Many inland lakes have undesirable aquatic plants. The amount of plants required to become a problem varies by lake based on factors like nutrient levels and water temperature, as well as tolerance of the lake user. If your lake has excessive nuisance plant levels, you should contact a lake management service provider to help determine whether control efforts are appropriate and, if so, how to proceed (see the “Management Plan” fact sheet for information on finding a lake management service provider). The state’s natural resource agency is also a good resource for technical advice on the management of nuisance and invasive aquatic plants.

Factors that may determine whether and how control is attempted include: What type of plants do the weeds consist of (native or invasive)? How much of the lake is populated by the weeds? Have the weed levels been staying about the same or increasing?