Aquatic Weed Control

A simple definition of an aquatic weed is a plant that grows (usually too densely) in an area such that it hinders the usefulness or enjoyment of that area. Some common examples of aquatic plants that can become weeds are the water milfoils, ribbon weeds, and pondweeds. They may grow in ponds, lakes, streams, rivers, navigation channels, and seashores, and the growth may be due to a variety of factors such as excess nutrients in the water or the introduction of rapidly-growing exotic species. The problems caused by aquatic weeds are many, ranging from unsightly growth and nuisance odors to clogging of waterways, damage to shipping and underwater equipment, and impairment of water quality.

It is difficult and usually unnecessary to eliminate weeds completely from a lake or stream. Therefore, aquatic weed control programs usually focus on controlling and maintaining the prevalence of the weeds at an acceptable level. The methods used in weed control may include one or a combination of the following: physical removal, mechanical removal, habitat manipulation, biological controls, and chemical controls.

Physical removal of weeds involves cutting, pulling, or raking weeds by hand. It is time-consuming and labor-intensive and is most suitable for small areas or for locations that cannot be reached by machinery. Mechanical removal is accomplished by specialized harvesting machinery equipped with toothed blades and cutting bars to cut the vegetation, collect it, and haul it away. It is suitable for off-shore weed removal or to supplement chemical control. Repeated harvesting is usually necessary and often the harvesting blades may be limited in the depth or distance that they can reach. Inadvertent dispersal of plant fragments may also occur and lead to weed establishment in new areas. Operation of the harvesters may disturb fish habitat.

Habitat manipulation involves a variety of innovative techniques to discourage the establishment and growth of aquatic weeds. Bottom liners of plastic sheeting placed on lake bottoms can prevent the establishment of rooted plants. Artificial shading can discourage the growth of shade-intolerant species. Drawdown of the water level can be used to eliminate some species by desiccation. Dredging to remove accumulated sediments and organic matter can also delay colonization by new plants.

Biological control methods generally involve the introduction of weed-eating fish, insects, competing plant species or weed pathogens into an area of high weed growth. While there are individual success stories (for example, stocking lakes with grass carp), it is difficult to predict the long-term effects of the introduced species on the native species and ecology and therefore, biological controls should be used with caution.

Chemical control methods consist of the application of herbicides that may be either systemic or contact in nature. Systemic herbicides are taken up into the plant and cause plant death by disrupting its metabolism in various ways. Contact herbicides only kill the directly exposed portions of the plant, such as the leaves. While herbicides are convenient and easy to use, they must be selected and used with care at the appropriate times and in the correct quantities. Sometimes, they may also kill non-target plant species and in some cases, toxic residues from the degrading herbicide may be ingested and transferred up the food chain.

Schmidt, J. C. How to Identify and Control Water Weeds and Algae. Milwaukee, WI: Applied Biochemists, 1987.

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