Department of Crop Sciences University of Illinois at Urbana-Champaign

Agronomy Day 2006

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Aquatic Weed Management

George Czapar George Czapar
Extension Educator,
Integrated Pest Management
University of Illinois Extension
(217) 782-6515

Although numerous plant species grow in ponds and are important components of the aquatic environment, excess weed growth has a detrimental effect. Management of aquatic weeds begins with prevention. Since aquatic weeds are more common in shallow, nutrient-rich waters, reducing sediment and nutrient loading is important. Maintaining a sod or grass cover around your pond will help reduce soil erosion and nutrient runoff.

figure 1

For weeds growing in small patches, mechanical controls such as pulling, dredging, or cutting weeds beneath the water surface can be used. Since most aquatic weeds are perennial, it is important to remove roots to prevent re-sprouting. As aquatic weed infestations become more severe, mechanical removal is less practical.

The most widely used biological control is the grass carp, which will consume submersed and free-floating plants. Only grass carp that are sterile (triploid), and cannot reproduce in nature, should be released. Check with your local fisheries department or the Illinois Department of Natural Resources for information and stocking rates.

figure 2
Watermeal and Duckweed

Several herbicides are labeled for aquatic weed control, but proper weed identification is critical. The types of weeds found in aquatic environments include 1.) algae, 2.) emergent or shoreline plants, 3.) rooted-floating plants, 4.) submersed plants, and 5.) free-floating plants such as duckweed and watermeal.

Late spring is usually the best time to apply aquatic herbicides. Weeds are young and actively growing, and the risk of oxygen depletion is less. When using any aquatic herbicide, always read the label directions, and follow all restrictions or waiting periods before using treated water. The goal for pond management is to achieve a balance. Some aquatic vegetation is desirable and trying to make a natural body of water look like a swimming pool should be avoided.

figure 3

The use of barley straw for controlling algae has received considerable attention, but results have been inconsistent. In replicated studies, barley and wheat straw appeared to have some effect on algal growth, but differences were not significant.