Manage Excessive Aquatic Plants in Ponds

Summer plant management promotes healthy ponds and prevents fish kills

June 29, 2015, 11:59 am | Allen Pattillo

PondweedAMES, Iowa – Recent weather conditions may have pond owners battling an abundance of aquatic plants. While the plants are a necessary and beneficial component of a natural ecosystem, too many can cause issues for pond owners – aesthetically, recreationally and biologically.

Extremely hot temperatures, high plant loads and overcast skies for extended periods can cause a fish kill by depleting all the oxygen in a pond. Iowa State University Extension and Outreach Fisheries Specialist D. Allen Pattillo recommends approximately 15–25 percent of the pond area contain large aquatic plants as a preventative measure to oxygen depletion.

“An overabundance of aquatic plants can strain a pond’s ecosystem and potentially lead to a fish kill,” said Pattillo. “During daylight hours, plants produce oxygen and raise the water pH, yet at the same time the plants respire, removing oxygen and adding carbon dioxide and lowering the pH. If this cycle isn’t balanced, issues can result.”

According to Pattillo, control of aquatic plant growth must be managed early in the growing season, during April or May. “Pond owners need to continually manage various strategies for controlling aquatic plant growth and removal of plant loads during summer months,” he said.

Pattillo notes aquatic plants are categorized into four major categories that occupy slightly different habitats – algae, floating plants, emergent or above-water plants and submersed plants. Each category requires a different management strategy to control the impact on pond ecology during the summer.

Biological Control Methods

Introducing an animal like grass carp that feeds on aquatic plants to control submerged plants is one method that can be used. In Iowa, the White Amur, or grass carp, usually eight to 10 inches in size, is typically stocked at four to eight fish per surface acre, depending on the amount and type of vegetation present. Grass carp can live for decades and reach huge sizes (greater than four feet long and more than 50 lbs.), but they are most effective at controlling vegetation within the first five to 10 years. The state of Iowa now requires all grass carp stocked into public waters be triploid or sterile so they cannot reproduce.

Mechanical Control Methods

Using tools such as rakes and shovels to remove aquatic plants is appropriate in very small ponds because it requires a lot of human labor to maintain. A way to make this more efficient is by using an aerator in the middle of the pond. This will force plants like duckweed to the edges for easier removal and to reduce their shading effects. With more light penetration, the impact of duckweed is minimized and other more desirable plants can be established.

Cultural Control Methods

Implementing water drawdowns, lowering the water level or partially drying the pond also are effective forms of control. Although they may be labor intensive initially, they prevent aquatic plants from getting started. Creating steep-sided ponds that reach a depth of three feet or greater relatively close to shore reduces the ability of rooted plants to establish because of light limitations. When using such methods, pond owners should take proper precautions as the deep-edged areas can be hazardous to children and those who cannot swim.

Pattillo also suggests installing rip-rap on the pond bank for some ponds. “The rocks will reduce the rooting ability of plants on the shore and reduce shoreline erosion, although this may cause problems for wildlife like turtles,” he said. “Aeration also can provide some temporary relief from aquatic plants absorbing nutrients like phosphorus. Pond dredging may be necessary every 20-30 years, or when ponds become shallow, to reduce sediment nutrient loads, which directly contribute to the fertility and plant biomass in a pond.”

Other prevention methods that alter the pondscape include installing sediment basins and planting buffer strips of prairie upstream of the pond to stop sedimentation and remove nitrogen and other nutrients from entering the pond. Ponds that are tile fed may benefit from the installation of a bioreactor filled with wood chips to denitrify the incoming water.

Chemical Control Methods

Applying chemicals to control aquatic plants is generally the last resort for pond owners. Treatments for specific plant species can be quite expensive with variable results. The pesticide may be toxic to fish and other aquatic organisms and may require a Category 5 Aquatic Pest Control certification to apply. Pattillo advises having chemicals applied by a trained professional. For these methods to be effective, pond owners should start implementing management strategies in the winter and early spring to get ahead of plant growth and prevent establishment.

Pond owners and those thinking of building ponds will find valuable information in the publications Building Quality Ponds and Aquatic Plant Management. Both are available to download free from the ISU Extension and Outreach Online Store. For more in-depth information, Pattillo recommends the Aquatic Pest Control Manual, available for purchase through the ISU Extension and Outreach Online Store.

Photo -- American Pondweed bloom in pond. By D. Allen Pattillo

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