Pond Weed Problems Increase with Hot Summer Temperatures

AMES, Iowa -- Iowa pond owners are seeing an explosion in aquatic plant growth during the hot summer months. Because of naturally high nutrient loads and high alkalinity from limestone bedrock, plants are thriving as the water warms, which can degrade pond aesthetics and recreation quality, according to Allen Pattillo, aquaculture and fisheries specialist with Iowa State University Extension and Outreach.  

“Generally, ponds with heavy nutrient loads and full sun exposure tend to have the most aquatic plant growth,” Pattillo said. “Approximately 25 percent pond coverage with aquatic plants is considered healthy and gives a good balance between ecosystem health, aesthetics and recreation.”

Aquatic plant management

Aquatic plants, although necessary for the pond ecosystem, can become a nuisance in ponds throughout Iowa during the hot summer months. Excessive plant growth can be managed through a variety of methods of varying cost and effectiveness. While long-term management should include a full watershed nutrient and water management strategy, many Iowans choose to control plant growth in the short-term using aquatic herbicides.  

“Just as there are many aquatic plant species, there are many aquatic approved herbicides, each with their own level of effectiveness for any given plant species,” said Pattillo. General guidelines for chemical application can be found in  Aquatic Plant Management, an Iowa State University Extension and Outreach publication.  

Pattillo warns that application of aquatic herbicides under high water temperatures can be a recipe for disaster. The breakdown of plant material by bacteria uses a considerable amount of oxygen, which is naturally scarce in warm water. Add natural plant decay to a forced plant die-off, and there may not be enough oxygen to support fish, thus leading to a fish kill.  

The aftermath of a fish kill is obvious by the appearance of white fish bellies floating on the pond’s surface and the smell of decay.  “During oxygen deprivation, fish will immediately begin to gulp air at the surface,” he said. “Observing low oxygen conditions before it’s too late and providing emergency aeration is critical to avoiding a fish kill.” Pattillo recommends that pond owners always have aquatic herbicides applied by a pond consultant with a Category 5: Aquatic Pest Control certification.  

Aquatic herbicides can be very expensive and Pattillo said they should be considered a temporary fix to the true pond issues of excessive nutrient loads.

Wetlands and buffer strips in the watershed are great management tools for reducing nutrients and sedimentation, and must be maintained for optimum efficiency. A nutrient and sediment management plan should be created for each pond to prolong the life of the pond and aesthetic and recreational value.

For additional questions, please contact Pattillo at 515294-8616 or pattillo@iastate.edu.