The city is escalating its war on invasive weeds that clog Lake Winona. For nearly 30 years, the curly-leafed pondweed plants have been cut back with a harvester, a sort of underwater combine that “mows” the weeds. On Monday, the City Council unanimously approved a contract with the Minnesota Department of Natural Resources (DNR) to add the herbicide imazamox to its anti-weed arsenal.
The city will spend up to $7,500 this year for the treatment and plans to continue the treatment for several years. The harvester will continue to run for the time being, though “ultimately the goal is to no longer use the harvester at all,” said Public Works Director Keith Nelson.
His department is recommending the chemical treatment because the harvester has chronic maintenance issues and because the mechanical “mowing” has provided a purely cosmetic benefit while the chemical treatment will control the weeds as much as possible, he explained.
City staff estimated the cost of a new harvester at $175,000.
The U.S. Environmental Protection Agency (EPA) approved the imazamox brand-name product Clearcast for use on aquatic plants in 2008. It is a broad-spectrum herbicide originally developed for weed control in soybean fields. It works by inhibiting a plant’s ability to create proteins. It is harmful to animals in large quantities, but such large quantities that the EPA deems it practically nontoxic. The substance typically breaks down in water within hours. The EPA reports that bioaccumulation (the build up of toxins going up the food chain) is a non-issue for imazamox.
A Syracuse Environmental Research Associates, Inc. (SERA) study on imazamox, found that data on the substance’s toxicity to aquatic plants was limited and that data on amphibian toxicity did not exist.
Curly-leafed pondweed is native to Eurasia, Africa, and Australia. It came to America in the 1800s as an aquarium plant. The vigorous plant colonizes in still water and chokes out native aquatic plants.