State begins treatment at Clear Lake for invasive aquatic plants

Throughout the winter, the Iowa Department of Natural Resources will monitor applications of an approved herbicide, in very low doses, to the water at Clear Lake to eradicate invasive plant species.

Eurasian watermilfoil and curly-leaf pondweed are non-native plants that are easily spread to wetlands and other lakes. The plants can also impact recreation on the lake.

Scott Grummer, a fisheries management biologist at Clear Lake with the DNR said the herbicide, Sonar A.S., will only impact the non-native plant species because it is being applied during a season when native aquatic plants have already died back.

Grummer said the DNR went out in several boats in early November to spread the herbicide in strips through the center of the lake. He said they let “Mother Nature do that initial mixing” to spread the chemical throughout the water.

The department is sampling and testing the lake water to keep herbicide concentrations at the recommended level of above 3 parts per billion, and to determine if additional “bumps” of the herbicide are needed. A bump, he said, will likely be applied in December, which could mean staff will have to drill holes into an iced-over lake.

While it’s unlikely many Iowans are slipping into their water skis this time of year, Grummer said the herbicide is applied in such a low dose that there are no water use restrictions in place at the lake.

“Even if we were in the middle of summer, there’d be no swimming, boating, water use restrictions at all,” Grummer said.

Fishing and irrigation are also still OK on Clear Lake during the treatment period, which is expected to continue until the middle of January.

Grummer said the herbicide is very “slow acting” so the “true evidence of success” for the treatment won’t be known until the department conducts a vegetation survey in 2026.

The herbicide starves the plants over a number of weeks by preventing them from producing a pigment necessary for photosynthesis.

Grummer said the treatment has been successful at a number of other Iowa lakes, including Crystal Lake, East Okoboji and Lake Cornelia.

“We’ve done enough of them, we know what works,” Grummer said. “As long as we can hold that concentration, we’ve been successful in other systems at eradicating this Eurasian watermilfoil, and that’s the goal with this project.”

Grummer said this project took about a year to plan and is funded by the DNR’s lake restoration program and support from local partners.

Eurasian watermilfoil grows at deep depths, as tall as 12 to 15 feet, which Grummer said can be a nuisance to water recreation.

The plant spreads via fragmentation. Like an easy-to-propagate houseplant, a small segment and some water is enough to grow roots and spread.

“All you need is a little piece to get a population started,” Grummer said.

This factor means it’s especially important for boaters and anglers to follow good boat hygiene practices. Grummer said the basics, regardless of if a known invasive species is present or not, are to clean, drain and dry before transporting anything from a lake or river.

“We probably have boaters come from all over Iowa (to Clear Lake) and it’s just a really needed project, just to kind of cut off these invasives the best we can,” Grummer said.

This article was first published by Iowa Capital Dispatch.