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Iowa DNR and local partners to manage the invasive curlyleaf pondweed at Iowa Great Lakes

  • 3/31/2020 12:42:00 PM
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Spirit Lake, Iowa - The Iowa Department of Natural Resources (DNR), working with the East Okoboji Lakes Improvement Corporation (EOLIC), will start the first phase of the curlyleaf pondweed management strategy for the Iowa Great Lakes in April.

About 60 acres of curlyleaf along shorelines will be treated with an herbicide approved for use on lakes to provide some relief around docks and boat ramps. “Our first use of the herbicide in 2018 treated the areas just outside of the dock on about 4.25 miles of shoreline. In 2019, we expanded that treatment to over 8 miles of shoreline,” says Mike Hawkins, Iowa DNR fisheries biologist. “In 2020, we will replicate last year’s treatment and make adjustments to our methods to maximize the value per acre.”  

“Another five miles of trails will be created using a commercial plant harvesting system in the coming weeks,” says Hawkins. “We are also doing 30 acre block treatments with both mechanical removal and herbicide in the north bay of East Okoboji Lake to reduce the amount of plant material that dies off in June and help prevent a fish kill from low oxygen.”

Curlyleaf pondweed is an invasive plant first discovered in the Okoboji chain in the mid 1900’s. It has been in all of the lakes since that time, but has caused navigation and lake access issues in recent years. Under the right conditions, this underwater plant can form dense mats making it hard to get through. A series of late and unseasonably warm fall seasons has allowed this plant to gain an advantage and increase in density.

“Unlike native aquatic plants, curlyleaf pondweed germinates in the fall and takes advantage of the long growing season created by mild conditions,” Hawkins says. “It grows under the ice and dies back in mid to late June. Once the plant starts to dominate, conditions for its growth improve even more because it clears the water and lets more sunlight shine through.” 

Because there are so many different types of aquatic plants, some people often confuse later growing native plants with curlyleaf pondweed which starts to die back in late June and quickly disappears. Hawkins states, “If you see aquatic plants in the lake after the first of July, those are native species and extremely important to keep the lake healthy.”

Although curlyleaf pondweed is widespread and causes problems throughout the United States, management strategies are mostly limited to spot treatments while attempts to eliminate it have failed. Larger treatment area control is being investigated and evaluations are underway in Iowa and other states.

“Over the past few years, we’ve worked with the community to design a strategy to treat the plant near shore and in a series of boating paths. We’ve done this with a combination of herbicide and mechanical harvesting,” says Hawkins. This year around 85 acres of the plant will be treated.

Terry Wilts, who has helped spearhead the local committee and is part of the EOLIC, says this isn’t an easy issue to tackle. “We’ve worked with the Iowa DNR, local cities, businesses, and municipal water systems to develop a plan that is safe, cost effective, and considers everyone’s concerns.”

Lakeshore property owners can remove plants around their docks and hoists without a permit.  The plants can be cut, raked, or harvested using mechanical means only. It is not legal for private citizens to treat plants with chemicals in public waters. 

“The use of herbicides by private individuals is dangerous and against the law,” Hawkins says. “Only certified professionals who are trained in aquatic application and have the necessary permits can apply herbicides to public lakes in Iowa. Aquatic herbicide application is strictly regulated to protect drinking water and all recreational users.”