DEC Shares Results of Invasive Herbicide Treatment in the Peconic River

After more than a decade of trying to remove a invasive floating primroses from the Peconic River by hand, the New York State Department of Environmental Conservation is heartened this year by the results of a selective herbicide program this past summer.

The DEC’s Aquatic Invasive Species Coordinator, Catherine McGlynn, detailed the results of the first year of the five-year project in two public informational forums in mid-December.

The floating plant, which has five-petaled yellow flowers that bloom all summer, “grows aggressively and forms dense mats. It displaces species that grow slower or lower in the water column,” Ms. McGlynn said. “Each year, the mats break down at the same time, which consumes oxygen and can cause fish kills and reduction in oxygen in general.”

The primrose, which the DEC calls by its scientific name, Ludwigia peploides, or just Ludwigia, was first found in the Peconic River in 2003, and the DEC and environmental groups have been enlisting volunteers to help pull the plants out of the river since 2006. But by 2022, when they began the herbicide program, Ludwigia was found from Connecticut Avenue in Calverton all the way east to Peconic Avenue, which crosses the river in downtown Riverhead. It wasn’t found downstream of the Grangebel Park dam, where freshwater begins to mingle with salt water, causing less ideal conditions for Ludwigia infestations.

In 2020, the DEC began a pilot project to treat a one-acre state-owned site known as Browns Bog with the herbicides Clearcast and ProcellaCOR, two highly targeted herbicides chosen for their effective control of this particular species, low use rates “on orders of magnitude below old herbicides,” according to Ms. McGlynn, because they are absorbed quickly by the plants and have a low aerial drift, meaning they aren’t carried in the air far from where they are applied. The compounds in the herbicides, which bind to an enzyme in the Ludwigia plant, are also photosensitive — they break down in exposure to light.

Ms. McGlynn said DEC scientists did environmental surveys of Browns Bog a week after the application and found no trace of the herbicide and no effect on other species in the river.

After producing a five-year management plan and obtaining eight required permits in 2022, the DEC embarked on its first large-scale herbicidal treatment of the river on July 27, 2022. 

The DEC conducted surface water sampling and biological surveys of the treated areas both before and after the application of the herbicide, in order to determine if the herbicide had any impact on the water or aquatic life in the river. They broke the section of the river they would be treating into nine “treatment blocks,” analyzing the treatment in each block separately.

They also performed what are known as “point intercept rake toss surveys,” essentially throwing a rake out a set distance from a boat, then pulling it in and characterizing the density of Ludwigia found in the area where the rake was tossed. They did this in numerous spots along the nine treatment areas before and after applying the herbicide. They saw reductions of 86, 68 and 50 percent in the density of Ludwigia growing in areas with the most dense Ludwigia infestations.

Ms. McGlynn said the ProcellaCOR seemed to break down more quickly than the Clearcast — its concentration was less than 1 part per billion within 24 hours of application, while Clearcast concentrations were 1 part per billion at 2/3 of the sites within five days, and at less than one part per billion a month after treatment. She hypothesized that tannins and turbidity in the water may have made it more difficult for sunlight to penetrate the Clearcast-treated areas and break down the chemicals.

Ms. McGlynn said data showed that, in the area that had the densest Ludwigia growth, dissolved oxygen in the river increased after the treatment.

The DEC plans to use the herbicides on returning Ludwigia again once per year for the next two years in an attempt to tamp down the infestation.

Maureen Dunn, a water quality scientist with Seatuck Environmental Association, questioned why the DEC was not removing the plants from the river after they were treated with herbicide, and asked if the DEC had tested whether the removal of the plants had an impact on nitrogen levels in the river.

“It’s the fundamental concept I have questions about,” she said. “The problem is created by impoundments there — there’s increased nitrogen in the water because there’s no movement and sedimentation. When you’re removing large amounts of vegetation, those plants were using up nitrogen in the water. Since 2006, you removed them by hand. That seemed like a great, successful bioextraction process. I still support removing the plants. That may well be what’s protecting the Peconic Estuary.”

Ms. McGlynn said the DEC could “certainly talk about nitrogen testing in 2023.”

More information on the DEC’s Peconic River Ludwigia control project is online at

Beth Young

Beth Young built her first boat out of driftwood tied together with phragmites behind her family’s apartment above the old Mill Creek Inn in Southold. Nowadays, she spends most of her time kayaking, learning about shellfish, writing newspaper stories, trying to sail a Sunfish, and watching the way the bay changes from day to day. You can send her a message at

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