Results are in for milfoil-killing herbicide used in Adirondack lakes

Additional lake associations seek ProcellaCOR permit for keeping invasive milfoil in check

By Zachary Matson

The herbicide ProcellaCOR proved effective at killing invasive Eurasian watermilfoil on the handful of lakes where it was used last summer, Adirondack Park Agency staff reported last week. 

The herbicide, which has been viewed both as a balm to communities struggling with the park’s most pervasive invasive plant and a troubling example of human meddling with nature, dissipated in the water as expected and appeared to have limited impacts to native plants. 

For some of the lakes that used the chemical, follow-up surveys later in the summer found the product had wiped out all of the invasive milfoil in its treatment zones. At most 1-2% of previously identified milfoil remained. 

Proponents of the herbicide have argued that by killing milfoil, native plants will have a chance to restore a more natural ecology. Lake associations have seen it as a welcome alternative to costly annual hand harvesting, which in many lakes has at best kept milfoil levels flat. 

Noting results

Groups on Chateaugay Lake, Brant Lake, Paradox Lake, East and West Caroga Lakes, Horseshoe Pond and Highland Forge Lake used the herbicide during the 2024 growing season, the most in a single summer since the product was first used in the Adirondacks on Minerva Lake in 2020. 

The APA presentation highlighted the results of required water sampling immediately following herbicide use and plant surveys that occurred in the months following the treatments.

The water monitoring showed that the product diluted into the water column as expected and was undetectable within three days at all of the lakes. The Lake George Association in its own monitoring found the product’s degradants in sediment and plant samples following its use in two Lake George bays.   

The plant surveys demonstrated significant invasive milfoil control, with all but a handful of small pockets of the plant decimated. The surveys also documented reductions in some native plants, especially in watershield and water lilies.   

“Survey says: the Eurasian watermilfoil was toast,” Aaron Ziemann, an APA water specialist, said while summarizing results from Chateaugay Lake. “Eurasian watermilfoil was controlled, the product degraded as anticipated, the non-target impacts were as expected but appeared to be limited.”

The herbicide’s reported success appeared to coincide with a natural decline in invasive milfoil throughout the park, Ziemann said. He said agency staff and others across the park observed “a marked decrease in Eurasian watermilfoil growth” beyond the lakes treated with herbicide, citing weather patterns and natural cycles as potential factors. 

“Professionals involved in the management of [Adirondack] lakes fully expect a rebound of Eurasian watermilfoil in the coming years, in the coming seasons,” Ziemann said.  

The latest in Lake George

The Lake George Park Commission in June released the herbicide into two north basin bays over the strident objections of the Lake George Association and some shoreline property owners. 

Commission officials have said the herbicide can be an effective tool to pair with hand-harvesting techniques in other infested bays but have not specified when or where they would use the product next. 

On Lake George: Why ProcellaCOR? Why now?

Dave Wick, the commission’s executive director, on Thursday said he did not plan to pursue another herbicide application this year. Instead, the park commission plans to work with the Lake George Association on an invasive species management plan as the two groups look to rehabilitate their relationship after multiple years of fighting in court and the court of public opinion. 

The plan, which the LGA had called for prior to the herbicide’s application, could prioritize other bays for milfoil control and establish a framework for when and where herbicide is appropriate and when other methods are preferred. 

Brendan Wiltse, the LGA’s new executive director, said the association is continuing to study how the herbicide affected the lake plants, wildlife and chemistry. The LGA is conducting its own monitoring and sharing findings with the park commission.  

“That work will help inform our understanding of whether or not ProcellaCOR is a tool the LGA will support,” Wiltse said. “We are interested to learn more and use the treatments that have occurred to understand if there are any long-term impacts to the ecology of the lake.”

Who’s making plans for 2025?

As of last week, APA had not received any applications to use the herbicide this summer, but some lake communities have expressed interest, according to APA records. 

The Mountain View Association in the northern Adirondacks and the Eagle Lake Property Owners group near Ticonderoga have both filed pre-application material to APA indicating an interest in using the herbicide. Chateaugay Lake, Highland Forge Lake and Caroga Lake, all of which used the herbicide in 2024, were considering phased applications that could be continued this year.

Residents around Eagle Lake, which rests just upstream of Paradox Lake outside the northern border of Pharaoh Lake Wilderness, grew concerned about invasive milfoil in the mid-2000s. But efforts to establish a management strategy fell apart, and the only active milfoil removal over the years came from the one-off efforts of residents, according to a pre-application filing. 

The lake association in recent years raised money and received a $30,000 state grant to develop a management plan and survey the lake last summer. That plant survey found invasive milfoil to be the most prevalent plant in the lake.  

Eurasian watermilfoil was first spotted on Mountain View Lake, which sits a few miles west of Chateaugay Lake, in the 1980s, and the lake association has contracted for hand removal since 2002, according to its pre-application material. The artificially-enlarged lake has also experienced water drawdowns from a town-owned dam that has killed off invasive milfoil in the past. The association seeks to potentially add chemical control to its arsenal.