WELLFLEET — The National Park Service plans to treat six Outer Cape kettle ponds for the invasive weed Phragmites australis, which is threatening the ecology of Herring, Gull, and Higgins ponds in Wellfleet and Truro’s Great, Ryder, and Horseleech ponds.

The treatments, scheduled for September, will use a surgically applied mixture of two herbicides, one of which is glyphosate — the controversial herbicide that caused a furor last year when Eversource announced it would spray the chemical to kill vegetation on power line corridors.

The Park Service’s purpose and application technique is different from the power company’s. But Deborah Games, who owns a house on Williams Pond in Wellfleet, is wary of the plan. On June 17, the Games family received a letter from the Park Service asking if they would like the Phragmites on their property to be treated with the herbicide imazapyr.

In a conversation with Stephen Smith, a National Park Service plant ecologist, Games learned that imazapyr was not the only herbicide the NPS planned to use. According to Smith, treatments would consist of both imazapyr and glyphosate. Smith told the Independent he did not mention glyphosate in the letter that went to residents on the pond because he simply forgot to do so.

Games responded to the NPS letter, declining the treatment, and stating her concern about the use of herbicides.

“The decision to treat the ponds with herbicides, including glyphosate, would cause greater harm to the aquatic environment and underground aquifer than the invasive species would,” she wrote.

Smith and other ecologists say the encroaching Phragmites is the greater threat, however, because it endangers the entire ecosystem surrounding the ponds.

According to a report by the Mass. Dept. of Conservation and Recreation, Phragmites “displaces native species, reduces biodiversity, offers little value for wildlife and chokes waterways.” It is dense and thick, Smith said, and once established can be “almost impossible to knock back.”

The NPS has been treating the kettle ponds of Truro and Wellfleet for Phragmites — also known as common reed — since 2005. Their treatment regime has evolved over that period, Smith said.

In the mid 1990s, a stand of Phragmites in Wellfleet’s Herring Pond had taken over 100 meters of shoreline. But after a 2006 herbicide treatment with glyphosate, the native plants came back.

The Park Service treated the reeds again in 2019, this time with both imazapyr and glyphosate and, according to a report by the NPS Invasive Plant Management Team, the combination proved much more effective, allowing the use of 50 percent less herbicide and almost permanently halting the growth of Phragmites.

Smith said the treatment method is very precise. The Plant Management Team at the NPS cuts each individual plant at chest height and injects the herbicide solution directly into its stem. The Park Service reported that after the 2019 treatment there was almost no regrowth of the invasive reeds and that other pond life was not affected.

“We saw complete recovery of the native vegetation,” Smith said.

The NPS treats Phragmites in two other ways: manually cutting them down deep in the water, and allowing salt water back into the system — an approach that can be used during tidal restoration projects. Cape Cod National Seashore Supt. Brain Carlstrom said chemical herbicides are used only as a last resort.

In this case, where those two methods could not be employed, Carlstrom said, protecting the ponds outweighs the risks that come with herbicide use.

​​Those risks are not entirely clear. The World Health Organization declared glyphosate a “probable human carcinogen” in 2015. But the following year the Environmental Protection Agency determined it is “not likely to be carcinogenic to humans at doses relevant to human health risk assessments.”

In 2020, the German pharmaceutical giant Bayer agreed to pay more than $10 billion to settle tens of thousands of claims that the weed killer Roundup causes cancer, according to the Washington Post. Glyphosate is the active ingredient in Roundup. That same year, the EPA reapproved registered uses of glyphosate.

In 2017, concern over a plan to use herbicides to control Phragmites as part of the Herring River Restoration Project prompted the executive council overseeing the proposal to vote against the use of the herbicide. That decision still holds.

In 2019, both Wellfleet and Eastham adopted policies that prohibit the use of glyphosate on town-owned property. These policies, however, do not regulate glyphosate use on private land, including land controlled by the National Park Service.

Smith, the NPS ecologist, said the park service is sensitive to concerns about herbicide use but that the concerns must be balanced with the health of the ponds.

“We listen to and appreciate the emotions of the people surrounding this kind of work, but for us it is too important to just let this stuff take over these ponds and completely change their character and nature,” Smith said. “It would be a tragedy.”