State to start chemical treatment of invasive weed in Payette Lake

State to start chemical treatment of invasive weed in Payette Lake

Chemicals will be used to kill an invasive plant around the shoreline of Payette Lake for the first time this month.

A contractor hired by the Idaho State Department of Agriculture will apply herbicides to about 43 acres of Eurasian watermilfoil on Friday, Sept. 11, according to Sydney Kennedy, an ISDA spokesperson.

“Our plants administrator will be on-site to oversee the treatment,” Kennedy said.

The treatment targets areas around the western shoreline of Ponderosa State Park, Pilgrim Cove and Wagon Wheel Bay.

Clean Lakes, Inc., the company contracted by the state, will use ProcellaCOR in the zones identified for treatment. The herbicide works by mimicking a plant growth hormone, causing the milfoil to die back to its roots.

Milfoil grows in shallow, sunlit water up to about 30 feet deep. Each time the plant is broken — by a swimmer, boater, or even wave action — the fragment drifts off and settles elsewhere on the lakebed to sprout a new plant.

Diver surveys over the last two summers showed that milfoil in Payette Lake more than doubled in the last year. About 320 tons of milfoil were observed in the lake this summer, compared to about 149 tons in 2025, according to ISDA.

Milfoil can grow three to five feet in length each summer. If left unchecked, it could degrade water quality and choke out boat propellers, fish habitat, and native plants.

Expected public impacts

The chemical, ProcellaCOR, is not expected to affect the City of McCall’s drinking water system, which draws water from Payette Lake through intake stations at Legacy Beach and Davis Beach.

The herbicide also does not have any restrictions related to recreation or fishing, although water from the treatment zones should not be used to irrigate broadleaf plants for about three days.

The irrigation restriction is expected to end about three days after treatment, once water samples show herbicide concentrations have fallen below two parts per billion.

Treatment notices were sent to all water rights holders within a quarter-mile of the treatment zones. Additional notices will be posted on public docks 24 hours before the treatments.

The state will collect lakebed sediment samples 30 days, 90 days, and one year after treatment to monitor for traces of the herbicide. The state is also collecting pre-treatment samples to establish baseline conditions.

ISDA: diver removal not enough

Milfoil management in Payette Lake has historically relied on suction dredges and diver teams to manually remove the invasive plant.

Those efforts have continued this summer in heavily trafficked parts of the lake, such as around marinas and public boat ramps. But ISDA officials said the infestation has grown too large to manage through diver removal alone.

It would take more than 43,000 diver hours to remove all milfoil currently in the lake, according to the agency.

Herbicide treatments are a key part of the state’s recently approved five-year management plan for milfoil in Payette Lake.

The plan was approved by ISDA Director Chanel Tewalt following a review by city, county, and state agencies that helped draft the plan. The group included the City of McCall, Valley County, Ponderosa State Park, the Big Payette Lake Water Quality Council, and the Idaho Department of Environmental Quality, among other agencies.

The state planned to test the use of herbicides on milfoil in Payette Lake last summer, but called it off amid public opposition. The five-year management plan approved by the state was then drafted alongside local agencies over the winter.

The plan calls for an “integrated” approach combining herbicide treatments and mechanical removal. Future treatment plans will be based on annual surveys completed each July, when new milfoil growth is typically about eight to 12 inches.

The plan’s success will be measured based on total infested acres in the lake, milfoil density in infested areas, and transitioning from large-scale treatments to localized maintenance.

Milfoil was first found in Payette Lake in the late 1990s. An annual removal program led by Valley County began in 2001 and continued through 2022, at which point ISDA assumed control of the program at the county’s request.