Officials to target aquatic nuisance in Boulder Reservoir with herbicide

DENVER ( KDVR ) — Two years after Colorado Parks and Wildlife detected Eurasian watermilfoil, an invasive plant , in the Boulder Reservoir, Northern Water and the city of Boulder are creating a targeted herbicide application plan as a roadmap toward herbicide treatments.

It could be implemented as soon as spring 2025.

The plant has spread “significantly” since its discovery in the water during the summer of 2022, according to the city of Boulder, moving into the western coves and the swim beach and impacting boating and swimming.

“(Eurasian watermilfoil) impacts the Reservoir in many ways, including wildlife habitats, water quality, and recreational swimming and boating,” said Parks and Recreation Director Ali Rhodes in a release. “Since it has become established, eradication is now unlikely, and we are working hard with partners to take care of this important water resource”

The city warned that if the plant continues to spread, there’s a chance it could impact the 63rd Street Water Treatment Plant intake, which is a backup water supply for Boulder, and the Boulder Creek Supply Canal outlet, which Northern Water uses to deliver water downstream.

To combat the nuisance plant, Northern Water and the city hired a subject-matter expert in aquatic nuisance species to recommend management options, including using a targeted herbicide within the next year to manage the plant’s spread and minimize its impacts. Northern Water will work with a licensed herbicide applicator and the city will create a targeted herbicide application plan for treatments, which will be applied on an as-needed basis.

Triclopyr selected as herbicide for Eurasian watermilfoil

Boulder and Northern Water have selected triclopyr as their herbicide of choice because it is effective and has the fewest impacts on water quality and is already used by Northern Water in areas adjoining the Boulder Feeder Canal upstream of the reservoir.

“Triclopyr’s use poses minimal risks to water quality and irrigation, and can be managed through city operational protocols, concentration thresholds, application areas and timing,” said Joe Taddeucci, Boulder director of utilities, in a release. “We’re looking to start its application in spring 2025, although this may be re-evaluated pending the conditions of (Eurasian watermilfoil) in the Reservoir.”

Northern Water and the city will track triclopyr’s effectiveness on the nuisance plant to determine where to apply the herbicide, and ultimately reduce its use over time. The city will continue to pull the weed by hand near the swim area and near water intakes and outlets. Boulder and Northern Water said they both aim to limit herbicide use while fighting Eurasian watermilfoil.

The city said it will continue to manage established plants through the existing watercraft inspection and decontamination program in addition to visitor education and preventing recreationists from possessing or launching watercraft in all areas except on the South Shore.

“Our goals are to continue to have the Reservoir be a safe water storage facility for the city and region and safe place for our community to play,” said Rhodes in the city release.

Additional information about the situation and triclopyr application logistics can be found on the city of Boulder’s website .

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