Boulder County advances drone herbicide plan despite health concerns

Boulder County, Colorado, is moving ahead with a plan to use drones to apply the herbicide indaziflam across about 800 acres of county open space in the Red Hill area, despite continuing opposition from residents and scrutiny of the chemical’s potential health and environmental effects.

The treatment area is west of U.S. Highway 36, near St. Vrain and Hygiene roads, and includes part of Heil Valley Ranch. County commissioners voted 2 to 1 on May 21, 2026, to revive the long-disputed project. The decision places the application within the county’s wider weed-control strategy under its 2024 Integrated Weed Management Plan.

A targeted approach to cheatgrass

The county’s plan is intended to control cheatgrass, an invasive annual grass that competes with native vegetation and is widely blamed for increasing wildfire risk across the western United States. Under the broader plan, Boulder County expects to treat approximately 3,000 acres with indaziflam by 2030.

County officials say drones are suited to the work because they can apply the herbicide in difficult terrain and use lower treatment volumes than ground-based spraying. The approach is also intended to target areas affected by cheatgrass rather than apply the product indiscriminately across the entire landscape.

• Initial treatment area: about 800 acres in the Red Hill open space area
• Broader county target: roughly 3,000 acres by 2030
• Herbicide: indaziflam, marketed as Rejuvra
• Target weed: cheatgrass

“spraying known carcinogens from the sky over our community in a broad way.” — Commissioner Ashley Stolzmann

Residents challenge the chemical and the process

Stolzmann was the only commissioner to oppose the May vote. Nearby residents and environmental groups have also objected to the project. Azuraye Wycoff, owner of Yellow Barn Farm near the planned treatment area, said many people in the community are “really livid” about the decision.

Nick DiDomenico, who farms at Elk Run Farm, has alleged that the county’s reliance on indaziflam reflects undue influence from Bayer. County officials and representatives of Envu dispute that allegation. Bayer developed indaziflam, while the Bayer spinoff Envu has marketed the product under the Rejuvra brand since 2022.

Dispute over PFAS classification

A central issue is whether indaziflam belongs among herbicides containing per- and polyfluoroalkyl substances, commonly called PFAS or “forever chemicals,” because the compound contains fluorine. Researchers affiliated with the University of Colorado have also said that some industry-funded studies examining indaziflam’s persistence in the environment did not adequately disclose ties to Bayer.

Envu and industry groups maintain that regulatory assessments support the product’s use. The disagreement leaves the county’s drone program facing opposition not only over how the herbicide would be applied, but also over how its risks should be characterized. The county’s decision nevertheless keeps the Red Hill project moving under its long-term strategy for managing invasive vegetation and wildfire-related concerns.