Written by Carey Gilliam.
Soon, the chemicals will start falling from the sky over the Red Hill area of Boulder County, Colorado.
The wildland is known for its biodiversity, home to hundreds of important species and a concentration of rare environmental resources. It is a key habitat for beloved herds of elk and bighorn sheep, with farms and ranchland lying at its base.
But any day now hundreds of acres of Red Hill are set to become the target for drone spraying of a chemical herbicide that comes with a host of concerns.
County officials and other proponents of the plan say it’s an effective and essential move that will wipe out invasive weeds and mitigate the risk of dangerous and costly wildfires that could use the unwanted vegetation as fuel.
They tout Boulder County as “ground zero” for research and testing of indaziflam – a relatively new herbicide developed by Bayer, the German chemical and pharmaceutical conglomerate – that has been approved for use across millions of acres of public lands as a tool for preserving native ecosystems.
But the drone spray project has ignited an outcry from many living and working in the area as scientists warn the chemical comes with serious risks for vulnerable species and poses health dangers for people and animals exposed to the chemical, including through water that runs off the steep slope into rivers and creeks and into groundwater supplies for crop irrigation. Some critics also question ties between Bayer and the county.
“There are a lot of people in our community who are really livid about it.” – Azuraye Wycoff, owner of Yellow Barn Farm.
“It really directly affects all residents downstream from this,” said Azuraye Wycoff, owner of Yellow Barn Farm, a regenerative agricultural operation tending to crops, sheep, pigs and other livestock located at the base of Red Hill. “There are a lot of people in our community who are really livid about it,” she said.
Opponents fear the work could start as early as Nov. 1 though some county communications indicate they will begin spraying in mid-November.
Adding to the worries is evidence that the indaziflam product to be used, sold under the brand name Rejuvra, contains per- and polyfluoroalkyl substances (PFAS), a class of chemicals linked to a range of health risks and known to persist in the environment. There have been years of ongoing federal and state actions to rein in PFAS contamination.
“Its toxicity is pretty extreme,” David Jenkins, recently retired from a senior role with the Bureau of Land Management, said of indaziflam. Safer methods should be used, Jenkins said.
“Delay the Spray”
The drone plan for Boulder County is not new. Spraying over Red Hill was supposed to start last year but county officials paused the plan in the face of a backlash from community residents. The county plan includes mitigation measures aimed at protecting people and protected species, but critics are not mollified.
They say particularly in a place like Boulder County, which boasts of its commitment to environmental health and sustainability, non-chemical measures should be pursued.
Overall, the county plan targets about 3,000 acres for spraying by 2030 and extends beyond Red Hill. A group of local environmental advocates sued in 2024 to challenge the county’s broad plans. They alleged the drone spraying of terrain adjacent to homes, organic farms, trails, and “water features” illegally contradicted labeling prohibitions of application that will contact people “directly or through drift.” But in an Oct. 9 ruling, a district court judge ruled against the plaintiffs.
Now, with the drone spray plan for Red Hill revived, community members are scrambling to find another way to delay the plan.
Opponents are pushing a “Delay the Spray” call for action to mobilize residents to contact the county. And two conservation groups have filed a notice of intent to bring a new lawsuit against Boulder County officials, this one alleging violations of the federal Endangered Species Act.
Even Boulder city officials have questioned the wisdom of the spray strategy after recently completing a series of multi-year studies about the impacts indaziflam could have on the area.
The city’s report warns of “regulatory testing that shows potentially serious health impacts to people and wildlife, and high toxicity to invertebrates.” The report cites published research showing indaziflam can have genotoxic effects and notes that the US Environmental Protection Agency (EPA) has identified concerns with the chemical as well. In a 2010 report, the EPA said indaziflam was “highly toxic” to fish, for instance. The agency also reported neurotoxicity seen in animal studies as well as impacts on the liver, stomach and reproductive organs.
The city’s study findings also highlight particular concerns about the unintended consequences that spraying indaziflam could have on the viability of native seeds. Survival of native plants is critical to ecological balance, and indaziflam, which the city study found persists in the soil for years, could threaten that balance.
“Indaziflam is very effective at killing all seeds — with just a few minor exceptions,” said Brett KenCairn, a senior policy advisor for the city of Boulder on “climate and resilience”. KenCairn declined to offer an opinion on the county’s spray plan but said in the aftermath of wildfires “we are desperately going to need that indigenous seed bank.”
