Boulder county, Colorado, is days away from launching drone-sprayed herbicide over Red Hill, a wildland area formally designated as one of the county's highest-biodiversity zones, home to elk herds, bighorn sheep, and rare ecological resources. The target chemical is indaziflam, sold under the brand name Rejuvra, developed by Bayer and now marketed by Envu after a 2022 asset sale. County officials frame the operation as essential wildfire mitigation, aimed at eradicating cheatgrass — an invasive weed that pervades more than 50 million acres across western mountain ranges and fuels more intense, more frequent fires. The plan has met sustained local resistance. Farmers, ranchers, and residents downstream of Red Hill's steep slopes fear chemical runoff into rivers, creeks, and groundwater used for crop irrigation. Yellow Barn Farm owner Azuraye Wycoff speaks for many: residents are "really livid." A first attempt to spray last year was paused after public backlash, but the county has revived the project with mitigation measures including no-spray buffer zones and drift-reduction protocols. The scientific picture is contested but increasingly unfavorable for the herbicide. A June 2025 paper by two University of Colorado researchers published in a peer-reviewed journal highlighted risks to ecological and human health, cited prior studies raising concerns about genotoxicity and neurotoxicity, and argued the cheatgrass rationale was overblown. Critically, the researchers noted that some published studies promoting indaziflam on public lands were authored by people associated with Bayer, drawing comparisons to tobacco, DDT, and OxyContin promotion in scientific literature. The city of Boulder's own internal report delivered a sharper warning. It flagged "potentially serious health impacts to people and wildlife, and high toxicity to invertebrates," noted that the EPA had labeled indaziflam "highly toxic" to fish in a 2010 report, and found the chemical persists in soil for years — threatening native seed banks critical for post-wildfire recovery. Senior city policy adviser Brett KenCairn noted that indaziflam is "very effective at killing all seeds, with just a few minor exceptions," precisely the property that makes it dangerous when native plant regeneration is needed most. Adding a layer of concern is testing that detected per- and polyfluoroalkyl substances (PFAS) in the Rejuvra product — the same class of persistent "forever chemicals" that federal and state regulators have spent years trying to contain. Meanwhile, Boulder county's plan extends well beyond Red Hill, encompassing more than 2,000 acres total. Envu defends the product's safety and efficacy, pointing to a "substantial body of independent scientific research" supporting indaziflam as a tool for managing invasive species. Legal efforts to stop the spraying have so far failed. A 2024 lawsuit alleging violations of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) was dismissed by a district court judge in October 2026. Opponents have now filed a notice of intent to sue under the Endangered Species Act and are mobilizing a "Delay the Spray" campaign urging residents to pressure the county directly. The core tension is structural: a county that brands itself as a leader in environmental health and sustainability is deploying a chemical its own neighboring city government warns could undermine biodiversity, contaminate water, and destroy the native seed reserves needed after the very wildfires the spray is supposed to prevent. The question is not whether cheatgrass is a problem — it is — but whether the cure introduces risks that compound over time in ways the county has not adequately weighed.
