A Boulder County environmental group sued the county in federal court Tuesday, seeking to stop plans to spray herbicide at three foothills open space properties over concerns that the project could harm a federally protected mouse.
The lawsuit, filed Nov. 10 in U.S. District Court in Colorado, alleges that spraying by drone at Red Hill, Hall Ranch and Rabbit Mountain would violate the Endangered Species Act by harming the Preble’s meadow jumping mouse, a threatened species native to Colorado and Wyoming.
The county plans to spray roughly 800 acres at Red Hill, west of U.S. 36 near St. Vrain and Hygiene roads, including parts of Heil Valley Ranch, as early as mid-November. It has not announced spraying schedules for the other two properties.
The lawsuit marks the latest escalation in a yearlong dispute over Boulder County’s use of Rejuvra, an herbicide containing the chemical indaziflam, to control invasive cheatgrass. The plant competes with native vegetation and can increase wildfire risk. County officials say the treatment is needed to restore native grasslands. Residents have raised concerns that the herbicide could reach nearby waterways and farms, harming native plants, soil health and human health.
Grounds for Action, a Boulder County nonprofit advocating for soil health, filed the lawsuit against the Board of County Commissioners and Jason Seuc, director of Boulder County Parks & Open Space. The group is asking a federal judge to block the spraying unless the county obtains authorization under the Endangered Species Act.
“Boulder County is about to spray herbicides by drone across some of the last remaining habitat for a threatened species. The Endangered Species Act doesn’t allow that, and we’re asking a federal court to stop it,” said Matthew Sandler, an attorney representing Grounds for Action, in a statement.
Boulder County defended its spraying plans in a statement to Boulder Reporting Lab Tuesday, saying the herbicides it selected are designed to target plants with minimal effects on mammals, including the Preble’s meadow jumping mouse, when applied according to label instructions. The county said it would maintain buffers around mouse habitat, use an additive to reduce spray drift and conduct soil testing after treatment.
“Cheatgrass is poor habitat for all local species, including Preble’s meadow jumping mouse, which is why treatment of cheatgrass infestations is so important,” the county said.
The Red Hill project has become a rallying point for activists questioning the science behind the spraying and the county’s approach to land management and public input. Commissioners first halted the spraying in December 2025, reversed course at a January administrative meeting that drew criticism over transparency, then paused it again in February. In May, commissioners Claire Levy and Marta Loachamin voted to proceed, while Ashley Stolzmann opposed it.
The spraying window runs from mid-November through late February, a period county officials say would maximize the treatment’s effectiveness while minimizing harm to native plants. No specific date has been announced.
Sandler said the group plans to seek a temporary restraining order and preliminary injunction if the county does not halt spraying in the mouse’s habitat. Such an order could block spraying while the lawsuit proceeds.
A fight over protections for a threatened species
In May, the county revised its spraying map to add buffers around mouse habitat. Stefan Reinold, the county’s deputy director of land management, told commissioners that the actual separation would be about 300 feet on either side of a dry drainage, or roughly 600 feet across.
The lawsuit argues that those precautions aren’t enough. It alleges that herbicide could spread beyond the buffers through runoff and spray drift, damaging vegetation the mouse relies on for food, shelter and survival during hibernation.
The complaint cites federal wildlife research showing that the mouse uses habitat hundreds of feet beyond streams and drainages, including areas where it builds nests and hibernates. It also points to warnings on Rejuvra’s label about the herbicide’s potential to reach surface water through runoff.
Federal rules allow certain weed-control activities that unintentionally harm the mouse, provided they meet pesticide labeling and other requirements. Grounds for Action argues the county’s plan fails to meet those conditions and would therefore need a federal permit. The county maintains that its spraying plan complies with the rules and that no permit is necessary.
Rebecca Dickson, a Grounds for Action member who also chairs People and Pollinators Action Network, has been advocating against the Red Hill spray since last year, citing concerns about potential impacts on nearby regenerative farms. She said the commissioners’ May decision to proceed convinced her that stronger action was needed.
“I love living here, I’ve never contemplated suing Boulder County,” Dickson said, but the back-and-forth on approvals eroded her trust. “For me and others, we’re saying we’ve got to do something. They aren’t listening to us.”
The new lawsuit follows an unsuccessful legal challenge to the county’s weed management plan, though it relies on a different federal law.
In October, a judge ruled for the county on the remaining claims in the earlier case, which raised questions about whether spraying violated restrictions on the herbicide’s label.
“Plaintiffs haven’t identified any endangered species whose habitats are in these areas, only threatened species,” the judge wrote.
The new lawsuit relies instead on the Endangered Species Act, which protects both threatened and endangered species. Sandler said the different legal approach could produce a different outcome.
“The Endangered Species Act is very strong at protecting both threatened and endangered species,” he told Boulder Reporting Lab.
A separate lawsuit filed by Dickson alleges that the county improperly withheld or redacted records about the herbicide’s potential effects on the mouse. A hearing is scheduled for Dec. 2.
What the science says about spraying
The legal dispute comes amid a broader scientific debate over Rejuvra’s effectiveness, its long-term environmental effects and whether herbicide is needed to control cheatgrass.
Boulder County research found that indaziflam is effective at reducing cheatgrass while causing limited harm to native species. Colorado Parks and Wildlife uses indaziflam on its land to control cheatgrass, and Trent Verquer, the agency’s grasslands habitat coordinator, previously told Boulder Reporting Lab that it was the most promising tool the agency had found for controlling the invasive grass.
A separate study by researchers from Colorado State University and the University of New Hampshire found that indaziflam could reduce cheatgrass cover by as much as 80% and encourage native plant growth, but also affected microorganisms that keep soil healthy.
Other research has raised concerns about how long the herbicide remains in the soil and what that could mean for native vegetation.
An independent review by Colorado State University and University of Colorado researchers, released by the City of Boulder in October, found that indaziflam did not prevent tall oatgrass, another invasive species, from germinating. Although the review did not examine cheatgrass, it also looked at how long the herbicide remains in soil.
The researchers estimated a half-life of 4.6 years, meaning half the chemical could remain in the soil after that time. The herbicide’s label recommends waiting at least two years before reseeding, but the findings suggest it could persist longer, potentially affecting new plant growth.
Grounds for Action cites the findings as evidence that indaziflam could harm vegetation the mouse relies on for food and shelter long after spraying.
The need for spraying itself is also disputed.
Tim Seastedt, a CU Boulder ecologist who studies invasive species, has challenged the county’s conclusions. His research at Spruce Gulch, in the foothills outside Boulder, suggests that cheatgrass can decline without herbicide. He argues that spraying at Red Hill is unnecessary.
In a separate literature review, Seastedt and fellow CU Boulder researcher Robert Brakenridge found mixed evidence of indaziflam’s effects on native plants. They also found no studies linking cheatgrass to increased wildfire risk along the Front Range, noting that much of the research on that connection comes from the Great Basin. The review also raised questions about undisclosed ties between researchers and herbicide manufacturers.
The review, published in Restoration Ecology, was removed from the publisher’s website on Nov. 3. The journal said it was investigating concerns about the paper’s content.
Jennifer Poore, a spokesperson for Envu, Rejuvra’s manufacturer, said the company contacted the researchers to challenge what it called inaccurate conflict-of-interest allegations involving two of its employees who, she said, were not given a chance to respond before publication. The company’s goal, she said, was to “correct the record and protect our colleagues’ professional integrity.”
Opponents propose grazing instead
County staff previously considered manual herbicide application and goat grazing but ruled them out because of the steep terrain and concerns about disease transmission to bighorn sheep, respectively.
Opponents are now proposing a pilot project using cattle fitted with electronic collars to graze designated areas of Red Hill instead of spraying herbicide.
Developed with nearby nonprofit Drylands Agroecology Research, the project would cost an estimated $70,000 to graze 200 acres beginning in January 2027, according to local philanthropist Lynn Israel, who is helping organize it.
Israel said she plans to present the proposal to county officials.
“We understand that the city and county have to be responsible. But we also believe that there’s other ways to be responsible,” she said.
