Illegal Cannabis Grows Leave a Toxic Legacy in California’s Forests

Hidden cannabis gardens on public lands can leave behind far more than damaged vegetation. Illegal cultivation sites may contain rodenticides, insecticides, fertilizers, irrigation equipment, trash and makeshift roads—hazards that can persist after growers have abandoned the land.

The danger became clear in a 2012 study published in PLOS ONE, led by researchers from the Integral Ecology Research Center, the University of California and other institutions. Scientists examined 58 fisher carcasses from California and found that 46—79 percent—had been exposed to anticoagulant rodenticides. Four deaths were directly attributed to rodenticide poisoning.

That distinction matters. The study documented widespread exposure and confirmed poisoning in some animals; it did not establish that every exposed fisher died from an illegal cannabis grow. Still, the findings identified a serious conservation threat to fishers, rare forest carnivores that live largely in remote public forests rather than near conventional agricultural areas.

Researchers suspected that trespass cannabis cultivation could be one source of the contamination. Follow-up investigations at grow sites found rodenticides and other toxic chemicals, including substances that are prohibited or tightly restricted in the United States. Poison can move through the food web when rodents consume bait and are then eaten by fishers, owls, bobcats, foxes, mountain lions or other predators.

The risks are not limited to fishers. The Integral Ecology Research Center has reported toxicants and other environmental hazards at illegal cultivation sites affecting a range of wildlife, including black bears, northern spotted owls, turkey vultures and other carnivores. Because these sites are often placed in isolated forests, the chemicals can be difficult to detect and remove before wildlife encounters them.

Illegal grows can also damage ecosystems without killing animals directly. Growers may clear native vegetation, build unauthorized roads, disturb steep slopes and divert water from small streams. Those activities can increase erosion, carry sediment and agricultural chemicals into waterways, fragment habitat and reduce flows during the dry season, when aquatic species are already under stress.

Research on water use has shown why the timing and location of diversions matter. A study of permitted and unpermitted cultivation in Northern California found that unpermitted farms extracted more water during the dry season than permitted farms in the study area. The researchers also found that water use was concentrated in a relatively small number of watersheds, meaning local effects could be substantial even when statewide totals appear modest.

California’s legalization of adult-use cannabis in 2016 created a regulated market, but it did not eliminate unlicensed cultivation. Farms operating outside the licensing and environmental-permitting system can avoid requirements designed to protect streams, soil and wildlife. The state’s California Environmental Monitoring and Assessment Framework identifies unlicensed pesticide use, habitat conversion, erosion and water diversion as important environmental concerns.

The pollution can remain after a site is shut down. A 2025 U.S. Geological Survey study examined former trespass cultivation sites on federal land in California and found evidence of persistent organic contaminants, including pesticides and other substances. Such findings underscore why eradication alone is not enough: abandoned sites may require hazardous-material removal, water-diversion repairs, erosion control and restoration of native vegetation.

Cleanup efforts are carried out by partnerships among wildlife agencies, land managers, law enforcement officers, researchers and local restoration groups. The California Department of Fish and Wildlife’s Cannabis Restoration Grant Program supports projects that remove infrastructure and contaminants, repair damaged watersheds and restore habitat on public, private and tribal lands.

Illegal cannabis cultivation is therefore not simply a criminal-justice problem. At poorly managed sites, it can become a long-term ecological problem involving toxic exposure, depleted streams, unstable slopes and costly restoration. The research does not suggest that every cannabis farm causes the same damage, but it shows why unregulated cultivation in sensitive forests demands sustained monitoring, enforcement and cleanup.