This is a story about marijuana that begins in a drawer of dead birds. In the specimen collections of the California Academy of Sciences, curator Jack Dumbacher picks up a barred owl—so named for the stripes than run across its chest—and strokes its feathers. It looks like a healthy enough bird, sure, but something nefarious once lurked in its liver: anticoagulant rodenticide, which causes rats to bleed out, and inevitably accumulates in apex predators like owls. The origin of the poison? Likely an illegal cannabis grow operation in the wilds of Northern California.

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“It's a mess out there,” says Dumbacher. “And it costs taxpayers millions of dollars to clean up the sites.”

Marijuana doesn’t just suddenly appear on the shelves of a dispensary, or the pocket of a dealer. Someone’s gotta grow it, and in Northern California, that often means rogue farmers squatting on public lands, tainting the ecosystem with pesticides and other chemicals, then harvesting their goods and leaving behind what is essentially a mini superfund site. Plenty of growers run legit, organic operations—but cannabis can be a dirty, dirty game.

Morgan Heim/ BioGraphic /California Academy of Sciences Morgan Heim/ BioGraphic /California Academy of Sciences

As cannabis use goes recreational in California, producers are facing a reckoning: They’ll either have to clean up their act, or get out of the legal market. Until the federal prohibition on marijuana ends, growers here can skip the legit marketplace and ship to black markets in the many states where the drug is still illegal. That’s bad news for public health, and even worse news for the wildlife of California.

If you’re buying cannabis in the United States, there’s up to a 75 percent chance that it grew somewhere in California. In Humboldt County alone, as many as 15,000 private grows churn out marijuana. Of those 15,000 farms, 2,300 have applied for permits, and of those just 91 actually have the permits.

Researchers reckon that 15 to 20 percent of private grows here are using rodenticide, trying to avoid damage from rats chewing through irrigation lines and plants. Worse, though, are the growers who hike into rugged public lands and set up grow operations. Virtually all of them are using rodenticide. “At very high doses the rodenticides is meant to kill by basically stopping coagulation of blood,” says Dumbacher. “So what happens is if you get a bruise or a cut it you would you would literally bleed out because it won’t coagulate.”