Lt. Patrick Foy, of the California Department of Fish and Wildlife, knows more than a few ways to track down a large-scale marijuana growing operation: Air surveillance, reports from neighbors and sometimes, dead fish.

“A bunch of fish may turn up dead in a creek, so we’ll go look, walk upstream, and inevitably run into a marijuana growth site,” says Foy.

Growers in remote areas, Foy explains, often end up destroying local creeks and other water sources, or using harmful pesticides to keep their plants healthy, which damages the surrounding environment.

As more states legalize marijuana, some environmentalists fear that more domestic pot growing means more ecological harm. As the Big Pot creates jobs, inspires entrepreneurs and, er … heals the sick, experts predict that the industry, which is worth roughly $2.6 billion this year, could grow to $35 billion by 2020 if it becomes legal at the federal level.

But the emerging “Green Rush” may not be so green after all.

“Contrary to the stereotype of marijuana growers as genial and environmentally conscious hippies, illegal marijuana growers are often heavily armed and operate with little or no regard for the environmental impacts of their operations,” wrote Eric Christensen, a partner at Seattle law firm Gordon Thomas Honeywell, in a recent blog post.

Marijuana can take a heavy toll from the environment. Outdoor farmers need to clear an area of land, which sometimes requires cutting down trees and other natural vegetation. In addition, many cultivators use pesticides and rat poison, which wash into waterways and leach into groundwater. A 2013 study found that rat poison used in illegal growing operations was making its way through the ecosystem in the Sierra Nevada and killing off wildlife.

But even when growing is legal and moved to indoor greenhouses, the business still has a major carbon footprint that troubles environmentalists. “The most important environmental cost of marijuana production in the legal Washington [state] market is likely to be energy for indoor, and to a lesser extent, greenhouse growing,” reads a 2013 report from a team of professors at the University of California, Berkeley.

Indoor cannabis cultivation requires high-energy lighting and air regulation that burns up energy much higher than household consumption. Mills writes that the lights are similar to those found in a hospital operating room, and the air is changed 60 times as often as in an ordinary home, according to Evan Mills, staff scientist at Lawrence Berkeley National Laboratory and consultant at Energy Associates, who wrote about the issue in a recent blog post.

“Elevated electricity demand growth has been observed in areas reputed to have extensive indoor cannabis cultivation,” he wrote.

Researchers at Berkeley estimate that if all legal marijuana production in Washington were moved indoors, it could bring the state’s energy consumption up 0.8 percent.

“As an activist myself, I am broken-hearted to see those involved in the criminal, illegal cultivation and sales on our public lands,” says Cheryl Shuman, a California marijuana business expert. “[California] is very fractured between the legally compliant market and the criminal market,” she said.

Shuman’s own medical marijuana company is run on a model similar to community-supported agriculture, known as a CSA, which allows for all customers to actively participate in the growing process. But she said it’s not always so straightforward, and it can be difficult for customers to know whether they’re getting their product from legal or illegal operations.