In 2010, an energy researcher named Evan Mills was surprised to walk into a plant nursery near his Mendocino, California, home and find, among the seedlings and bags of soil, a display of gigantic 1,000-watt lightbulbs — a more powerful version of bulbs commonly used to light highways at night.

What he found — after interviewing grow-light sellers, reading trade journals and equipment manuals, poring over crop-yields analyses and case studies of growers’ energy use, and scouring law enforcement reports — is that together, legal and illegal cannabis growers use about 1 percent of all American energy.

Most of the industry’s sky-high emissions comes from indoor farms, which grow two-thirds of U.S. cannabis under artificial lights, air conditioning and irrigation. There’s an obvious solution here: Mills estimates the industry could cut three-quarters of its emissions by growing outdoors, where sunlight and rainfall free.

That’s partly the legacy of a rogue industry gone mainstream: For one, Flax said, some businesses tried to re-create their homegrown operations at factory scale. He remembers one converted warehouse where growers had plugged in 30 kitchen toaster ovens to dry their weed, instead of using a standard industrial dryer.

“It’s another looming energy issue,” said Mills. “Cannabis right now is the dominant part. But if the proponents of indoor agriculture got their way, it would be overshadowed gradually by all these other crops.”