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

He asked the nursery owner what they were for. “He gave me kind of a side eye and then explained, ‘Well, this is for cannabis growing, you idiot. That’s what everyone around here does,’” Mills said.

The federal government rarely funds research on marijuana — a substance it officially ranks as more dangerous than fentanyl, cocaine and meth — let alone its energy use. So, Mills, then a senior scientist at the Lawrence Berkeley National Laboratory, spent nights and weekends outside of work on a years-long quest to build what many growers, regulators, and researchers consider the most comprehensive model of the energy required to power the American cannabis industry.

What he found — after interviewing grow-light sellers, reading trade journals and equipment manuals, poring over crop yield 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. That’s more than cryptocurrency mining or all other crops combined, according to a paper Mills published in February, an update to his original 2012 study.

The industry’s greenhouse gas emissions warm the planet about as much as those of 10 million cars. For a daily user who buys cannabis grown indoors, their pot’s carbon footprint is nearly half the carbon footprint of their entire home, according to Mills.

“Consumers don’t know any of this,” he said. “They know that a car is labeled with how many miles per gallon it gets, or a refrigerator has an Energy Star label, but there’s zero consumer information about cannabis.”

Most of the industry’s sky-high emissions come 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 rain are free.

However, the apparent solution is far from easy. Businesses are reluctant to give up indoor farms that can produce six or more harvests a year with precise potency, opting instead to start over in outdoor fields that may yield only one or two crops a year. Outdoor fields must also contend with weather and wild pollinators that make their product less predictable.

Consistency is the key to success. Consumers know they can expect a high-quality product,” said Bonnie Bahlmann, who co-owns a small indoor cannabis farm in Boulder, Colorado, called In The Flow. “These things are just challenging to achieve outdoors.”

Instead, indoor operations like Bahlmann’s are working to reduce their emissions by upgrading to more energy-efficient equipment and purchasing renewable energy.

Those efforts could put a dent in cannabis emissions — if they catch on with companies and criminals across the industry. Upgrading all indoor farms to use more efficient equipment would reduce emissions by about a tenth, Mills said. Growing cannabis strains that produce more buds with less light could cut emissions by a sixth, and powering all indoor operations with solar energy would reduce emissions by half.

If the industry can’t move outdoors, these tactics are its best hope of going green.

Growing cannabis outdoors

No. 9 Collection is one of the largest outdoor cannabis farms in Massachusetts, a state that does more than any other to lower licensing fees and simplify regulations for outdoor growers. Each summer, 3,500 marijuana plants crawl up trellises across the 14-acre farm near Buzzards Bay.

Tending to them is precise and delicate work. Within the industry, outdoor farms have a reputation for growing cheap weed for the sake of extracting its THC, CBD, and other psychoactive ingredients for edibles, oils, and other products. But No.9 Collection prides itself on selling a premium product to compete with lab-grown indoor cannabis.

Throughout the growing season, 15 workers armed with pruning shears scrutinize each six-foot plant for any leaf out of place or sign that it isn’t getting enough nutrients or water. “You’re going down the rows all day just manicuring these plants,” owner Krishna Gandhi said. During the fall harvest, a team of 40 cuts down the stalks, hangs them to dry for a week in a greenhouse, and then carefully plucks each bud.

Then there’s the weather: During No.9 Collection’s first year in business in 2021, once-in-a-century rains soaked Massachusetts and flooded the farm. “It was a total disaster. I was sleeping in the parking lot in my van just trying to get things to work in the field,” Gandhi said.

But it was no use. There was hardly any harvest that year.

Indoor growers don’t have these headaches. Instead of contending with muggy Massachusetts summers, they can create the perfect arid conditions for marijuana plants to thrive indoors, with a consistent 12 hours a day of artificial light and an ideal blend of nutrients to maximize bud growth and THC content.

“Some of these strains that are out there that are pushing the upper limits — close to 30 percent THC by weight — that would be tough to do outdoors because you can’t control as much,” Gandhi said. “Outdoors, we aim to reach 25 percent.”

As a result, indoor cannabis can be as much as 10 times more profitable than outdoor cannabis for similarly sized businesses, according to Chelsea Haskins, who chairs the cannabis cultivation committee of the National Cannabis Industry Association.

“I don’t think it’s viable for businesses to make a full switch from indoor to outdoor cultivation and still survive,” she said. “The more realistic view is: How can we make indoor cultivators more energy-efficient?”

Making indoor farms more efficient

Boulder County, in Colorado, was one of the first local governments to establish rules aimed at limiting indoor cannabis growers’ emissions.

“When the industry was early, there was a shortage of technical expertise,” said Ron Flax, a Boulder County official who helped launch the program. “They were just sort of winging it.”

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

Patchwork laws around marijuana were another barrier. Many engineers refused to work with the cannabis sector, an industry that is still federally illegal.

That’s where regulations like Boulder’s have helped. The county remains a model for reducing indoor cannabis emissions, experts say.

In 2014, two years after Colorado legalized marijuana, Boulder tacked a 2-cent-per-kilowatt-hour fee onto cannabis companies’ electricity bills. It used the money to hire energy efficiency consultants to suggest ways to save power and to buy energy monitoring equipment used to inspect grow houses. Later, growers that paid for upgrades got a break on their energy fees.

Bahlmann, of Boulder’s In The Flow, was one of the first growers to accept Boulder County’s offer to help upgrade her indoor farm, which she and three co-founders had retrofitted in 2015 from a former mechanic’s shop.

Bahlmann and her partners didn’t have a lot of cash to work with: Unable to get a bank loan, they had already emptied their savings accounts and maxed out their credit cards to scrape together $200,000 to get the business running. However, by utilizing rebates from the county and their power company, they had to pay only about a quarter of the more than $100,000 required to replace their energy-hogging high-pressure sodium lights with new, energy-efficient LED bulbs.

“That lightened the load of the up-front costs,” Bahlmann said.

Today, In The Flow operates out of a new location, featuring three grow rooms equipped with LED lights that switch on and off every 12 hours on a rotating schedule, ensuring that none are running simultaneously. That eases the strain on the area’s power grid and lowers the farm’s electricity bill. Employees wear headlamps when they need to work in dark or unlit rooms.

The company also buys renewable energy credits from Jack’s Solar Garden, a community solar provider, to offset its electricity use. Bahlmann estimates the farm has cut its electricity use by about a fifth since it started making upgrades.

Other local governments, including Denver, Portland, San Francisco, and Boston, as well as states such as Illinois, California, and Colorado, have created their own rules requiring indoor growers to report their energy use and, in some cases, offset emissions or upgrade to more efficient equipment.

A budding problem

Finding a way to limit cannabis emissions is becoming more urgent as the blossoming industry grows to absorb more power year after year. When Mills first modeled the cannabis industry in 2012, he estimated the legal and illegal markets grew 10,000 metric tons of cannabis per year.

Since then, he estimates, cannabis production has nearly tripled to 24,000 metric tons of weed a year — enough, in theory, to roll every American adult 90 generous joints. (In practice, it’s not all joints. Some of that output will be turned into drinks, gummies, brownies, vapes, and other extracts, while some will wind up in police evidence lockers or landfills.

As 24 states, Washington, D.C., and Guam have legalized recreational marijuana, companies have rushed in to set up Walmart-size factory farms and take over the market, leading to overproduction and a lot of wasted weed, Mills said. Meanwhile, many state and local governments — worried about security, smells, and complaints from scandalized neighbors — have banned outdoor cannabis farming or regulated it so heavily that it becomes impractical.

As companies develop indoor farms for lettuce, tomatoes, strawberries, and other crops, cannabis could be a window into the future.

“It’s another looming energy issue,” said Mills. Cannabis is currently the dominant part. However, if the proponents of indoor agriculture were to prevail, it would be gradually overshadowed by all these other crops.

https://www.washingtonpost.com/climate-solutions/2025/03/23/cannabis-energy-use-climate-change/