A few years out of graduate school in botany, Paul Gray found himself tinkering with the lights for growing Green Crack. Yeah, that Green Crack: the strain of weed so strong that none other than Snoop Dogg—according to ganja lore—bestowed it such a name. Gray wasn’t just messing around, though. He found that growing Green Crack under light-emitting diode lamps could make the already potent strain even stronger.

Gray works for Illumitex, one of many LED companies catering to the cannabis industry. As more states legalize recreational and medical marijuana use, the business is growing and professionalizing. And LEDs, for their part, have recently gotten cheap enough to be more than a novelty. The once-underground world of indoor cannabis growing is coming out into the light—a soothing, magenta glow from LEDs.

LEDs offer two main advantages: One, they give off specific wavelengths of light that can be fine-tuned to the plant and its stage of growth. That’s why you see so much blue mixed with red, or magenta, light. (More on that later.) And two, they use way less energy—up to 60 percent less than traditional bulbs, by some accounts. Indoor marijuana farms are a notorious energy suck, and stories abound of illegal farms getting busted by their electric bills. So yes, illegal farms have an obvious incentive to cut their power bills. So do legal growers, though, who can save money and burnish their eco-conscious reputations at once.

Blue Plus Red

When Tweed, Canada’s largest pot grower, was setting up shop, its plant scientist turned to, where else, the Internet. “I can’t tell you how much time I spent in the beginning looking at forums just trying to get ideas,” says Katya Boudko, who heads up R&D for the company.

Boudko found that conventional weed-growing wisdom pointed to high-intensity discharge lamps, basically the lights you see shining down on stadiums. Growers typically use metal halides for the early vegetative or “veg” phase, and high-pressure sodium lights for the flowering phase.

This advice actually originates in peer-reviewed research, sort of. In the 1990s, scientists at NASA’s Advanced Life Support Program began studying how astronauts could one day use LEDs to grow food in space. They knew that plants tend to use blue and red light for photosynthesis. (The green reflects off the leaves.) They had good red LEDs twenty years ago, but they didn’t quite have blue. “You can grow plants under just red, but they would not grow normally. They would become very stretched out,” says Ray Wheeler, a plant physiologist at NASA's Kennedy Space Center.

So what does this have to do with veg and flowering? Well, metal halides tint blue, which seems to induce the plant to grow leaves. And high-pressure sodium lights tint orange—the red component seems to help plants flower. But now, good blue LEDs exist (they even netted their inventor a Nobel prize last year), and LEDs offer a much more precise way to tune wavelengths for a plant. Tweed currently uses the classic set up—metal halides and high pressure sodium—but Boudko is actively experimenting with LEDs in her R&D department. The visual quality and cannabinoid content of the LED-grown weed is higher, she says—though Gray cautions that not all lighting affects different strains equally.

Meanwhile, one of Wheeler’s former NASA colleagues now works on lighting solutions for Colorado pot growers.

Energy Savings

Metal halide and high-pressure sodium lights are less efficient because they convert a lot of the energy they use to heat rather than light. For Steve Cantwell, who lived in the Nevada desert, it was brutal. “When you flick those things on, you turn the grow room into an Easy-Bake Oven,” he says.

Cantwell started growing pot for himself with a medical marijuana license. But when he started his pot-growing company, Green Life Productions, he turned to LEDs. Without the extra heat from lights, he also pays less to cool his 15,000-square-foot grow room.

Of course, LEDs can help grow more than just pot. FarmedHere, the US’s oldest indoor farm, started in 2010 with fluorescent lights. They eventually switched to LEDs, which were twice as expensive upfront. But the company is now saving $45,000 a year on energy bills, says FarmedHere’s president, Megan Klein. When astronauts on the International Space Station grew and ate their own veggies earlier this year, the grow lights were LEDs

Sunlight is free, of course, and it’s hard for even LEDs to compete with free. Large-scale agriculture probably isn’t coming indoors—at least not until humans build a really big spaceship, or colonize another planet. But for very niche plants—like FarmedHere’s salad greens and herbs—LEDs are changing the economics. And weed, long grown indoors, is leading the way.