RiverRock is a marijuana company in Denver, CO, that was founded in 2009 by an enterprising medical malpractice attorney some four years before recreational weed become legal in the state. Today, RiverRock operates two dispensaries, grows its own, and makes edibles, extracts, and concentrates. It used to cultivate all its cannabis indoors — a quantity John Kocer, RiverRock's CEO, wouldn't specify, but says comprises between 3 percent to 5 percent of the state's $14 million monthly weed market.

A year and a half ago, the company shifted a large portion of its grow operations to a 27,000-square-foot greenhouse. In simple terms, a greenhouse is an outdoor, semipermanent structure with translucent ceilings and walls, through which light can filter. It's the same kind of thing that conventional farmers use to grow flowers and vegetables. RiverRock's is particularly state-of-the-art, with automated humidity and temperature controls and a special blackout system that can create pitch-dark conditions in the middle of a summer evening.

Pitch-dark conditions in the middle of a summer evening

The fact that RiverRock is using a greenhouse to grow pot may not seem that extraordinary, until you realize that until recently, most marijuana was grown indoors to stay hidden from view. But in a monumental shift in the cannabis industry, that's about to change.

"This is the trend for the future," Kocer says. "We're the only industry on the planet that grows indoor under light. Tomatoes, flowers, you name it, people don't grow indoors."

And there's good reason other industries don't: it's expensive to grow indoors, where powerful artificial lights — and massive air-conditioning systems used to counteract the heat from said lights — require massive amounts of energy. By harnessing the free power of the sun, growers can save as much as 90 percent on their electricity bills. RiverRock's monthly electricity bill is $25,000 a month, only $2500 of which is used in its greenhouse, versus its residual indoor grow operations which run up the bulk of that bill.

Not surprisingly, RiverRock isn't the only cannabis grower going "green." In Colorado, industry consultants and greenhouse suppliers estimate there are 10 marijuana greenhouse operations of similar scale to RiverRock's, with several even larger ones in development. RiverRock has plans to triple its greenhouse capacity in the coming months, which will double its weed production. (Although Washington state also recently legalized marijuana, Colorado has progressed much more quickly in setting up its legal cannabis marketplace.)

Until now, high-grade pot was almost exclusively grown indoors. "The reason why indoor cultivation became the cultivation technology of choice was because this was illegal for so long and indoor is easier to hide," says Kris Krane, a consultant for the marijuana industry who also runs an incubator for startup cannabis companies.

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Patient, "Wade", inside RiverRock's greenhouse (RiverRock).

Now, even though pot is still federally prohibited, Washington and Colorado have fully legalized it, and 20 other states (and DC) have approved it for medical use. If Colorado is an example, a regulated, legal pot marketplace will mean growers are less concerned about shielding their plants from view, and more motivated to explore cost saving opportunities.

Carefully monitored conditions of light, temperature, and ventilation

Whether indoors or in greenhouses, growing top-grade cannabis with high THC content requires carefully monitored conditions of light, temperature, and ventilation. Cannabis thrives in warm, moist conditions: RiverRock's greenhouse is kept at 71 degrees, with 40 percent humidity, and is watered via a drip system from overhead plastic tubing. Although every factor in cannabis growth needs to be tightly controlled, humidity is arguably the biggest challenge, according to Zev Ilovitz, president of the Richmond, CA-based Envirotech Greenhouse Solutions, whose company has designed and installed many small greenhouse projects for cannabis growers, and is currently involved in some of the new, larger operations being developed in Colorado. "Cannabis is particularly susceptible to fungal disease," he says. "You have to have a good venting system."

When growing in the wild, Cannabis plants produce buds as the days become shorter. To get plants to bud, a grower must simulate 12-hour "nights," by blacking out some of the daylight. This is relatively easily achieved in a warehouse, but to do it in a greenhouse, you need a retractable blackout curtain. Some blackout curtains are automated, and can be rolled over the greenhouse like a garage door, while others are manually hung.

Indoor growers' greenhouse gas emissions are equivalent to that of 3 million cars.

Beyond saving money, marijuana greenhouses impose a smaller environmental footprint — an issue that's become an increasing concern in the cannabis industry. One independent study published in 2011 by Evan Mills, a scientist at Lawrence Berkeley National Laboratory, looked at the yearly energy consumption of indoor pot-growing operations (based on federal estimates of total national marijuana production, from legal and black-market sources). He estimated that annual energy expenditures were $6 billion — six times that of the entire US pharmaceutical industry. Indoor growers' greenhouse gas emissions, he reported, are equivalent to that of 3 million cars.

Although marijuana greenhouses eliminate the need for many energy sucking lights, some growers opt for a hybrid approach, using some grow lights to manipulate the plants' cycles. Marijuana, for instance, will grow higher during longer daylight hours, as opposed to budding during shorter daylight hours. So growers who want to encourage the plant's initial growth phase in the middle of winter, say, may artificially produce more "daylight" hours through the use of lights. In February, residents of the southern Colorado town of Penrose complained: "Neighbors in the town said the smell from the plants is too strong and the bright lights from the greenhouses are too invasive at night," according to the Denver-based local CBS news affiliate.

A $5 million pot 'superstore' has been proposed for the small town of Eagle, CO.

Still, any reduction in energy cost is likely to make greenhouse cultivation an attractive option in the competitive world of weed farming. "Eventually a lot of these warehouses where people are paying top dollar for rent will get squeezed out by greenhouses," says Jay Czarkowski, managing partner of the Boulder, CO-based cannabis business consultants, Canna Advisors. "Greenhouses allow producers to ... have more competitive pricing, too."

Nexus Corp., a traditional high-end greenhouse designer, fabricator, and supplier that has worked for clients including the University of California at Berkeley, reports that it's receiving an increasing number of inquiries from the marijuana industry. "As long as it's a legal crop, we'll do it," says Craig Humphrey, vice president of engineering at Nexus. "They just need to prove where they are located. We're not selling to a customer who lives in a state that doesn't have it legalized in some manner."

Meanwhile, Colorado is already in full expansion mode. A $5 million pot "superstore" has been proposed for the small town of Eagle, CO, which would incorporate a 45,000 square foot greenhouse (in addition to a 6,000-square-foot retail store, and a 3,750-square-foot "prohibition museum"). Silverpeak Apothecary, a ritzy dispensary in Aspen, CO, in January erected a 25,000-square-foot greenhouse, called High Valley Farm.

On a national scale, marijuana greenhouses, like wind turbines or giant satellite dishes, may one day transform cheap land that sits on the outskirts of cities. And Ilovitz of Envirotech, envisions a time when there may also be "microbrewery style" greenhouses attached to marijuana stores in cities. "Greenhouses," he says, "really reflect the fact that the industry is stepping out into the light."