Indoor farming is an industry that continues to grow — and not just your greenhouse variety, either — but a close cousin that uses no sunlight or pesticides, little water, and a whole lot of technology.

Inside an old ginger ale factory in Millis, Freshbox Farms has 8-by-40 foot containers lined up, each connected to a water purification and air filtration system. Inside every container is a hydroponic farm growing an acre’s worth of leafy greens. But, before we can even enter one, the company’s senior vice-president of plant science and product development, Deane Falcone, and I suit up in lab coats and hair nets so we don’t bring in insects that can damage the plants.

Inside, rows of lustrous, leafy green romaine lettuce greet us from trays of water. Each section is in a different stage of growth ranging from two weeks to harvest time, which occurs at five weeks. The company also monitors the soil and air conditions, which is captured every 30 seconds with software created in-house

This romaine lettuce receives 16 hours of fake sunshine from LED lights and uses about 18,000 gallons of water in an entire year, a fraction of what traditional lettuce farms use. Hydroponic farming has long been regarded for its water efficiency but critics of indoor farming like this say the cost to power the LED lights is hard to justify. Falcone agrees that it’s costly but cites Haitz’s Law, which predicts the efficiency of LED lights to increase exponentially every decade.

“So we're paying for a lot of electricity to power up a lot of the LED lights,” he said. “The thing that we always point to is the fact that the efficiency of the LEDs is increasing every couple of years.”

The cost factor is something the founders of Freight Farms, a local startup, considered when they started turning shipping containers into pre-fabricated vertical farms that require only one layer of lighting on each side. They’re called Leafy Green Machines.

It was one of the draws for Bobby Zuker and Chris Mutty, who bought one of these containers. Together they launched Greenline Growers, right out of a former taxi depot in Coolidge Corner, Brookline.

We started in December of 2015,” Zuker said. “That was when we got our first Freight Farm. And then by March we were actually selling to restaurants.”

Greenline Growers now has three vertical farms. Freight Farms sells these Leafy Green Machines to budding farmers like Zuker and Mutty for $85,000. They also provide training and now an app that allows users to control settings remotely.

They've taken a lot of different technologies and aggregated it and made it fairly simple to automate it and then we just do the farming part.” Zuker said.

Tim Griffin of the Friedman School of Nutrition Science and Policy at Tufts, said this latest iteration of urban agriculture could diversify the food supply as weather becomes more erratic.

I see it as being it's another way that we grow food and if it works really well with particular foods, then we should be thinking about how we should be thinking in a positive way,” he added. “And then [we should] also always [be] thinking about who has access to what and who does not.”

Even though we won’t be seeing corn or wheat grown in these things, that doesn’t rule out strawberries and even someday rice and grapes to go with the ever-growing varieties of lettuce.