The reformed marijuana grower and the ex-Wall Street banker make an unlikely duo, working side by side in an old South Philadelphia factory building where - despite the chill outside - the air is warm, humid, and sweetened by hundreds of basil plants.

Lee Weingrad, the grower, and Jack Griffin, the businessman, have great hopes for this "vertical" farm, where hydroponic herbs, microgreens, and tomatoes crowd together in troughs stacked almost to the ceiling. This is Metropolis Farms. Griffin says it's a model for a network of operations that will change the face of urban farming - heck, of the produce industry.

"Most produce is coming from thousands of miles away," he said. "We'd like to have a farm in each neighborhood."

While fields are encrusted with frost and ice, ambitious growers are turning underused spaces across the region - a former mall, a factory basement, an industrial chicken house - into year-round edens, powered by grow lights and heaters and the belief that even the grittiest warehouse can, with technology and care, be fertile ground.

"This has the potential to bring a lot of our produce back to being very local, even when you live in a very urban environment," said Steven Hughes, a biologist who runs the aquaculture program at Cheyney University.

Some, like Metropolis, are growing hydroponically, which means plants are grown in nutrient-enhanced water. Hughes said interest in aquaponics - which adds fish to fertilize the plants - had been even greater. Entrepreneurs and venture capitalists see in both a way for industrial agriculture to infiltrate the buy-local movement.

Still, Hughes said, fewer than 10 companies in the area are successfully growing indoors year-round.

"This is a business that requires that you pay attention to not just the biology, but also the marketing," Hughes said. "Many operations that fail did not pay proper attention to their marketing, or they were undercapitalized."

Among the early successes is Herban Farms on Cheyney's campus. The farm sells more than 7,000 basil plants a week - grown in water fertilized by koi - to Acme, Giant, and Wegmans stores.

On an even larger scale, there's BrightFarms, a New York start-up that opened its first hydroponic greenhouse in Yardley in 2013 and now sells about 800,000 containers of greens a year to 70 area supermarkets. It has raised $40 million and is expanding to the Chicago and Washington areas.

"Demand is increasing a lot faster than we can meet it," said BrightFarms chief executive Paul Lightfoot. "The supermarkets want to have a consistent supply and meet the demand for local, and it's not easy for them to do that. And, right now, the West Coast is struggling with drought, which is playing havoc with their quality."

No wonder more start-ups are jumping in.

In Harleysville, Brian Haentze and Jamie Romani have converted an old chicken house on Haentze's family farm into a skylighted, 4,000-square-foot, aquaponic greenhouse. This year, their first full season, they expect their Evergreen Lane Farm to produce thousands of heads of lettuce, strawberries, and microgreens for their client, the Desmond Hotel in Malvern.

And they're marketing aquaponics kits, which can produce vegetables as well as fish, to schools and home growers. (An on-site hatchery sells fingerling tilapia, which grow to full size and can be eaten.)

In Cheltenham, FOCUS Foods is finalizing plans to turn 110,000 square feet of former mall space into an aquaponic farm, providing lettuce, basil, baby spinach, arugula, and bok choy to an adjacent ShopRite, said cofounder Julia Kurnik.

The system will use LED bulbs in lieu of sunlight. But, she said, it will use just 5 percent of the water a traditional farm would - and no gasoline for shipping.

Seed money came from business-plan competitions, said Kurnik, a recent Wharton MBA. Now, she's courting funders.

"There is growing passion and interest among investors," she said. "It's not like a health-care or tech start-up, or an app, where it's something everyone understands. But there's a lot of growth."

Griffin, of Metropolis Farms, said it was an easy sell when he has explained he could grow faster, cheaper, and in higher density than you could outdoors.

He is partnering with the regional supermarket chain NetCost Market this year to open a 100,000- square-foot vertical farm in Northeast Philadelphia. Two additional farms, planned for Chicago, will be built in the spring, he said.

"Think of how many buildings have spaces available," he said. In the face of future food shortages, "these solutions have to happen. There aren't enough farms."

So, why not grow hard-to-find mushroom varieties in, say, the basement of a factory in Juniata?

That's what's happening at Mycopolitan Mushroom Co. In two plastic-covered tunnels within the echoing basement, farmers produce 250 pounds of pioppini, king trumpet, nameko, and pom pom mushrooms weekly for restaurants, including Kensington Quarters, Aldine, and Urban Farmer.

Compared to buying land and building a farm, "this seemed to be the most viable option," said farmer Tyler Case. Besides, clients "appreciate that they can ask for something in a pinch, and we're close enough to deliver."

Still, the year-old start-up fluctuates between profitability and just breaking even. In the heat of July and August, the managers shut down and, instead, forage for chanterelle, porcini, and maitake. And there are unique challenges, like hauling away the mushroom scraps an ordinary farm could just dump outside. (They're starting a compost-bin sideline to absorb some of it.)

The collision of idealism and reality has crushed some farms along the way.

Union Farms, which aimed to turn an Olney warehouse into a microgreen farm, recently folded in planning stages. An investor fell through, farmer Peter Usilton said.

Another venture, Urban Food Lab - a dream of Steven Williams, of Partnership Community Development Corp. in West Philadelphia - started four years ago and is still struggling to get off the ground. Williams said energy costs proved unsustainable, though he hoped to restart the operation. Meanwhile, the storefront that housed the farm recently was slated for sheriff's sale for tax delinquency.

Still, the idea continues to pique imaginations.

A Newark, Del., start-up, AquaJar, recently raised $46,348 on crowdfunding sites for Mason-jar aquaponics kits.

And at Temple's Ambler campus, an aquaponics lab started in 2013 on the initiative of a technology staff member, Mike Bavas, now hosts a curriculum of credit and noncredit classes. (Next up: a noncredit, online class, March 10-31.)

Thomas Bilotta, who teaches there (and who now has two aquaponics systems in his living room), said the allure was simple: food security.

"To be able to build a system in your basement that can give you fish as well as vegetables is mind-blowing to some people."