On a 1,600-square-foot-rooftop in Guangzhou, China, 14 hydroponic tanks produce hundreds of pounds of vegetables a year, with a potential profit of over $6,000 annually—almost twice the 2015 annual minimum wage in the city, which has one of the highest monthly minimum wages in the country. The hydroponic tanks are part of study that shows residents and developers in Guangzhou that their rooftop space might be worth some green.

A paper published this past July the journal Agronomy for Sustainable Development reports that growing leafy greens in rooftop hydroponic systems can not only produce a steady supply of vegetables—it can also be cheaper than buying store-bought alternatives.

It’s one of the first studies outlining a comprehensive business model for hydroponic rooftop farming, a method cropping up in the US, the European Union, and Canada. This type of farming all but removes soil from the equation, with each plant housed in plastic containers hovering above a tank, their stringy roots dangling into a circulating pool of nutrient-boosted tap water.

Rooftop farming could also create jobs and reduce the carbon footprint of transporting foods into cities, says Wanquing Zhou, a research associate at the Worldwatch Institute. Those are essential side effects, considering the rapid urbanization currently underway in China: By 2020, Guangzhou’s population is expected to nearly double from 9.62 million in 2010 to 15.17 million—almost equivalent to adding the entire population of New York City.

“There is a need for rooftop farms not only in Chinese cities, but all major cities that have the resources—rooftop spaces, water, sunlight—and yet are heavily dependent on food produced long distances away,” Zhou says.

For the two-year study, researchers constructed a “screenhouse”—a semi-enclosed structure with a roof—on top of a two-story building inside the South China Botanical Garden. Surrounded by screens to ward off armies of insects that thrive in Guangzhou’s summer subtropical climate, 14 hydroponic tanks inside the screen house nursed and fed a forest of seven different greens, including caraway, potherb mustard, and Italian lettuce. The crops were then rotated based on their natural growing seasons—from November through March, bouquets of crown daisy and Italian lettuce populated the screenhouse, while summer and fall months brought waves of leaf mustard.

Lufa Farms Bok choy growing in a hydroponic rooftop farm in Montreal, Canada.

After calculating the cost of building the screenhouse and tanks, rent, labor, utilities, seeds, fertilizer, and other equipment, the team from the Chinese Academy of Sciences South China Botanical Garden and the Zhong Kai University of Agriculture and Engineering found that six out of the seven vegetables were cheaper to produce than to purchase at a local store. Each kg of crown daisy, for example, was $1.78 cheaper than its market price while each kg of leaf mustard was $3.71 cheaper. Due to its short growing season and low yield per tank, caraway was the one green that was less expensive to buy.

The team also tested two greens and found that they contained fewer contaminants than their market counterparts—including pesticides, nitrate, lead and arsenic. If the finding holds up for the other five vegetables, the hydroponic screenhouse model could be particularly relevant for urban farming. New York, Philadelphia, Detroit and Shanghai have all had traces of lead contamination pop up in their urban soils.

According to Dickson Despommier, a microbiologist at Columbia University and author of The Vertical Farm, hydroponic systems coupled with screen houses allow farmers to control virtually every detail of crop production, from the pesticides used to what nutrients the plant is soaking up. “The next thing you know, you’re feeding your crops with absolutely pure ingredients,” Despommier says. “You can expect a higher quality crop as a result.” Hydroponic farms are also lighter than traditional soil-based rooftop farms—important when trying to grow on buildings with weight limits for their roofs.

But, as Brad Rowe, a horticulturalist from Michigan State University specializing in green roof technology, points out, the use of hydroponic systems has its drawbacks. Compared to soil-based rooftop gardens, hydroponic operations, especially if they are enclosed by a screen, might not reduce the heat island effect—in which dense cities experience higher temperatures compared to less-developed areas around them—as effectively as open-air greenroofs. They also wouldn’t help reduce and reuse stormwater runoff, another benefit of soil-based gardens, said Rowe.

Rowe and lead author David Ow say that hydroponic systems are more suited for leafy greens and tomatoes than for slow-growing, heavy crops like watermelons or rice. Leafy greens and tomatoes are ideal because of their fast growing times and large yield. “It doesn’t have to be a complete farm,” Ow says. “We are looking it as a supplement for other kinds of existing agriculture.”

New York-based Gotham Greens and Montreal-based Lufa Farms are two examples of what these plant-topped buildings would look like.

Lufa Farms An aerial view of Lufa Farms’ first greenhouse in Montreal.

Founded in 2009, Gotham Greens now has four rooftop hydroponic greenhouses in New York and Chicago sprouting leafy greens, herbs, and tomatoes. Its largest operation, located on top of a manufacturing plant in Chicago, measures over 75,000 sq ft and produces up to 10 million heads of leafy greens and herbs a year, the largest and most productive rooftop farm in the world. Lufa Farms is credited with opening the world’s first commercial rooftop greenhouse in 2011 in Montreal, clocking in at 32,000 sq ft. Since then it has added another location just north of Montreal, producing over 40,000 lbs of fresh produce a year between the two.

Both of the farms’ expansions can be credited to a growing desire for local, sustainably produced food in Canada and America—and the price consumers will pay for it. But both also had a hard time getting off the ground with an unproven business model. Banks, developers, and zoning offices all expressed skepticism because of the high startup cost and lack of profitable precedents. “It’s been a long haul for us,” says Lauren Rathmell, greenhouse director at Lufa Farms. “The first three to four years were finding a building that would be suitable for our greenhouse.”

Along with his study, Ow hopes that successful enterprises like Gotham Greens and Lufa Farms will show developers that there is a real incentive to incorporate rooftop farms into plans for future properties, especially as the estimated 2.4 million acres of rooftop space in China continues to expand.

Zhou, of the Worldwatch Institute, has the same hope. “It takes time and the right people to trigger a high-level policy push,” Zhou says—that could include anything from government subsidies to changing zoning regulations to accommodate urban farming. “But as the need for alternative food systems grow in China, rooftop farming will have an important role to play in the future.”