On Chicago’s south side, a sprawling new factory is making toxin-free soap for Method . Built on a former brownfield, it doesn’t look much like a traditional manufacturing plant. Instead of smokestacks, a massive wind turbine and solar trees sit in the parking lot. And on the roof, is the world’s largest rooftop greenhouse.

The farm–at 75,000 square feet, bigger than a city block–is run by Gotham Greens, which first launched greenhouses in Brooklyn on top of a former bowling alley and on a Whole Foods that stocks greens from the roof downstairs in its produce section. Now they’ve decided to expand and open the largest urban farm of its kind in the world.

“I think we’ve been able to prove this model that commercial scale local agriculture can be done profitably,” says co-founder Viraj Puri. “There’s this growing consumer demand for locally-grown produce in terms of quality, nutrition, there’s all the other macro trends we’re seeing in places like California with the drought, and long distance transportation. So it seemed to us if this model is successful in New York City, why can it not also be successful in large metropolitan areas?”

In Chicago, where frigid winters mean that produce is often shipped from thousands of miles away, the new greenhouse will help supply more than a million pounds of leafy greens and herbs to local stores every year.

It isn’t the only urban farm in the area; nearby, a former packing plant turned into a vertical farm a few years ago. But that farm, like many others launching in cities, runs on artificial light. Gotham Greens argues that isn’t ideal for plants.

“The results are overwhelmingly superior when growing under natural lighting, in terms of crop quality, crop consistency, and yield,” says Puri. The company is participating in a study with Rensselaer Polytechnic Institute that it says will back up those claims.

Of course, using natural light instead of hundreds or thousands of LEDs also saves energy. The new greenhouse can run on fully on renewables. It has advantages over traditional farms as well; it reuses all of the water used to irrigate the crops, doesn’t need pesticides, and takes a fraction of the land.