dbar yields(Amy Braga @ Braga Photography)

A ledger tracking dbar's rooftop yields.

For a small but growing number of urban U.S. restaurants whose chefs are concerned about serving the finest, freshest local ingredients available, the distance from farm to table is shrinking. It can be as short as a flight of stairs; stretch several flights; or extend a few blocks away. But the distance is, increasingly, vertical, because in this latest iteration of urban farming, the gardens are on the roof.

Four years ago Rick Bayless, award-winning chef, owner of Frontera Grill, Topolobampo and the brand new Xoco in Chicago, cookbook author, and host of PBS' "Mexico - One Plate at a Time," installed a garden on the roof above his restaurants. A celebrated supporter of urban agriculture, Bayless says, "I think it's really important to prove to people it's easy to grow things even in an urban setting... even in downtown Chicago."

Rick Bayless(Courtesy of Rick Bayless)

Bayless (pictured): "I think it's really important to prove to people it's easy to grow things even in an urban setting."

In 60 self-watering EarthBoxes, he grows five varieties of chiles and 12 kinds of tomatoes to serve Frontera Grill. "It's a salsa garden, minus the herbs. Herbs are more fragile, and the garden gets 100 percent sun all day long," Bayless explains. With Xoco's construction behind him, though, Bayless plans to begin growing herbs next summer.

Frontera Grill's "Rooftop Salsa," served with grilled fish, is on the menu from the first harvest through the end of the growing season. Usually, the tomato harvest begins in early July. This year, because spring and early summer were unusually cold, the first tomatoes did not arrive until early August.

Bayless also has a production garden at his home, about 10 minutes away from the restaurants, where he grows all the greens and edible flowers used at Topolobampo -- roughly $25,000 worth of produce annually. Bill Shores, who designs and manages urban landscapes and edible gardens, works at the production garden and visits the rooftop garden roughly once a week. Bayless and other Frontera chefs work in the rooftop garden, too. "It's also important for the chefs to have direct access to food at the source," he says.