When Michelle Obama planted a vegetable garden on the White House lawn this spring, I took it as yet another symbolic gesture about local food and healthy eating that wouldn't amount to much. ("Let's hear it for vegetables!" she hollered to the fifth-graders who came to help her dig. "Yay!"). But gardening seems to be a hot topic in the West Wing of the White House, too. In his first major speech on urban policy last month, President Obama went out of his way to cite urban agriculture as a best practice for cities and metro areas.

The Obamas are tapping into a growing conversation about urban farming. But for the most part, harvesting soybeans in the shadow of skylines remains an idealistic vision, espoused by local-food die-hards known as "locavores." Growing fruits and vegetables on vacant city land, they say, is good for the environment, good for health and good eating.

For urban agriculture to work, however, we'll need to stop talking about it in utopian terms. Instead, let's talk about money. That's what Roxanne Christensen does in Philadelphia. Several years ago, she set out with the Philadelphia Water Department to prove that a half-acre of urban land could yield crops worth $50,000. She planted 60 types of vegetables on a tiny PWD plot and raised only high-value crops that grow quickly. In its fourth year, the parcel produced gross sales of $68,000. "The only way for urban agriculture to establish itself on any scale is to be a viable business proposition," says Christensen, who has written a book on small-plot farming.

Shrinking cities such as Baltimore, Cleveland and Detroit could learn a thing or two from this. Typically, they view their vacant land as a cost--someone must be paid to mow all those weedy lots. But what if cities viewed this land the way Christensen does, as a resource that can generate $68,000 by the half-acre? Local governments could help ramp up micro-farming to a scale where it's not only profitable but also creates jobs. When a group of architects and planners recently studied how to create a "leaner, greener" Detroit, they concluded that agriculture could become a big local industry if 10,000 acres--less than half its vacant land--were converted to farm uses.

Not everyone thinks urban farming is such a good idea. Kaid Benfield of the Natural Resources Defense Council wrote on his blog recently that this "sounds like giving up on cities and downtowns altogether." He's right: There is something odd about creating farmland in the city even as developers gobble it up in the country. But for a place in as bad a shape as Detroit is, it's hard to see what else there is to do.