The food movement places a lot of emphasis on urban farming, usually meaning the conversion of empty lots to community gardens, though sometimes it's rooftop gardens and occasionally vertical farming. That sequence, lots, roofs, vertical, represents my degree of sympathy with it: a good deal of sympathy for lots and very little for vertical.
Even the conversion of empty lots is a limited expedient; such lots are mostly doomed by market forces and cultural factors. Cultural factors in that a society like the English, for example, can emphasize and preserve allotment gardening. I doubt we can create such an emphasis. Market factors in that the same forces which eliminated the 261 farmers (owners and tenants) the 1920 census found in the District of Columbia will continue to operate. Urban land is too valuable, so I don't expect any self-sufficient farm to be created and to last in DC. Any farming/gardening will have to be an adjunct to some bigger institution.