Examples of green walls and vertical gardens exist. The primarily ornamental, but studies at the University of Waterloo have shown that green walls can be integrated in to the ventilation to help filter air, features can be used as extra space for the production of food. These examples can take advantage of already existing structures to provide agricultural space on or in structures. These walls also provide the ability to filter partially polluted water(gray water). Instead of being a drain on the hydro system they would boost the cities filtration system.

Picture on the right from Vertigarden a commercial product in England

On the left a green wall at Queens University
http://en.wikipedia.org/wiki/Living_wall

Bottom a green wall on the Musée du Quai Branly, in Paris
http://archrecord.construction.com/features/digital/archives/0702dignews-1.asp