It's a lovely idea and a great way to bring nature back to urban dwellers... but, each human requires 1 acre to grow enough food for a year, and four to grow in sustainable crop rotation. That means the island of Manhattan could, if fully utilized for solar gain, support 2500 to 10,000 of its 1.6 million people. As a supplement, urban gardens are great. In a time of breakdown, it is useless without dire depopulation.