I think I have to agree with Craig on this one. In an urban environment I just can't see a need for resorting to eating plants or shrubs.

In the case of a disaster where you'd have to find some form of nutrition (i.e., some form of multi-day event like Katrina), I'd suggest that vending machines, unmanned stores and even abandoned homes are perfectly acceptable sources of food. You may need to make reparations later, but I think surviving such a disaster comes first.
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Victory awaits him who has everything in order — luck, people call it. Defeat is certain for him who has neglected to take the necessary precautions in time; this is called bad luck. Roald Amundsen