You're right of course, resorting to savaging in homes would be the highest risk and therefore an absolute last resort.
I guess a problem with discussing this is actually trying to picture a situation we'd find ourselves in where, in an urban environment, we'd actually have to forage for food to survive off of.
I'm trying to image a scenario where I'd actually resort to picking dandelions to survive. I'd assume that I'd have to be in a position to understand I'd be without help for more than you'd normally be prepared for (i.e., this is a highly unlikely scenario I've found myself in).
It would also have to be something that didn't occur while at home, but where I couldn't expect to be able to make it to any sort of friendly location within many days.
You're likely right that something like Katrina wouldn't fit this criteria; so were likely talking about something more catastrophic; maybe like that asteroid mini-series just on TV
So maybe we should step back and first ask in what situation would we find ourselves where we would have to start grazing through the landscaping plants?