Actually I would have to disagree with you there, most cities don't have edible and useful plants of any significance. And the towns and cities that do would be subject to seasonal variation. i.e. they would be only available during a 1-2 months during the year. Its tough enough finding edible food in the wilderness let alone a large tract of land covered in motorways, high rise buildings, high density housing and industrial sites. Many cities do not even have any large parkland areas and recreational areas offering wooded areas of any size large enough for even 1 person to find enough wild food to last even a few days even during those couple of favourable months of the year. Even good agricultural land using modern industrial farming methods is quite barren of wild edibles unless the famer has set aside land strips around the field specifically for wildlife i.e. hedgerows

If survival in the urban area required the aquisition of high energy nutrious food then searching out wild edibles would be way down on the activities list, whereas supermarket and domestic bin raking skills during the night would be much more productive in the urban survival setting. If food was in such short supply then hunting urban wild life such as pigeons, squirrels etc taken using an air gun or sling shot would be next on the activities list. At this point in time a serious effort would be needed to get out of dodge. Urban survival would most likely require 'Hobo' thinking rather than any 'Ray Mears wild foods' thinking.