The warmest zone, 11b, is in Key West, Florida, where the coldest annual lows range from 45 F to 50 F. On the 2012 map, northern Minnesota had a much more extensive and continuous zone 3a.

Yet there is one important limiting factor to growth that these maps don’t acknowledge and that is humidity. This is amply illustrated if you spend a day in Seattle followed by a day in Juneau ...