There is a wealth of material about frost tolerance of plants, and even lists of the lowest temperatures given plants can withstand. But this information is highly suspect for container plants, whose roots can actually experience freezing temps. The recent cold snap here in Texas has me moving ALL my container plants into a protected area, even though those plants are listed as being highly freeze tolerant. Would be nice to have some better information along these lines. I've heard some hand-waving arguments that container plants should be treated as being hardy only to two USDA zones higher than the plants in the ground. Of course, down here, the ground freezes only down to an inch or two, so roots of in-ground plants can almost always be considered protected.