The American Horticultural Society has recently devised a heat zone map. It sections the US into 12 zones. Both the heat and cold hardiness maps are only indicators. How well an individual plant will survive depends on additional factors such as the overall health of the plant, plant maintenance techniques, planting site and plant maturity. Some diseases may actually predispose plants to winter injury.
Plants weakened by drought or root rots are more prone to cold injury.
Generally more mature plants will survive cold stress better than newly established plants. A deep snow cover helps plants to survive severely cold temperatures. Gradually colder temperatures are less stressful on plants than a sharp drop in temperature. Hardiness is also affected by the return of warm temperature. A few days of warm weather in mid to late winter can reduce plant cold hardiness significantly especially in trees and shrubs.