Most garden reference books, and catalogs use USDA plant hardiness zone ratings. We managed to beat the average this last week. We can grow plants rated to zone 5 or less. Sometimes we can stretch a zone 6 plant into our area under ideal growing conditions or in a protected area. Winter cold is not the only criteria for plant survival.
Summer heat is also critical.
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.