Hardiness Zones, Gardening Zones, Growing Zones and Plant Zones refer to defined geographic regions that can support specific plants, flowers and trees. The zones define a minimum range of temperatures that a plant or tree can survive safely in that zone. The most commonly used Hardiness Zones were defined by the USDA. The USDA provides a hard copy map that displays these zone throughout the US. The last update to this map covers minimum temperatures from 1961 to 1990.