The US Department of Agriculture Plant Hardiness Zone Map, revised in 1990, shows the lowest temperatures that can be expected each year in the United States. These temperatures are referred to as "average annual minimum temperatures" and are based on the lowest temperatures recorded for each of the years 1974 to 1986. The different zones represent areas of winter hardiness for agricultural and natural landscape plants.
For each plant we offer, we include a range of hardiness zones, for instance, Zones 3-8. This means that as far as temperature goes, a particular species once established and properly sited should be able to grow anywhere on the map within Zones 3 to 8. This is very general and one would need to take into account specific cultural and habitat requirements. So use a zone to determine minimum temperature thresholds and our ranges to determine broad climate tolerances.