Hardiness ratings are traditionally based upon the ability of plants to survive winter temperatures. However, many plants that do well in cold climates do poorly in warm ones. In each plant description we have given the range of zones in which that plant can be expected to grow well.
We’ve arrived at these zone designations by our own experience, communications with other gardeners, and reference books. They are not foolproof. Cold hardiness depends on many factors besides air temperatures, such as wind exposure, soil moisture, and snow pack. Survival in warm climates is often adversely affected by high humidity and excessive summer rainfall.
USDA Zone maps are useful, but climates can vary greatly over a small geographic area. Consult your state agricultural extension agent (if there isn’t one in your county, call your state university) or local nurseries to help in finding your zone.