The U.S. Department of Agriculture has developed a nationwide system of hardiness zones, indicating the average coldest winter temperatures. This map designates 11 major zones, with the lowest numbers denoting the lowest average temperatures. The numbering system has been widely adopted by the horticultural industry, and zone information for individual perennial plants is included in most garden catalog descriptions (as in “this variety hardy in zones 6–8” or “to zone 8”).
We container gardeners have much more control over the vagaries of winter than do our in-the-ground-garden counterparts because containers can be more easily protected against or moved away from severe weather conditions.