Climate Map
The Department of Agriculture has developed a map to show the hardiness of plants based on your geographical location. These zones are based on average low tempertures. If a plant says hardy in Zone 6 for instance, then it can make it through the winters in 7, 8, 9, and 10.
I refer to my climate map weekly to see what plants are hardy in my zone.
Herbs Annuals Perennials Moongarden Climate Map Bibliography Winter Treatments Queen's Gardens