The USDA (United States Department of Agriculture) has divided the United States into hardiness zones (also referred to by some as growing zones). These numbered zones include regions with similar minimum average annual temperatures. The lower the zone number is, the lower the winter temperatures in that zone. Each zone is on average 10°F warmer or colder than the adjacent zone. Zones can be further divided into sub-zones. For example, zone 6a and 6b are adjacent zones with a 5°F differential in average annual extreme minimum temperature.