Hardiness zones are a system of mapping developed by the United States Department of Agriculture (USDA) to help gardeners know what kinds of perennial plants will thrive in their region. The map of the territorial United States is divided into 11 areas, each with a number. The zones correspond to the lowest temperatures regularly recorded in that area, with the zone number ascending with the temperatures: Zone 1 has the coldest winters and zone 11 has the warmest. There's a ten degree difference between zones. For example, zone 5 has average winter temperatures between -10 and -20 degrees Fahrenheit (-23 to -29 degrees Celsius), while in zone 6, the coldest temperatures are usually between 0 and -10 degrees Fahrenheit (-18 to -23 degrees Celsius).
If you look at a colored map of the zones, it looks rather like an upside-down rainbow with colder areas inland and North, while the warmer areas are southern and coastal.
Annual plants die off in the winter, so the lowest temperatures aren't particularly significant for them, but when you're thinking about what perennials are appropriate for your garden, it's important to know if they'll survive the cold. This is usually more important than what heat your plants can tolerate, which is why the zones are based on coldest temperatures. Many plants you can buy will have a tag telling you how to look after them and what zone they'll grow best in. Gardening books and Web sites can give you similar information.
Bear in mind that the USDA hardiness zones are a just a guide; they don't take into account the parameters of your exact location. This is especially true in the more mountainous parts of the western U.S. where the zone chart is less accurate. Also consider that the hardiness zones relate to one feature of the environment, and you need to factor in other elements as well, such as wind protection, hours of sunlight, soil types and drainage. Only when you consider all the factors together, can you make the best choices for your garden.