The United States Department of Agriculture (USDA) provides gardeners around North America with numerous references, including a hardiness zone map which gardeners can use to determine what sorts of plants are most likely to thrive in their area. The USDA zones are broken up by temperature: each zone is 10 degrees Fahrenheit (approximately five degrees Celsius) colder than the zone below it on the map, and that much warmer than the zone above it. Many plants are hardiness tested and list the USDA zones that they do well in, so that gardeners can make decisions about what to plant, and when.
The USDA zones incorporate data which has been collected since the 1930s. The first hardiness map was released in 1960, and was continually updated until 1990, when major changes were made, including the addition of an 11th zone to the original 10. Also, USDA zones two through 10 were further divided to reflect a five degree temperature variance, which can make a big difference to some plants. Because of this division, a total list of the USDA zones starts with Zone One, the coldest, with temperatures that can reach -50 degrees Fahrenheit (-45.6 degrees Celsius) and goes through USDA zones 2a, 2b, 3a, 3b, 4a, 4b, 5a, 5b, 6a, 6b, 7a, 7b, 8a, 8b, 9a, 9b, 10a, 10b, and 11, the warmest, with a winter temperature that exceeds 40 degrees Fahrenheit (4.5 degrees Celsius).
Once a gardener determines what zone he or she lives in by consulting the local garden shop or using a zone finder tool on the Internet, plants which will thrive in that zone can be sought out. The zone designation information on a plant will usually indicate which USDA zones the plant can winter over in, as well as USDA zones that the plant can be grown in as long as it is in a greenhouse during the worst of the cold. For gardeners living in cold areas where many plants have difficulty during the winter, the USDA zones take a great deal of guesswork out of gardening.
The USDA zones do have some flaws. Although they take average temperatures into account, they do not look at things like humidity, heavy rain, high wind, day length, and elevation. This means that the information may not always be accurate for your zone, especially if you live at high elevation; local garden shops can usually make more solid recommendations for plants which will do well, as can other gardeners living in your conditions. In addition, numerous gardening forums discuss plant hardiness in different conditions, and may be more accurate yardsticks, especially if you live in the Western States. Western gardeners can also take advantage of the Sunset Western Climate Zones map, since they incorporate more factors.