Gardeners can use the United States Department of Agriculture Plant Hardiness Zone Map to determine which plants are most likely to thrive in their regions of the country. They can search the map, which is available at PlantHardiness.ARS.USDA.gov, by ZIP code, town or city or by clicking on it to focus in on a specific region.
The Plant Hardiness Zone Map divides the country into zones based on the average annual extreme low temperature in an area. A plant described as "hardy to zone 6b" can tolerate a minimum temperature of -5 degrees Fahrenheit, for example. Once gardeners know their zones, nurseries can tell them which plants are recommended for those zones. Gardeners can also find information about which plants thrive in their zones from the American Horticultural Society at AHS.org.
One drawback of the Plant Hardiness Zone Map is that it does not take into account the average extreme high temperatures for a region, which also affects how well plants grow in certain areas. The American Horticultural Society offers a Plant Heat Zone Map that breaks the country into zones based on extreme high temperatures. Gardeners can search this map, which is available at AHS.org, the same way they would the USDA Plant Hardiness Zone Map.