Successful regional gardening hinges on knowing what plants are suited for your zip code's climate. What flourishes in Boston may wither in Fresno, Calif., or Cheyenne, Wyo., since the cities don't share identical growing conditions. Or the same plant could require different levels of care in different locations. To help us know what plants will thrive in our backyards, the U.S. Department of Agriculture (USDA) devised a cheat sheet, called the Hardiness Zone Map. Most plants you buy come with a reference tag that details optimal sunlight, season and watering schedule. Plants may also have a number listed beside a color-coded map of the United States. That number and map refer to hardiness zones.
The USDA's map divides the United States into 11 hardiness zones. Hardiness zones, numbered one through 11, denote the lowest temperature ranges typical for that region and are ranked from coldest to warmest. Neighboring zones are 10 degrees higher or lower than each other. All zones, except for 1 and 11, also are split into "a" and "b" subregions, which are separated by 5 degrees. Why do hardiness zones only measure cold extremes? Plants are more sensitive to cold than heat.
For example, peonies grow in hardiness zones 3 through 8, which means the flowers can withstand cold temperatures from 20 to minus 40 degrees Fahrenheit (minus 6.6 to minus 40 degrees Celsius). You can feasibly plant them if you live in Austin, Texas, since your plant hardiness zone would be 8b. In normal weather, the mercury should drop no more than to 15 or 20 degrees Fahrenheit (minus 6.7 to minus 9.4 degrees Celsius). But you'd have to leave them behind if you moved to chilly Pinecreek, Minn. There, the hardiness zone is a 2b, and it could reach minus 45 to minus 40 degrees Fahrenheit (minus 40 to minus 42.7 degrees Celsius).
When using the hardiness zones as your gardening guides, remember that it applies only to perennials. Annual plants die at the conclusion of their growing seasons, rendering the zone distinctions irrelevant. The USDA hardiness zone map also isn't flawless. Its accuracy varies depending on your location. If you live in the flatter geography of the Eastern and Plains states, the zones are fairly reliable. But as you travel west, the map skews somewhat because of the mountainous geography. The interaction of the higher elevations and weather moving eastward from the Pacific Ocean outweighs the influence of temperature on plants [source: National Gardening Association].
In addition to cold temperatures, a gardener must consider factors such as water, light, ambient temperature and soil acidity. As with hardiness zones and climate, those elements will shift as you move among gardening regions.