One of the most important pieces of information every gardener should know is the U.S. Department of Agriculture plant hardiness zone for their garden. Almost every plant or seed packet you purchase carries a tag that includes USDA zone ratings as part of the plant's description. By knowing your USDA zone and what it means for you as a gardener, you can select plants suited to thrive in your garden.
Reading the USDA Zone Map
The USDA Hardiness Map divides the United States into 13 numerically-designated zones, based on the average annual extreme minimum temperature. Each color-coded zone includes a 10-degree-Fahrenheit range, with 5-degree subdivisions designated by lower-case letters "a" and "b." Zone 1a is the coldest zone subdivision, where winter extremes average 55 degrees below zero. Zone 13b is the warmest sub-zone, with average winter lows of 65 to 70 degrees above zero. Find your location on the map -- available on the internet, at cooperative extension offices and most garden supply stores -- and you have your USDA plant hardiness zone.
Understanding What Zones Mean
USDA zones don't reflect the lowest possible temperatures in any area. Instead, they average information over a thirty-year period to give you a good guide of extremes your garden many face. By knowing the upper and lower limits of a plant's hardiness zones ratings, you can determine if a plant should survive winter temperatures in your area. Zone information also tells you if your garden is cold enough for the plant to bloom or bear fruit well. Lilacs (Syringa spp.), for example, are generally hardy from USDA zones 3 through 8, depending on the variety. They withstand USDA-zone-3 lows, but don't bloom well in areas warmer than USDA zone 8.
Putting Zone Information to Work
USDA hardiness zones are intended as guides for gardeners, but they aren't written in stone. Plants rated beyond your standard limits, high or low, can often survive and flourish when placed in protected spots or given extra winter protection. A sunny, spot next to a south-facing facing wall can create a microclimate that's consistently warmer than the rest of your garden. Low-lying areas where frost settles can result in colder mini-zones, especially in shaded areas where moisture accumulates. By pairing the individual microclimates of your garden with USDA plant hardiness zone information, you can focus on plants that should flourish at your home.
Other Growing Zone Guides
While the USDA hardiness zone map addresses cold hardiness and chilling needs, many southern gardeners deal with heat challenges. The American Horticultural Society publishes the AHS heat zone map and plant ratings to help southern gardeners key in on heat tolerance. For gardeners in the western United States, Sunset climate zones take factors such as rainfall, humidity and length of growing season into consideration, along with high and low temperatures. In western regions -- where sharp climatic differences are common within short distances due to slope, rainfall and the impact of coastal humidity -- Sunset zone ratings help gardeners chose plants suited for the rigors of their region.