What is the USDA Plant Hardiness Zone Map?
For a perennial, shrub or tree to successfully grow year after year, it must be able to withstand year-round conditions in an area. One major condition that must be taken into account is temperature extremes. Gardeners need a way to compare their garden climates with the climate where a plant is known to grow well. That’s why climate zone maps were created, says Tony Kahtz, with University of Illinois Extension and Land of Lincoln AmeriCorps. Climate zone maps show the minimum and maximum temperature extremes a plant can withstand. The USDA plant hardiness zone map is the most used resource of any map. It divides the United States into 11 zones. On average, each zone is 10 degrees Fahrenheit warmer or colder than the adjacent zone. Excluding zones 1 and 11, all other zones are divided into two sub-zones such as 2a and 2b, 3a and 3b. The letters “a” and “b” further break down temperature variations by increments of 5 degrees. For example, zone 3a’s lowest temperature is 35 to 40 degrees
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