The USDA Hardiness Zone Map

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The map divides North America into 11 individual planting zones; each growing zone is 10°F warmer (or colder) in an average winter than the adjacent zone.  In the description of plants or plants growing in a specific gardening catalog, chances are it’s referring to the USDA Hardness Zone Map.  This guide, growers use to find out what plants are suitable for their local areas pertaining to hard freezes during the coldest months of the year

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What bring about Cold Hardiness?

In early fall, there are shorter days and cooler temperatures, this is when many species of plants acquire cold hardiness. 

 

Extremely cold weather early in the fall may bring serious injury to plants even though the temperatures may not reach the average lowest temperature for your zone.  Likewise, exceptionally warm weather in midwinter followed by a sharp cold snap back to seasonably cold weather may cause injury to plants as well kill them.