Understanding the United States Department of Agriculture Hardiness Zone Map can mean the difference between success and failure in your garden. This USDA map divides North America into 13 separate planting zones; each growing zone is 10°F colder in an average winter than the adjacent zone. If you see a hardiness zone in a plant description from a gardening catalog, chances are it refers to this map.
The latest version of the USDA Hardiness Zone Map was released in 2012. The map was jointly developed by the USDA’s Agricultural Research Service which is the USDA’s chief scientific in-house agency and Oregon State University’s PRISM Climate Group which has expertise in meteorology and climatology, geography, and statistical modeling. Updates to the zones were a result of new and more sophisticated algorithms that included changes in elevation, nearness to large bodies of water, and terrain position. Additional input was provided by horticultural experts.
So why do we need this map? A gardener or landscaper needs to understand that choosing plants, vegetables, and flowers should correspond to the appropriate growing zone. The information provided by this map helps mitigate plant losses and reduce expenses. There are some things that are not covered by this map. Climate parameters that a farmer, grower, or landscaper may need to take under consideration include humidity, precipitation, soil type, and soil drainage.
There is no print version of the map. The online interactive map enables searching by State or zip code and it can be found here.