A hardiness zone map is a map that splits the US or any state or other country into zones based on coldest temperature. These maps can be very useful for gardeners because they give one indication of how well plants will grow within a specific area and whether the temperature would be so cold that it might kill certain plants. By knowing where you live, you can look at a hardiness zone map (there are many online ones) and determine whether a plant you want will grow well or might die in your zone.
There are actually a few versions of the hardiness zone map for different areas. In the US, plant stores may use the US Department of Agriculture (USDA) hardiness zone map, but in some of the Western United States, some stores still go by Sunset Magazine’s climate map, which was developed over forty years ago. If you’re ordering from a seed or gardening catalog and you live in a western state, you may want to see which criteria is being used by the catalog or by your local nursery.
The USDA hardiness zone map is based on studies of climate patterns over more than a decade, but the map was last updated in the early 1990s. It divides the US into eleven climate zones. Zone 1 can mean temperatures reach below – 50 F (-45.56 C). Zone 11 means temperatures do not drop below 40 F (4.44 C). There is considerable variation in climate in the vast amount of land that makes up the US, and each consecutive zone slightly raises lowest temperature.
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There can be very complex forms of hardiness zone maps, and some get into extreme detail, even within a state. For example, California has several zones. The lowest temperature in the Sierra Mountains is going to be quite different than temperature in San Diego.
Plants are then rated by zone to tell those selecting them if they can expect a plant to do well given the lowest temperatures in their area. Sometimes zone doesn't matter much, especially when you're planting annuals like pansies or impatiens. As long as you have relatively warm summer temperatures these plants will do fine. When you're planting perrenials, plants that last from year to year, you want to make sure the plants are hardy enough to survive the cold temperatures in your area and live to bloom another year.
While gardeners say a hardiness zone map can be helpful, it isn’t the only consideration when you’re choosing plants. Different types of soil will help plants grow or prevent them from prospering. Amount of light, wind, rainfall and numerous other factors also need to be considered.
What a hardiness zone map can accomplish is to give you basic ideas about what plants will and won’t do well in your area. As you become more skilled at gardening, you then need to think about the needs of each different type of plant your grow. When you fine tune your gardening skills to things like analysis of soil, water needs, and pest reduction or weeding needs, you’ll be able to brag about most plants you grow and how well they are doing.