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Knowing Your USDA Zone

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When you flip through any garden book or catalog, you are almost bound to see a little notation in every plant description about its “zones.” These can be mysterious for even an experienced gardener, since the zones given for any one plant may vary from book to book and catalog to catalog.

So what exactly do they mean, and how do we figure out what zones to rate a plant for?

A climate zone should describe the coldest to warmest temperature that a plant can tolerate. We are accustomed to thinking of a climate as being too cold for a certain plant, but rarely do we think about the fact that some plants also need a certain amount of cold temperature exposure in order to perform well. For instance, without going through a couple months of cold weather, many daffodils and tulips won’t bloom. The same holds true for peonies, and other plants enjoyed by northern gardeners. These plants need to spend some time sleeping - going dormant - and they don’t do that until cold temperatures signal to them that it is time.

By the same token, some plants are heat sensitive. In some areas of the country, the sun is so hot that it can scald a plant, leaving it looking less than its best. So that plant either needs a lot more shade and water than it may need in the north, or would be best left to cold climate gardeners. How heat sensitive or sun-sensitive a plant is will determine the highest zone that is good for that plant. How cold sensitive a plant is will determine the lowest zone that a plant will thrive in. Any colder and the root system will freeze - and the plant will die.

The U.S. Department of Agriculture has divided the country up into different zones, ranging from 1-10. One is the coldest area, and 10 the hottest. They decided which areas of the country belonged in a specific zone by looking at only one thing - the average coldest temperature in that area.

However, you may have noticed that temperatures seem to be rising, that winters haven’t been as cold as they used to be and summers seem hotter. Is it us? Or is it true? The Arbor Day Society believes that there has been a distinct warming trend, and recently issued its own climate zone map based on this. Using their map you may find that you are in a higher zone than the USDA map would indicate. This map was based on the most recently compiled average annual minimum temperatures recorded by U.S. weather stations.

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