What Are Hardiness Zones?It seems every gardening book and nursery catalog refers to plant hardiness zones, also known as climate zones or growing zones. If you're new to gardening, you may be wondering what all the fuss is with these zones, and how to find out which zone you are gardening in. Basically, plant hardiness zones are a guide to help you know which plants will grow where you live, so you don't plant things that will soon die just because they can't manage your region's temperatures. Plants vary in the temperature extremes they can endure. Basic laboratory testing can determine the lowest sustained temperature a particular plant type can withstand, but, as gardeners, we still need to know how these measurements relate to our own gardens.
How Many Zones Are There?The USDA plant hardiness map divides North America into 11 hardiness zones. Zone 1 is the coldest; zone 11 is the warmest, a tropical area found only in Hawaii, coastal Southern California and southernmost Florida. In between, the zones follow a fairly predictable pattern across the continent, though a closer look will reveal scattered patterns of variations. Generally, the colder zones are found at higher latitudes and higher elevations.
Why Were Those Zones Developed?In an attempt to answer this question, years ago botanists and horticulturists started gathering weather records throughout North America to compile a database to show the average coldest temperatures for each region. These records were condensed into a range of temperatures and transformed into various zones of plant hardiness. Maps were then made to show the lines between these temperature zones. The climatic studies and maps were undertaken by two independent groups: The Arnold Arboretum of Harvard University in Cambridge, Massachusetts, and the United States Department of Agriculture (USDA) in Washington, D.C. The two maps reflected some variances, but in recent years, the differences between the Arnold Arboretum and the USDA have narrowed. Today, the USDA map, which was last updated and released in 1990 (based on weather records from 1974-1986), is generally considered the standard measure of plant hardiness throughout much of the United States. Hence we have the USDA Plant Hardiness Zones. A similar map for Canada has been issued by the Canadian government's agriculture department.
How Useful Are The Plant Hardiness Zones?Well, just think about this: The average minimum temperature is not the only factor in figuring out whether a plant will survive in your garden . Soil types, rainfall, daytime temperatures, day length, wind, humidity and heat also play their roles. For example, although both Austin, Texas and Portland, Oregon are in the same zone (8), the local climates are dramatically different. Even within a city, a street, or a spot protected by a warm wall in your own garden, there may be microclimates that affect how plants grow. The zones are a good starting point, but you still need to determine for yourself what will and won't work in your garden.
Applying Zone ReferencesPlant encyclopedias may refer simply, for example, to "Zone 6," which generally means that the plant is hardy to that zone (and will endure winters there), and generally can withstand all the warmer zones below. More detailed information may indicate a range of zones (i.e., "Zones 4-9"), which means the plant will only grow in those zones, and will not tolerate the colder and warmer extremes outside them. But remember, zones are only a guide. You may find microclimates that allow you to grow more than the books say you can; by the same token, you may find to your dismay that some precious plant -- one that's "supposed" to be hardy in your zone -- finds its way to plant heaven instead.
The USDA Plant Hardiness Zone MapAs mentioned previously the zones maps vary according to which organization developed it. You may look at three different maps, and it suggest three different zones. This particular map does not show it, but each zone is also broken up into 'a' and 'b' sections. For example, there is a 2a and a 2b; the 'a' is usually the most northern section of the zone. The zone map can be very confusing, but is a very good guide towards figuring out what can and can not grow in you locality. I have included a zone look-up here in this post so that you may look up your zone if you do not currently know. The results are very interesting. I live in Hampton, Virginia, which is zone 7b, but the zone look-up tool says that I am zone 8. Like I said, the zone you live in depends on who you ask; that is why it is strongly suggested as just a guide. The zone look-up tool will redirect you to arborday.org. If you do not wish to be redirected, please do not use the zone look-up tool.
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