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Gardening Articles: Flowers :: Perennials

A Gardener's Guide to Zone Maps

by Joseph F. Williamson with NG

The 1990 USDA zone map

What zone do you live in? This may seem like an odd question, but the answer is one of the most important guides to knowing which plants will thrive in your home or school garden. A variety of maps, including some new ones, can help you find the information you need.

Understanding climate zones is critical for selecting permanent landscape plants. If you want a shrub, perennial, or tree to survive and grow year after year, the plant must tolerate year-round conditions in your area: the lowest and highest temperatures, the amount and distribution of rainfall (or availability of irrigation), and soil conditions.

In eastern North America, the most commonly used zone map is the United States Department of Agriculture (USDA) Hardiness Zone Map, devised in 1960 and revised in 1990. In the West, most gardeners use the zone maps featured in Sunset's Western Garden Book. The National Garden Book (Sunset Publishing Corporation, 1997; $40 hardbound, $35 paper) extends the western maps to include the eastern half of the country. In addition, the American Horticultural Society has published a heat-zone map. On the Internet, the most commonly used zone map is from National Gardening.

This article explains the origins and uses of these climate zone maps, including those four.

Which Zone Map?

Gardening references include many kinds of climate zone maps. For instance, vegetable gardeners might consult maps that show the average dates of first and last frosts or perhaps the amount of sunshine or rain their regions receive. But the most common zone maps are those that show where various permanent landscape plants can adapt.

Perhaps the safest course to ensure plant adaptability is to grow only plants native to your particular region. Native plants have proved they can survive in your climate. However, plants don't stay in their regions of origin any more than gardeners do. Plants native to China, Siberia, and Mexico thrive alongside each other in many American gardens. Furthermore, a California gardener may want to grow a plant that's native to the Great Plains. In such cases, gardeners need a way to compare their garden climates with the climate where the plant is known to grow well. Zone maps provide critical climate information.

Making Sense of Zone Maps

Zone maps are fairly recent innovations. German geographer Alexander von Humboldt created the first one in the early nineteenth century. Humboldt used lines, called isotherms, to delineate regions of equal temperatures. In 1927, horticulturist Alfred Rehder created a system of eight isothermic zones for most of the United States, with each zone separated by temperature differences of 5oF. He was the first to relate average winter minimum temperatures to the hardiness of specific plants.

In 1931, German geographer-meteorologist Vladimir Koppen published a zone map divided into regions that share similar native vegetation. By studying and classifying vegetation types, Koppen ensured that his map accounted for rainfall, humidity, elevation, soils, and the myriad other factors that affect plant growth. The map shown on the facing page, Ecoregions of the United States, is a Koppen-type map and was created by Robert G. Bailey for the U.S. Forest Service in 1980. On it, zones with names like "tropical" and "temperate" are completely distinct, even though parts of them may have similar minimum winter temperatures. All of the climate zone maps in use today derive from these two prototypes.

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