Gardening Articles: Flowers :: Perennials

A Gardener's Guide to Zone Maps (page 2 of 4)

by Joseph F. Williamson with NG

The Arnold Arboretum Map

In 1938, horticulturist Donald Wyman expanded Rehder's isotherm map to create the Arnold Arboretum (AA) Map. Its temperature bands differ by 5°F, 10°F, and 15° F, depending on latitude. For example, average winter minimums on the AA map's zone 2 range from -35° F to -50° F, a spread of 15° F. On the same map, zone 7 winter lows are typically between 5° to 10° F, a spread of only 5° F.

Gardeners often confuse the AA map with the USDA map (see below), which it closely resembles. For instance, zone 6 on the AA map has average minimum temperatures between 5° and -5° F. USDA zone 6 is 0° F to -10° F: a difference close enough to be missed in casual examination. More than one gardener has looked up a plant in Michael Dirr's Manual of Woody Landscape Plants (Stipes Publishing, 1990; $40) and assumed that its AA zones are USDA zones.

The 1990 USDA Zone Map

This map is the most recent and most widespread incarnation of Humboldt's isotherm map. Developed by H. Marc Cathey, it divides North America into 20 separate zones. They are numbered 1 through 11, as in an earlier version, but now zones 2 through 10 are subdivided into "a" and "b" regions. Each zone is 5° F warmer (or colder) in an average winter than the adjacent zone. The USDA map is the one most gardeners in the eastern United States rely on, and the one that National Gardening and most national garden magazines, catalogs, and books currently use. Similar plant hardiness zone maps for Europe and China have also been developed.

The greatest virtues of the USDA map are its widespread use and the fact that many plants have been categorized according to its zones.

The USDA map does a fine job of delineating the garden climates of the eastern half of the country. That area is comparatively flat, so mapping by isotherms is mostly a matter of drawing lines approximately parallel to the Gulf Coast every 120 miles or so as you move north. The lines tilt northeast as they approach the Eastern Seaboard. They also demarcate the special climates formed by the Great Lakes and by the Appalachian mountain ranges.

But this map has shortcomings. In the eastern half of the country, the USDA map doesn't account for the beneficial effect of a snow cover over perennial plants, the regularity or absence of freezeothaw cycles, or soil drainage during cold periods. And in the rest of the country (west of the 100th meridian, which runs roughly through the middle of North and South Dakota and down through Texas west of Laredo), the USDA map fails completely.

Many factors besides winter lows determine western growing climates. Weather comes in from the Pacific Ocean and gradually becomes less marine and more continental as it moves over and around mountain range after mountain range. The criteria that define the different western zones are winter cold, summer heat, amount and duration of precipitation, humidity, seasonal winds, and number of sunlight hours.

In the East, nature provides 16 to 44 inches of rain during the growing season, enough water to grow many kinds of plants and crops without irrigation. And humid air often enhances the moisture during this period. If no rain falls for a month, it's called a drought, and many unfortunate farmers lose their shirts.

In the same season, the West gets from a fraction of an inch to maybe 10 to 11 inches of rain. That's not enough to grow a lawn, a flower garden, or a vegetable garden without regular irrigation, but it is enough to grow western native plants. That's why, in the late twentieth century, many western gardeners have turned to native plants to provide foliage and flowers in the dry season.

Additionally, eastern climate definitions aren't useful for the low-elevation West because winters are mild there. Vancouver, Seattle, Portland, San Francisco, Fresno, Los Angeles, San Diego, Phoenix, and Tucson all experience average winter lows of 27° to 48° F (with all-time lows mostly in the high teens and low 20s). And between 27° and 32° F, plant damage from freezing is comparatively unimportant. In most places, freeze damage is not significant enough to qualify as the single defining climatic factor -- but it's the primary criterion for USDA zones.

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