There are very few rules in gardening, but perhaps the most important one is to work within your zone. Choosing plants that grow and thrive in your particular climate is the very first step toward having a beautiful garden.
Gardeners need a way to compare their garden climates with the climate where a plant is known to grow well. That's why climate zone maps were created. Zone maps are tools that show where various permanent landscape plants can adapt. 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, such as the lowest and highest temperatures and the amount and distribution of rainfall.
First published in 1960 and updated in 1990, this map is based on average annual minimum temperatures recorded throughout North America. By using the map to find the zone in which you live, you can determine which will survive in your garden because they can withstand these average minimum temperatures.
The USDA Hardiness Zone Map is one of several maps developed to provide this critical climate information. It is the one most gardeners in the Eastern United States rely on, and the one that most national garden magazines, catalogs, books, and many nurseries cite. This map divides North America into 11 separate zones. Each zone is 10 degrees F warmer (or colder) in an average winter than the adjacent zone.
The USDA map does a fine job of delineating the garden climates of the Eastern half of North America. That area is comparatively flat, so mapping is mostly a matter of drawing lines approximately parallel to the Gulf Coast every 120 miles of 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 freeze-thaw 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.
Many factors beside winter lows, such as elevation and precipitation, determine growing climates in the West. Weather comes in from the Pacific Ocean and gradually becomes less marine (humid) and more continental (drier) as it moves over and around mountain range after mountain range. While cities in similar zones in the East can have similar climates and grow similar plants, in the West it varies greatly. For example, the weather and plants in low elevation, coastal Seattle are much different than in high elevation, inland Tucson, Arizona, even though they're in the same zone USDA Zone 8.
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To garden successfully in the West, it is essential to understand the attributes of the climate zone in which you live. Sunset magazine classifies 33 western zones, numbered from harshest (Zone 1) to mildest (Zone 33), and organized by region from north to south. The boundaries of each of these unique zones are a function of six geographic and climactic factors: latitude, elevation, ocean influence, continental air influence, mountains and hills, and local terrain. Taken together, these factors determine what will grow well in your garden and what won't.
Learn more at Sunset »
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