What are Zone Maps?
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.
The 1990 USDA Hardiness Zone Map
The USDA Hardiness Zone Map is one of several maps developed to provide this
critical climate information. The USDA map 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 currently use. This map
divides North America into 11 separate zones. Each zone is 10?F warmer (or
colder) in an average winter than the adjacent zone. (In some versions of
the map, each zone is further divided into "a" and "b" regions.)
Great for the East
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 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.
Zone Map Drawbacks
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.
Problems in the West
Many factors beside winter lows, such as elevation and precipitation,
determine western 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. |
