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.
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is a journey: "We hope you enjoy yours"! |
The 14
regions of the National Gardening Zone Map descend directly from the Koppen-type
map that organizes regions of similiar native vegetation. It's usefulness
comes into play when the zones are correlated to the USDA map. When a plant
or activity is recommended for a particular region and zone within that region,
the gardener has significantly more assurance of success compared to advice
for the USDA Zone alone.
