|
What are Hardiness Zones?
Introduction
This map supersedes U.S. Department of Agriculture
Miscellaneous Publication 814, "Plant Hardiness Zone Map," which
was revised in 1965. This 1990 version shows in detail the lowest
temperatures that can be expected each year in the United States, Canada,
and Mexico. These temperatures are referred to as "average
annual minimum temperatures" and are based on the lowest temperatures
recorded for each of the years 1974 to 1986 in the United States and
Canada and 1971 to 1984 in Mexico. The map shows 10 different zones,
each of which represents an area of winter hardiness for the plants of
agriculture and our natural landscape. It also introduces zone 11 to
represent areas that have average annual minimum temperatures above 40 F
(4.4 C) and that are therefore essentially frost free.
How
to Use the New Map
Zones 2-10 in the map have been subdivided into
light- and dark-colored sections (a and b) that represent 5 F (2.8 C)
differences within the 10 F (5.6 C) zone. The light color of each
zone represents the colder section; the dark color, the warmer section.
Zone 11 represents any area where the average annual minimum temperature
is above 40 F (4.4 C). The map shows 20 latitude and longitude lines.
Areas above an arbitrary elevation are traditionally considered unsuitable
for plant cropping and do not bear appropriate zone designations.
There are also island zones that, because of elevation differences, are
warmer or cooler than the surrounding areas and are given a different zone
designation. Note that many large urban areas carry a warmer zone
designation than the surrounding countryside. The map-contains as
much detail as possible, considering the vast amount of data on which it
is based and its size.
In
using the map to select a suitable environment for a landscape plant,
today's gardeners should keep in mind the following:
Stress
Factors. We
became aware of additional stresses to plants during the 1970's.
Acid rain, gaseous and particulate pollution, security lighting, and
toxic wastes, among many other stress factors, have significantly
increased the potential for unsatisfactory performance of landscape
plants. We need to document the tolerances of plants to these
factors.
New
Plant Management Systems .
New techniques of planting, transplanting, watering, fertilizing, and
providing pest control measures have done much to increase the vigor of
landscape plants. But used unwisely, these same measures can
reduce plant hardiness.
Artificial
Environments. We
have pushed the use of plants into totally artificial environments such
as expressways, malls, elevated decks, and buildings where plant roots
are totally removed from the ground and its warming influence. The
assortment of plants that can adapt to such environments is proving to
be very restricted. Hardiness ratings alone are inadequate to
guide landscapers in selecting the most successful plants.
USDA
Hardiness Zones and Average Annual Minimum Temperature Range
| USDA
Zone |
Temperature
Range |
Example
Cities |
|
1 |
Below
-50 F (below
-45.6C) |
Fairbanks,
Alaska
Resolute,
Northwest Territories (Canada) |
| 2a |
-50
to -45 F (-42.8
to -45.5 C) |
Prudhoe
Bay, Alaska
Flin
Flon, Manitoba (Canada) |
| 2b |
-45
to -40 F (-40.0
to -42.7 C) |
Unalakleet,
Alaska
Pinecreek,
Minnesota |
| 3a |
-40
to -35 F (-37.3
to -39.9 C) |
International
Falls, Minnesota
St.
Michael, Alaska |
| 3b |
-35
to -30 F (-34.5
to -37.2 C) |
Tomahawk,
Wisconsin
Sidney,
Montana |
|
4a |
-30
to -25 F (-31.7
to -34.4 C) |
Minneapolis/St.Paul,
Minnesota
Lewistown,
Montana |
| 4b |
-25
to -20 F (-28.9
to -31.6 C) |
Northwood,
Iowa
Nebraska |
| 5a |
-20
to -15 F (-26.2
to -28.8 C) |
Des
Moines, Iowa
Illinois |
| 5b |
-15
to -10 F (-23.4
to -26.1 C) |
Columbia,
Missouri
Mansfield,
Pennsylvania |
| 6a |
-10
to -5 F (-20.6
to -23.3 C) |
St.
Louis, Missouri
Lebanon,
Pennsylvania |
| 6b |
-5
to 0 F (-17.8
to -20.5 C) |
McMinnville,
Tennessee
Branson,
Missouri |
| 7a |
0
to 5 F (-15.0
to -17.7 C) |
Oklahoma
City, Oklahoma
South
Boston, Virginia |
| 7b |
5
to 10 F (-12.3
to -14.9 C) |
Little
Rock, Arkansas
Griffin,
Georgia |
| 8a |
10
to 15 F (-9.5
to -12.2 C) |
Tifton,
Georgia
Dallas,
Texas |
|
8b |
15
to 20 F (-6.7
to -9.4 C) |
Austin,
Texas
Gainesville,
Florida |
| 9a |
20
to 25 F (-3.9
to -6.6 C) |
Houston,
Texas
St.
Augustine, Florida |
| 9b |
25
to 30 F (-1.2
to -3.8 C) |
Brownsville,
Texas
Fort
Pierce, Florida |
|
10a |
30
to 35 F (1.6
to -1.1 C) |
Naples,
Florida
Barstow,
California |
| 10b |
35
to 40 F (4.4
to 1.7 C) |
Miami,
Florida
Coral
Gables, Florida |
|
11 |
above
40 F (above
4.5 C) |
Honolulu,
Hawaii
Mazatlan,
Mexico |
What's
My Zone Click here and type in your postal code.
|
