USDA Zones

The United States Department of Agriculture (USDA) has divided up the North American continent into "zones", which reflect the historical minimum winter temperatures for different parts of the continent. The maps and much of the information below was obtained from a page on the USDA web site.

Introduction

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 11 different zones, each of which represents an area of winter hardiness for the plants of agriculture and our natural landscape. Note that zone 11 represents areas that have average annual minimum temperatures above 40 F (4.4 C) and that are therefore essentially frost free.

How to Use the 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.

Map of North America (173K)

More Detailed Map of 48 Contiguous U.S. States (166K)

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USDA Hardiness Zones and
Average Annual Minimum Temperature Range

USDA Zone Temperature Range Example Cities
1 Below -50 F
(below -45.6 C)
Fairbanks, Alaska
Resolute, Northwest Territories (Canada)
2a -50 to -45 F
(-45.5 to -42.8 C)
Prudhoe Bay, Alaska
Flin Flon, Manitoba (Canada)
2b -45 to -40 F
(-42.7 to -40.0 C)
Unalakleet, Alaska
Pinecreek, Minnesota
3a -40 to -35 F
(-39.9 to -37.3 C)
International Falls, Minnesota
St. Michael, Alaska
3b -35 to -30 F
(-37.2 to -34.5 C)
Tomahawk, Wisconsin
Sidney, Montana
4a -30 to -25 F
(-34.4 to -31.7 C)
Minneapolis/St.Paul, Minnesota
Lewistown, Montana
4b -25 to -20 F
(-31.6 to -28.9 C)
Northwood, Iowa
[city], Nebraska
5a -20 to -15 F
(-28.8 to -26.2 C)
Des Moines, Iowa
[city], Illinois
5b -15 to -10 F
(-26.1 to -23.4 C)
Columbia, Missouri
Mansfield, Pennsylvania
6a -10 to -5 F
(-23.3 to -20.6 C)
St. Louis, Missouri
Lebanon, Pennsylvania
6b -5 to 0 F
(-20.5 to -17.8 C)
McMinnville, Tennessee
Coatesville, Pennsylvania
7a 0 to 5 F
(-17.8 to -20.5 C)
Oklahoma City, Oklahoma
Collingswood, New Jersey
7b 5 to 10 F
(-17.7 to -15.0 C)
Little Rock, Arkansas
Griffin, Georgia
8a 10 to 15 F
(-12.2 to -9.5 C)
Tifton, Georgia
Dallas, Texas
8b 15 to 20 F
(-9.4 to -6.7 C)
Gainesville, Florida
Austin, Texas
9a 20 to 25 F
(-6.6 to -3.9 C)
St. Augustine, Florida
Houston, Texas
9b 25 to 30 F
(-3.8 to -1.2 C)
Fort Pierce, Florida
Brownsville, Texas
10a 30 to 35 F
(-1.1 to 1.6 C)
Naples, Florida
Barstow, California
10b 35 to 40 F
(1.7 to 4.4 C)
Miami, Florida
Coral Gables, Florida
11 Above 40 F
(Above 4.5 C)
Honolulu, Hawaii
Mazatlan, Mexico

Sunset Zones

The Sunset Western Garden Book has created a different zone system for the Western US, and it is said to be in some ways more useful than the USDA zones. However, references to zones in gardening books and on nursery plant labels always refer to the USDA zone system.

Frost-Free Season

Note that the USDA zone has no relationship to the length of frost-free growing seasons. Check with your local weather service, nursery, or Extension Service to find out the average frost-free growing season in your area, including the usual date of the last frost in spring and the first frost in autumn.