Plant hardiness zones are determined according to the expected lowest annual temperatures for a given region. The USDA divides the country into 11 hardiness zones. A wide range of growing conditions besides temperature also affects which plants will grow within a hardiness zone, such as the amount of rainfall and availability of water, altitude, soil types and lengths of growing seasons. Specific growing conditions control which species will grow in a particular area, or microclimate, within each zone.
USDA Zone 11
The warmest planting zone in the U.S. has tropical temperatures that remain above 40 degrees Fahrenheit all year. The southernmost tip of Florida, most of Hawaii, and southern coastal areas of California are in zone 11.
Zones 9 and 10
Zones 9 and 10 are the next warmest planting zones. They include higher altitudes of Hawaii, southern Florida and southern coastal areas along the Gulf of Mexico. Most of California and coastal areas of Oregon and Washington fall into zone 9. The lowest temperatures in zone 9 average 20 to 30 F, and in zone 10 the average lowest temperature is 30 to 40 F.
Zones 7 and 8
More frigid temperatures are found in zones 7 and 8. Those areas have average lowest temperatures from 0 to 10 F and 10 to 20 F, respectively. The zones make a curve across the Mid-Atlantic states on the east coast, and across the southern states as far north as the Ohio River valley region. Zones 7 and 8 then sweep across the southern plains states and up through the areas west of the Rocky Mountains.
Zones 5 and 6
Zones 5 and 6 fill in the eastern and midwestern states up to the Great Lakes, and create a belt across the plains states and around the western Rocky Mountains. The average lowest temperatures in zones 5 and 6 are from -20 to -10 F and from -10 to 0 F. Protected valley areas and altitude create diversity in the mountainous regions. The open plains areas are plagued with harsh winter winds, and the eastern areas in these zones can have lake-effect snowfall from the humid air that blows across the Great Lakes.
Zones 3 and 4
Zone 4 has average lowest temperatures of -30 to -20 F, and it includes northern New England states along the St. Lawrence River and most of Maine. Zone 4 also covers most of Montana, Wyoming, North and South Dakota, and parts of Minnesota and Wisconsin. Zone 3 is even more frigid with lowest temperature averages of -40 to -30 F. Zone 3 fills in some areas along the U.S.-Canada border.
Zones 1 and 2
Zones 1 and 2 are in central and northern Alaska. Temperatures in zone 2 reach -50 to -40 F, and zone 1 is below -50 F. Alaskan zones are banded from these coldest areas by zones 3 and 4 on the coastal areas, with southern coasts and the Aleutian Islands in the zones 5, 6 and 7 range. A few small areas in the southernmost coastal area of Alaska reach a zone 8 rating.
Other Zone Maps
The USDA hardiness zone map may be the most commonly used, but it is not the only map of planting zones in the U.S. Resource maps at the Arbor Day Foundation present interesting temperature change information from 1990 to 2006. The American Horticultural Society resource map indicates heat zones in the U.S.
Sunset resource maps were created with great detail to incorporate winter low and summer high temperatures, length of growing season, rainfall amounts and timing of rainfall, and humidity. The Sunset resource maps establish 45 different US zones based on precise growing conditions.