Illinois climate

Illinois Plant Hardiness Zones

Illinois’s towns run from zone 5a to zone 7a: the average coldest winter night is 4.9°F at Cairo and -17.8°F at Elizabeth.

Hardiness zones in Illinois

In USDA’s colors, neighboring states washed out. Zones follow elevation, coasts and cities down to half a mile. Drawn by WeatherOverTime from the USDA-ARS and Oregon State University PRISM Climate Group 2023 GIS data; this is not the official USDA Plant Hardiness Zone Map.

Map of the 2023 plant hardiness zones across Illinois, from zone 5a to zone 7a, with the state’s border drawn.
  • 5a
  • 5b
  • 6a
  • 6b
  • 7a

Open the live map with plant hardiness zones

Towns by zone

Each town’s 1991-2020 average annual extreme minimum at its center, read from the map’s 800 m grid. Warmest zone first.

7a Zone 7a, 0 to 5°F 104 towns

6b Zone 6b, -5 to 0°F 127 towns

6a Zone 6a, -10 to -5°F 425 towns

5b Zone 5b, -15 to -10°F 315 towns

5a Zone 5a, -20 to -15°F 22 towns

Counties

At the county’s center; a county with hills or a coast spans several zones.

CountyZone at the centerAverage coldest night
Adams County6a-6.6°F
Alexander County7a3.6°F
Bond County6b-2.2°F
Boone County5b-13.2°F
Brown County6a-7°F
Bureau County5b-11.3°F
Calhoun County6b-4.2°F
Carroll County5a-16.8°F
Cass County6a-7.2°F
Champaign County6a-8.3°F
Christian County6a-5.7°F
Clark County6b-4.9°F
Clay County6b-2.1°F
Clinton County6b-2.2°F
Coles County6b-4.9°F
Cook County6b-4.8°F
Crawford County6b-3.9°F
Cumberland County6b-4.4°F
DeKalb County5b-13.1°F
DeWitt County6a-8.8°F
Douglas County6a-6.8°F
DuPage County5b-10.2°F
Edgar County6a-7.1°F
Edwards County6b-0.5°F
Effingham County6b-3.6°F
Fayette County6b-3.1°F
Ford County6a-9.5°F
Franklin County7a0.2°F
Fulton County6a-9.7°F
Gallatin County7a1.8°F
Greene County6a-5.3°F
Grundy County5b-10.5°F
Hamilton County6b-1°F
Hancock County6a-8.3°F
Hardin County7a2.1°F
Henderson County6a-10°F
Henry County5b-11.9°F
Iroquois County5b-10.4°F
Jackson County7a0.4°F
Jasper County6b-3.5°F
Jefferson County6b-2°F
Jersey County6b-3.5°F
Jo Daviess County5a-17.4°F
Johnson County7a1.7°F
Kane County5b-12.6°F
Kankakee County5b-10.1°F
Kendall County5b-10.8°F
Knox County5b-11.1°F
Lake County6a-8.6°F
LaSalle County6a-9.7°F
Lawrence County6b-2.4°F
Lee County5b-13.7°F
Livingston County6a-9.8°F
Logan County6a-7.8°F
Macon County6a-6°F
Macoupin County6b-4.4°F
Madison County7a0.5°F
Marion County6b-1.7°F
Marshall County5b-10.7°F
Mason County6a-8.8°F
Massac County7a3.2°F
McDonough County6a-9.6°F
McHenry County5b-12.4°F
McLean County6a-9.2°F
Menard County6a-7.5°F
Mercer County5b-11.9°F
Monroe County7a0.8°F
Montgomery County6b-4.4°F
Morgan County6a-6.3°F
Moultrie County6a-6°F
Ogle County5b-14.4°F
Peoria County5b-10.6°F
Perry County7a0.5°F
Piatt County6a-7.6°F
Pike County6a-6.7°F
Pope County7a2.1°F
Pulaski County7a3.8°F
Putnam County5b-11°F
Randolph County7a0.6°F
Richland County6b-2.4°F
Rock Island County5b-11.8°F
Saint Clair County7a0.9°F
Saline County7a1.2°F
Sangamon County6a-5.8°F
Schuyler County6a-7.6°F
Scott County6a-6.5°F
Shelby County6b-4.8°F
Stark County5b-11.9°F
Stephenson County5a-15.3°F
Tazewell County6a-9.2°F
Union County7a1.4°F
Vermilion County6a-8.4°F
Wabash County6b-1.1°F
Warren County5b-11.2°F
Washington County6b-0.9°F
Wayne County6b-1.1°F
White County6b-0.3°F
Whiteside County5b-14.3°F
Will County6a-9.1°F
Williamson County7a0.8°F
Winnebago County5b-14.2°F
Woodford County5b-10.1°F

The zones

A plant hardiness zone is set by the average of each winter’s single coldest temperature, 1991 to 2020: zones are 10°F apart and each half zone, a and b, 5°F. It says how cold a winter night usually gets, not the coldest it has ever been, and nothing about summer heat, rain or soil. Plant labels give the coldest zone a perennial survives in.

ZoneAverage coldest nightIn Celsius
5a-20 to -15°F-28.9 to -26.1°C
5b-15 to -10°F-26.1 to -23.3°C
6a-10 to -5°F-23.3 to -20.6°C
6b-5 to 0°F-20.6 to -17.8°C
7a0 to 5°F-17.8 to -15.0°C

Sources

Hardiness zones: USDA Plant Hardiness Zone Map, 2023, Agricultural Research Service, U.S. Department of Agriculture, from the GIS data of the PRISM Climate Group, Oregon State University, which may be freely reproduced and redistributed. Drawn by WeatherOverTime from the USDA-ARS and Oregon State University PRISM Climate Group 2023 GIS data; this is not the official USDA Plant Hardiness Zone Map.