Using the 1990 USDA Hardiness Zone classifications, this updated interactive version covers the state of Illinois and was compiled using National Weather Service climate data through 2009 and covering a broader time frame than the 30 year range used in the current USDA map.
Plantmaps Zone 4a-30°F to -25°F
Plantmaps Zone 4b-25°F to -20°F
Plantmaps Zone 5a-20°F to -15°F
Hardiness Zones, Gardening Zones, Growing Zones and Plant Zones refer to defined geographic regions that can support specific plants, flowers and trees. The zones define a minimum range of temperatures that a plant or tree can survive safely in that zone. The most commonly used Hardiness Zones were defined by the USDA and contain data from the period 1960-1990. This updated version of the map was created by Plantmaps and includes climatology data through December 2009. This map also uses a wider range of climate data than the current USDA map, which limits the zones to 30 years of data.