Summary: the median dates for fall frost in Illinois range from early October in northern Illinois, to mid-October for central Illinois, and late October for southern Illinois.
Frost is the formation of thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans. If a frost period is severe enough to end the growing season or delay its beginning, it is referred to as a “killing frost”.
Frost in both spring and fall can be a concern to farmers, landscapers, and gardeners. However, we usually do not directly measure frost at weather stations in Illinois. Sometimes observers may note the presence of frost in their comments on the forms. To get around the lack of direct observations, we use a temperature threshold of 32° for frost and 28° for a hard freeze.
The actual date varies from year to year. For tender plants in the fall, subtract two weeks from the average date to be on guard against an early frost.
Open areas are usually the first to experience frost, while areas under trees are more protected. Homeowners can protect plants by covering them when a frost is expected. Plants near heated buildings sometimes are spared too. Because of the abundance of trees and warm buildings, we see frost arrive in town a little later than in the countryside.
Here is the median date of the first occurrence of 32 degrees in fall. Additional maps for spring and fall, 32 and 28-degree thresholds, etc. can be found on the frost webpage.