Illinois Frost Dates and Growing Season

The occurrence of frost in both spring and fall is of interest to farmers, landscapers, and garderners. Frost in Illinois is usually not measured directly at weather stations. Instead, it is inferred from the air temperature - when the air temperature crosses the threshold of 32°F.

• Date of Last Spring Frost
• Date of First Fall Frost
• Growing Season - the period between the last spring and first fall frost

The actual date varies from year to year. For tender plants, add two weeks to the average date in the spring to protect against the possibility of late season frost. In the fall, subtract two weeks from the average date to protect against an early frost.

Although 32°F is the temperature traditionally used to identify frost, visible frost can be seen on the ground and objects when temperatures are slightly above 32°F on calm, clear nights that allow cold, dense air to collect near the ground. Under these conditions, the temperature near the ground actually can be a few degrees cooler than at the 5-foot height of the official National Weather Service thermometer.

Open, grassy areas are usually the first to experience frost, while areas under trees are more protected because the trees help prevent the heat from escaping. Homeowners can protect tender plants by providing this same type of protection if they cover their plants when a frost is expected. Plants near heated buildings sometimes are spared too. Because of the abundance of warm buildings and trees in towns, they tend to experience frost less often than those living in the country.