Frost in spring is a concern to farmers, landscapers, and gardeners. 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.
The average date of the last spring occurrence of 32°F ranges from April 7 in far southwestern Illinois to April 28 in northern Illinois (see map below). The actual date can vary from year to year. The spring dates are getting earlier by about 5-10 days over the last few decades.
The actual date varies from year to year. For tender plants, add two weeks to the average date in the spring to protect against late season frost.
Although 32°F is the temperature traditionally used to show 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.