Illinois Frost Dates & Growing Season Guide
Illinois (IL) spans USDA hardiness zones 5a, 5b, 6a, with growing seasons ranging from 165 days in Rockford to 188 days in Springfield. The average growing season across the state is approximately 180 days.
Illinois City Frost Dates
The table below shows the average last spring frost date, first fall frost date, growing season length, and USDA hardiness zone for each city. Click a city name for detailed planting calendars and zone information.
Planting Windows for Illinois
Based on average frost dates, here are the recommended planting windows for each city. The indoor seed start date is approximately seven weeks before the last spring frost. Transplanting should occur about two weeks after the last frost. The last direct sow date for fall crops is ten weeks before the first fall frost.
| City | Start Seeds Indoors | Transplant After | Last Direct Sow |
|---|---|---|---|
| Chicago | March 2 | May 4 | August 13 |
| Springfield | February 24 | April 27 | August 9 |
| Rockford | March 10 | May 12 | August 1 |
| Peoria | February 28 | May 1 | August 6 |
Gardening in Illinois
Illinois benefits from rich prairie soils that are among the most fertile in the world. The state stretches across zones 5a to 6b, with growing seasons ranging from 165 days in Rockford to 188 days in Springfield. Chicago's urban heat island extends its season compared to surrounding suburban areas. Illinois gardeners can grow a full range of temperate vegetables, and the deep, loamy soils require less amendment than most states. Hot, humid summers make good airflow and pest monitoring essential. Corn, tomatoes, peppers, and green beans perform excellently. Plant cool-season crops by early April and set out warm-season transplants after mid-May in central Illinois.
Understanding Illinois's Hardiness Zones
Illinois includes USDA hardiness zones 5a, 5b, 6a. These zones indicate the average annual extreme minimum winter temperature for each area, which determines which perennial plants, trees, and shrubs can survive winter without protection. When purchasing trees, shrubs, or perennial flowers, always check that the plant is rated for your hardiness zone or a lower (colder) zone number.
Tips for Using Illinois Frost Dates
These frost dates represent long-term averages and should be treated as guidelines rather than guarantees. In any given year, the actual last spring frost or first fall frost may arrive one to three weeks earlier or later than the average. Factors that affect your specific location include elevation above the city center, proximity to water, slope direction, and whether you are in an urban or rural area. South-facing slopes and areas near large pavement or building masses tend to be warmer than surrounding open land. Low-lying valleys and exposed hilltops are often colder than mid-slope positions.
To protect against late spring frosts, keep row cover fabric, old bed sheets, or frost blankets on hand. Monitor your local weather forecast daily as the average frost date approaches. When frost is predicted after you have transplanted, covering plants in the late afternoon traps ground heat and can protect against temperatures down to about 28 degrees Fahrenheit. For fall season extension, the same covers protect mature plants from early frosts, often buying several additional weeks of harvest.