Frost dates in Illinois

Where the numbers come from

Sources and method

Frost dates. U.S. Climate Normals 2020: U.S. Annual/Seasonal Climate Normals (1991-2020), NOAA National Centers for Environmental Information (Palecki, Durre, Applequist, Arguez and Lawrimore, 2021). These are 30-year station statistics for 1991 to 2020. "Frost" here means an air temperature of 32°F or lower at the station. We use 6,539 stations that have at least 10 years of record and a published date; 176 more are stations where a freeze is too rare for NOAA to date, and 180 are stations cold enough to have frost risk all year.

Probability level. The headline dates are NOAA's 50% level: in half of years the last spring freeze comes later than that date, and in half of years the first fall freeze comes earlier. The 10% and 90% levels are shown next to them. For tender plants, the 10% spring date is the safer one to plan from: a freeze follows it in only 1 year in 10.

Which station. Your ZIP code is matched to the nearest of those stations by straight-line distance from the ZIP code's centre point (GeoNames, geonames.org, CC BY 4.0; 40,979 ZIP codes in the 50 states and DC). Nearest is not always most alike: a station on a hill, in a valley or by the water can differ from your garden by a week or more. That is why the station's name, distance and elevation are shown with every result, along with two more stations nearby. NOAA makes the same point: the normals are limited between stations as distance grows horizontally and vertically.

Seed-starting offsets. Growing Vegetables: When to Plant Your Vegetable Garden, a fact sheet from University of New Hampshire Extension. It gives each crop's weeks from seed to transplant and when it is safe to set out or sow relative to the last frost. The method is theirs too: take the planting date and count back the seed-to-transplant weeks. Where the fact sheet gives a range of planting dates, the start-indoors dates count back from the earliest one. The figures used, as the fact sheet states them:

• Basil: Seed to transplant 6-8 weeks; set out 2 weeks after last frost.
• Bean: Plant seed in garden after last frost.
• Broccoli: Seed to transplant 4-6 weeks; set out 2-4 weeks before last frost.
• Brussels sprout: Seed to transplant 4-6 weeks; set out 2-4 weeks before last frost.
• Cabbage: Seed to transplant 4-6 weeks; set out 4 weeks before last frost.
• Cauliflower: Seed to transplant 4-6 weeks; set out 2 weeks before last frost.
• Celery: Seed to transplant 10-12 weeks; set out after last frost.
• Cucumber: Seed to transplant 3-4 weeks; set out 1-2 weeks after last frost.
• Eggplant: Seed to transplant 8-10 weeks; set out 2-3 weeks after last frost.
• Kale: Seed to transplant 4-6 weeks; set out 4 weeks before last frost.
• Kohlrabi: Seed to transplant 4-6 weeks; set out 4 weeks before last frost.
• Leek: Seed to transplant 8-10 weeks; set out 4 weeks before last frost.
• Lettuce: Seed to transplant 4-5 weeks; set out 3-4 weeks before last frost.
• Melon: Seed to transplant 3-4 weeks; set out 2 weeks after last frost.
• Onion: Seed to transplant 8-10 weeks; set out 4 weeks before last frost.
• Parsley: Seed to transplant 9-10 weeks; set out 2-3 weeks before last frost.
• Parsnip: Plant seed in garden after last frost.
• Pepper: Seed to transplant 8-10 weeks; set out 2 weeks after last frost.
• Potato: Plant seed potatoes in garden 1-2 weeks before last frost.
• Pumpkin: Seed to transplant 3-4 weeks; set out 2 weeks after last frost.
• Radish: Plant seed in garden 2-4 weeks before last frost.
• Spinach: Seed to transplant 4-6 weeks; set out 3-6 weeks before last frost.
• Summer squash: Seed to transplant 3-4 weeks; set out 2 weeks after last frost.
• Sweet potato: Plant slips in garden 1-2 weeks after last frost.
• Swiss chard: Seed to transplant 4-6 weeks; set out 2 weeks before last frost; or plant seed in garden 4 weeks before last frost.
• Tomato: Seed to transplant 6-8 weeks; set out 1-2 weeks after last frost.
• Turnip: Plant seed in garden 2-4 weeks before last frost.
• Winter squash: Seed to transplant 3-4 weeks; set out 2 weeks after last frost.
• Zucchini: Seed to transplant 3-4 weeks; set out 2 weeks after last frost.

Not on the calendar. The same fact sheet times these by soil temperature, not by frost date, so they get no date here:

• Pea: as soon as the soil thaws.
• Beet: when the soil warms to 45°F.
• Carrot: when the soil warms to 60°F.
• Corn: when the soil warms to 60°F.
• Cucumber, pumpkin, summer squash, winter squash, zucchini: direct-sow when the soil warms to 70°F.

Rutabaga is left out: the fact sheet gives it New Hampshire calendar dates, not a frost offset.

The fact sheet was written for New Hampshire. The offsets are relative to the last frost, so they carry to other places with a spring frost, but where winters are mild many cool-season crops are grown in fall and winter instead. Your state's extension service has the local calendar. Dates are 30-year statistics, not a forecast: check the forecast before setting tender plants out.