Planting calendar for Bolingbrook, IL
Direct answer. In Bolingbrook, IL, the median last 32°F date at ROMEOVILLE WFO, IL US (USC00117457) is April 20, and the median first 32°F date is October 22. The station is 5.9 miles from the city point. Crop windows come from University of Illinois Extension. Where that guide counts weeks from a frost date and does not name a 10%, 50%, or 90% level, those weeks are applied to these median dates.
Median last 32°F April 20. Median first 32°F October 22. Frost dates for Bolingbrook.
Outside the median season Growing seasonTicks: 10%, 50%, and 90%
Last 32°F in spring
NCEI’s probability is the chance the last spring minimum of 32°F or lower occurs on or after the date.
| Probability | Date | How to read it |
|---|---|---|
| 90% | April 7 | Earlier date. A last 32°F on or after this day is common. |
| 50% | April 20 | Median. |
| 10% | May 6 | Later date. A last 32°F on or after this day is uncommon. |
First 32°F in fall
NCEI’s probability is the chance the first autumn minimum of 32°F or lower occurs on or before the date.
| Probability | Date | How to read it |
|---|---|---|
| 10% | October 7 | Earlier date. A first 32°F on or before this day is uncommon. |
| 50% | October 22 | Median. |
| 90% | November 3 | Later date. A first 32°F on or before this day is common. |
32°F growing-season length
Days between the last spring 32°F and the first autumn 32°F. The figure is a length the season meets or exceeds at that probability, from NCEI, not a subtraction done on this site.
| Probability the season is at least this long | Days |
|---|---|
| 10% | 202 |
| 50% | 184 |
| 90% | 162 |
28°F threshold
| 10% | 50% | 90% | |
|---|---|---|---|
| Last, on or after | April 23 | April 8 | March 27 |
| First, on or before | October 19 | November 1 | November 15 |
| Season at least | 226 days | 206 days | 186 days |
36°F threshold
| 10% | 50% | 90% | |
|---|---|---|---|
| Last, on or after | May 17 | May 1 | April 17 |
| First, on or before | September 28 | October 12 | October 24 |
| Season at least | 180 days | 162 days | 142 days |
Station ROMEOVILLE WFO, IL US (USC00117457). Completeness flag R, 23 years used for the 32°F dates. NCEI’s annual probability of at least one day at or below 32°F is 100%. Freeze dates are conditional on that threshold occurring. Elevation 201.2 m. The same probability language applies to the 28°F and 36°F rows when those dates passed the same quality checks.
Questions
- Which frost date do the crop weeks use?
- Weeks after the frost-free date use the median last 32°F date, April 20, when the guide does not name a probability. Weeks before the first freeze use the median first 32°F date, October 22. Both medians are 50% dates in the NOAA 1991–2020 normals for the station on this page. The page says when that anchor was used.
- Where do the crop dates come from?
- Crop dates come from the extension guide cited on this page, not from a second calendar invented here. The publisher and the title are printed with the source note. A month without a day is expanded to the first and last day of that month, and the page says so. A phrase without two dates stays a phrase.
- Is this planting calendar a forecast?
- No. The frost dates are 1991–2020 climate normals at the station 5.9 miles from the city point. The crop windows are the cited guide’s ranges, shifted only when that guide itself used weeks. A cold spring can still fall past the median. The National Weather Service forecast covers a particular night. This page does not.
Crops in the cited guide
Bolingbrook, IL
Zone 6aAverage annual extreme minimum -10°F to -5°F
Outside the median season Growing seasonTicks: 10%, 50%, and 90%
Illinois. Nearest qualified station: ROMEOVILLE WFO, IL US (USC00117457), 5.9 miles. Median last 32°F April 20. Median first 32°F October 22. The 50% growing-season length is 184 days.
Open the ZIP page · Crop guides for Illinois
32°F probabilities at this station
| 10% | 50% | 90% | |
|---|---|---|---|
| Last 32°F on or after | May 6 | April 20 | April 7 |
| First 32°F on or before | October 7 | October 22 | November 3 |
Column order matches the NCEI file: for the last freeze, 10% is the later date and 90% is the earlier date. How to read the probabilities.
Planting calendar
Windows below are from University of Illinois Extension, When to Plant, retrieved 2026-10-10. The planting chart is divided into Northern Illinois, Central Illinois, and Southern Illinois. Median dates for the last 32-degree freeze on the same page: Northern Illinois April 8-29; Central Illinois April 8-15; Southern Illinois April 1-8. The page also says southern Illinois can generally start crops two weeks earlier than central Illinois and northern Illinois about two weeks later; the crop rows below are the chart dates, not a recalculation of that sentence.
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State crop pages: Bush beans · Beets · Broccoli · Cabbage · Carrots · Swiss chard · Sweet corn · Cucumber