Illinois frost dates: J
| City | Last frost | First frost | Season | Zone |
|---|---|---|---|---|
| Jacksonville | Apr 17 | Oct 21 | 187 d | 6a |
| Jacob | Apr 9 | Oct 22 | 196 d | 7a |
| Janesville | Apr 15 | Oct 24 | 192 d | 6a |
| Jerseyville | Apr 14 | Oct 19 | 188 d | 6b |
| Jewett | Apr 13 | Oct 22 | 192 d | 6b |
| Johnsonville | Apr 9 | Oct 24 | 198 d | 6b |
| Johnston City | Apr 4 | Oct 28 | 207 d | 7a |
| Joliet | Apr 19 | Oct 25 | 189 d | 6a |
| Jonesboro | Apr 8 | Oct 29 | 204 d | 7a |
| Joppa | Apr 3 | Oct 27 | 207 d | 7a |
| Joy | Apr 22 | Oct 19 | 180 d | 5b |
| Junction | Apr 7 | Oct 26 | 202 d | 7a |
| Justice | Apr 10 | Nov 1 | 205 d | 6a |