Illinois frost dates: I
| City | Last frost | First frost | Season | Zone |
|---|---|---|---|---|
| Illinois City | Apr 18 | Oct 24 | 189 d | 5b |
| Illiopolis | Apr 14 | Oct 24 | 193 d | 6a |
| Ina | Apr 4 | Oct 28 | 207 d | 6b |
| Indianola | Apr 19 | Oct 19 | 183 d | 6a |
| Industry | Apr 16 | Oct 23 | 190 d | 6a |
| Ingleside | May 1 | Oct 11 | 163 d | 5b |
| Ingraham | Apr 13 | Oct 22 | 192 d | 6b |
| Ipava | Apr 17 | Oct 19 | 185 d | 6a |
| Iroquois | Apr 23 | Oct 14 | 174 d | 5b |
| Irving | Apr 13 | Oct 24 | 194 d | 6b |
| Irvington | Apr 9 | Oct 24 | 198 d | 6b |
| Island Lake | May 1 | Oct 11 | 163 d | 5b |
| Itasca | Apr 16 | Oct 28 | 195 d | 6a |
| Iuka | Apr 9 | Oct 26 | 200 d | 6b |
| Ivesdale | Apr 23 | Oct 13 | 173 d | 6a |