| 2023 Hardiness Zone: | Zone 6b: -5°F to 0°F |
| 2012 Hardiness Zone: | Zone 6a: -10°F to -5°F |
| 1990 Hardiness Zone: | Zone 1b: -15F to -10F |
| Average First Frost Date: | Oct. 21 - Oct. 31 |
| Average Last Frost Date: | Apr. 11 - Apr. 20 |
| Koppen-Geiger Climate Zone: | Dfa - Humid Continental Hot Summers |
| Ecoregion: | 72f - River Hills |
| Heat Zone Days: | 46 - 60 days Over 86°F |
Annual Climate Data for ZIP Code 63366 - O Fallon, MO | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg Min Temp (°F) | 19 | 24 | 33 | 43 | 53 | 62 | 67 | 65 | 57 | 45 | 35 | 24 | 44 |
| Avg Max Temp (°F) | 38 | 44 | 55 | 67 | 76 | 85 | 89 | 88 | 80 | 69 | 55 | 42 | 66 |
| Avg Precip (In.) | 1.95 | 2.16 | 3.43 | 3.85 | 4.29 | 3.68 | 3.99 | 3.16 | 3.13 | 2.92 | 3.61 | 2.7 | 38.88 |