O'Fallon, Missouri: planting zone & frost dates
O'Fallon is in USDA plant hardiness zone 6b, where the average coldest winter night reaches -5 to 0°F (-20.6 to -17.8°C). The typical last spring frost is around Apr 16 and the first fall frost around Oct 20.
Frost dates
| Probability | Last spring frost (32°F) | First fall frost (32°F) |
|---|---|---|
| Safer (10% risk) | Apr 26 | Oct 12 |
| Typical planning date (30% risk) | Apr 16 | Oct 20 |
| Median (50%) | Apr 10 | Oct 26 |
How to read this: after the spring "10% risk" date, a 32°F frost happens in only about 1 year in 10. In fall, the "10% risk" date is the early end — a frost before it happens about 1 year in 10.
Frost data comes from NOAA station WELDON SPRING NWS, MO (USC00238805), 6.3 mi (10.1 km) from the city center and 13 ft lower than the city center (-4 m). Record used for the 1991–2020 normals: 29 years.
Why your yard may differ from the station: microclimates.
Hardiness zone across O'Fallon
All 6 ZIP codes covering O'Fallon are in zone 6b.
Vegetable planting calendar for O'Fallon
Dates are counted from the 30%-risk frost dates above (last frost Apr 16, first frost Oct 20). Shift a week later in cold spots or low-lying ground.
| Crop | Start indoors | Transplant / plant out | Direct sow | Fall crop sow/plant |
|---|---|---|---|---|
| Tomato | Feb 19 – Mar 5 | Apr 23 – May 7 | — | — |
| Pepper | Feb 5 – Feb 19 | Apr 30 – May 14 | — | — |
| Lettuce | Mar 5 – Mar 19 | Apr 2 – Apr 23 | Mar 19 – Apr 30 | Aug 11 – Sep 8 |
| Green Beans | — | — | Apr 23 – May 28 | Jul 28 – Aug 11 |
| Sweet Corn | — | — | Apr 23 – May 21 | — |
| Zucchini & Summer Squash | Mar 19 – Mar 26 | Apr 23 – May 7 | Apr 23 – May 14 | Jul 28 – Aug 11 |
| Cucumber | Mar 19 – Mar 26 | Apr 30 – May 7 | Apr 23 – May 14 | Jul 28 – Aug 11 |
| Carrot | — | — | Mar 26 – Apr 30 | Jul 28 – Aug 11 |
| Onion | Jan 22 – Feb 5 | Mar 19 – Apr 2 | — | — |
| Potato | — | Mar 19 – Apr 9 | — | — |
| Basil | Mar 5 – Mar 19 | Apr 30 – May 14 | Apr 30 – May 21 | — |
| Spinach | — | — | Mar 5 – Apr 2 | Aug 25 – Sep 22 |
| Kale | Feb 19 – Mar 5 | Mar 19 – Apr 9 | Mar 19 – Apr 9 | Aug 11 – Sep 8 |
| Broccoli | Feb 19 – Mar 5 | Apr 2 – Apr 23 | — | Jul 28 – Aug 11 |
| Peas | — | — | Mar 5 – Mar 26 | Aug 11 – Aug 25 |
| Eggplant | Feb 5 – Feb 19 | Apr 30 – May 14 | — | — |
| Cabbage | Feb 19 – Mar 5 | Mar 19 – Apr 2 | — | Jul 28 – Aug 11 |
| Radish | — | — | Mar 12 – Apr 30 | Sep 8 – Oct 6 |
| Strawberry | — | Mar 19 – Apr 9 | — | — |
| Beets | — | — | Mar 26 – May 7 | Aug 11 – Aug 25 |
Nearby cities in Missouri
- St. Louis — zone 7a, last frost Apr 7, first frost Oct 31
- Columbia — zone 6b, last frost Apr 12, first frost Oct 25
- Springfield — zone 6b, last frost Apr 16, first frost Oct 22
- Independence — zone 6b, last frost Apr 16, first frost Oct 22
- Lee's Summit — zone 6b, last frost Apr 13, first frost Oct 23
- Kansas City — zone 6b, last frost Apr 10, first frost Oct 29
More: all Missouri cities · other zone 6b cities: Columbus, OH, Indianapolis, IN, Boston, MA, Detroit, MI, Kansas City, MO, Cincinnati, OH, Pittsburgh, PA, Buffalo, NY
O'Fallon gardening questions
When is the last frost in O'Fallon, MO?
Plan around Apr 16. At NOAA's Weldon Spring NWS, MO station, 6.3 mi from the city center, there is a 30% chance of a 32°F frost after that date. Half of springs have their last frost by Apr 10, and after Apr 26 the risk drops to about 1 in 10. A hard freeze (28°F) usually ends earlier, around Mar 31. The 90% and 10% dates are 31 days apart, close to the 36-day median for cities on this site.
When should I plant tomatoes in O'Fallon, MO?
Start seeds indoors Feb 19 – Mar 5 and set transplants outside Apr 23 – May 7, once nights stay above 50°F and the soil is near 60°F. That leaves 187 days between the 30% frost dates, enough for most varieties including 80–85-day slicers. More: tomato planting dates by zone.
How long is the growing season in O'Fallon?
About 199 days without a 32°F frost in a typical (median) year. In 9 years out of 10 the season lasts at least 178 days, and in 1 year in 10 it runs 220 days or more. In fall there is a 10% chance of frost by Oct 12 and a 30% chance by Oct 20. That ranks 6th longest of 7 Missouri cities on this site.
When should I plant a fall garden in O'Fallon?
Count back from the first fall frost (Oct 20): transplant broccoli Jul 28 – Aug 11; sow kale Aug 11 – Sep 8; sow lettuce Aug 11 – Sep 8; sow radishes Sep 8 – Oct 6. Hardy crops keep growing through light frosts; the median 28°F hard freeze here is Nov 5. How the math works: succession and fall planting.
Did O'Fallon's hardiness zone change on the 2023 map?
Yes. Most of O'Fallon was zone 6a on the 2012 map and is zone 6b on the 2023 map — half a zone warmer, like about 56% of U.S. ZIP codes. The shift comes from a newer 1991–2020 averaging period plus more weather stations and better mapping; it does not mean last frost comes earlier. Why the 2023 map is warmer.
How does O'Fallon compare with St. Louis, MO?
O'Fallon is zone 6b and St. Louis is zone 7a. The typical last spring frost in O'Fallon (Apr 16) comes about 9 days later than in St. Louis, MO (Apr 7), 28 miles away, and the first fall frost is Oct 20 versus Oct 31. O'Fallon sits about 155 ft higher, which explains part of the gap. The two cities use different NOAA stations.