O'Fallon, MO 63366 Frost Dates
63366, near O'Fallon, MO: the frost window
Typically, the last spring freeze lands around 04/10, roughly 225 days out. Fall's first freeze usually shows up around 10/26, roughly 199 days after the last spring frost.
O'Fallon sits in USDA Zone 6b (-5 to 0°F average annual minimum). The dates above are NOAA's 1991-2020 normal at the nearest reporting station, a 30-year average, not a forecast.
Stretching the season past the first frost
O'Fallon's growing season runs out in roughly 8 weeks — a cold frame or greenhouse kit is the usual way to keep cool-weather crops going past the first frost.
Why 04/10 isn't a guarantee
04/10 is the middle of 30 years of NOAA records, not a promise. Getting tender plants in by 03/26 carries real risk, frost has hit that late in about 9 of 10 years here. Wait until 04/26 and only about 1 year in 10 sees frost after that.
O'Fallon against the rest of the dataset
At roughly 199 frost-free days, O'Fallon places 17,984 of 40,283 ZIP codes in this dataset. Within MO, it ranks 411 of 1,129.
Learn more
Two guides worth reading on 63366: zone vs. frost date, on why Zone 6b and O'Fallon's frost date answer different questions, and the frost range, on how to read the range around O'Fallon's dates instead of trusting one.