Missoula, MT 59803 Frost Dates
59803, near Missoula, MT: the frost window
Around 05/10 is the usual last spring frost, about 255 days away. Fall's first freeze usually shows up around 10/01, roughly 143 days after the last spring frost.
Zone 5b (-15 to -10°F average annual minimum) comes from the USDA Plant Hardiness Zone Map; the frost dates above are the 1991-2020 normal at the nearest reporting NOAA station for Missoula, not a forecast for this year.
Stretching the season past the first frost
Missoula's growing season runs out in roughly 5 weeks — a cold frame or greenhouse kit is the usual way to keep cool-weather crops going past the first frost.
How much risk comes with planting early here
05/10 is the middle of 30 years of NOAA records, not a promise. Getting tender plants in by 04/27 carries real risk, frost has hit that late in about 9 of 10 years here. Wait until 05/28 and only about 1 year in 10 sees frost after that.
Missoula against the rest of the dataset
About 143 days separate Missoula's last and first frost, putting it 36,489 of 40,283 ZIP codes nationally. Within MT, it ranks 41 of 392.
Learn more
Two guides worth reading on 59803: zone vs. frost date, on why Zone 5b and Missoula's frost date answer different questions, and the frost range, on how to read the range around Missoula's dates instead of trusting one.