Great Falls, MT 59405 Frost Dates
Frost dates for Great Falls, MT, ZIP 59405
Around 05/06 is the usual last spring frost, about 251 days away. The first fall frost typically arrives around 10/01, about 147 frost-free days later.
The USDA Plant Hardiness Zone Map puts Great Falls in Zone 4b (-25 to -20°F average annual minimum); frost dates are NOAA's 1991-2020 station normal, not this year's forecast.
What to do before the first frost
Zone 4b around Great Falls has about 5 weeks left before the first fall frost — that's the usual window to get a cold frame up if you want to keep harvesting past it.
Why 05/06 isn't a guarantee
05/06 is the middle of 30 years of NOAA records, not a promise. Plant by 04/23 and you're betting against frost in roughly 9 years out of 10. 05/19 is the safer bet, historically only a 1-in-10 chance of frost that late.
How Great Falls compares
About 147 days separate Great Falls's last and first frost, putting it 35,832 of 40,283 ZIP codes nationally. Within MT, it ranks 27 of 392.
Learn more
zone vs. frost date explains why Zone 4b and Great Falls's frost date answer different questions, and the frost range explains how to read the range around Great Falls's dates instead of trusting one.