St. Ignatius, MT 59821 Frost Dates
Frost dates for St. Ignatius, MT, ZIP 59821
The last spring frost here typically falls around 05/12, about 257 days from today. The first fall frost typically arrives around 10/02, about 142 frost-free days later.
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 St. Ignatius, not a forecast for this year.
What to do before the first frost
Zone 5b around St. Ignatius 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 05/12 isn't a guarantee
05/12 is the middle of 30 years of NOAA records, not a promise. Plant by 04/28 and you're betting against frost in roughly 9 years out of 10. 06/01 is the safer bet, historically only a 1-in-10 chance of frost that late.
How St. Ignatius compares
St. Ignatius's growing season runs about 142 days, ranking 36,774 of 40,283 US ZIP codes measured here, longest to shortest. Within MT, it ranks 57 of 392.
Learn more
zone vs. frost date explains why Zone 5b and St. Ignatius's frost date answer different questions, and the frost range explains how to read the range around St. Ignatius's dates instead of trusting one.