Colstrip, MT 59323 Frost Dates
Colstrip, MT: when does the frost end?
Around 05/15 is the usual last spring frost, about 260 days away. Fall's first freeze usually shows up around 09/30, roughly 138 days after the last spring frost.
Zone 5a (-20 to -15°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 Colstrip, not a forecast for this year.
Stretching the season past the first frost
With roughly 5 weeks until Colstrip's first fall frost, a row cover or cold frame is what stretches the last few weeks of harvest here.
Why 05/15 isn't a guarantee
05/15 is the middle of 30 years of NOAA records, not a promise. Getting tender plants in by 05/02 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.
Colstrip against the rest of the dataset
About 138 days separate Colstrip's last and first frost, putting it 37,401 of 40,283 ZIP codes nationally. Within MT, it ranks 71 of 392.
Learn more
Before leaning on 59323's figures: local microclimates explains what one yard in Colstrip can differ from the 59323 normal, while zone vs. frost date explains why Zone 5a and Colstrip's frost date answer different questions.