Colstrip, MT 59003 Frost Dates
When can you plant in Colstrip, MT?
Typically, the last spring freeze lands around 05/25, roughly 270 days out. The first fall frost typically arrives around 09/19, about 115 frost-free days later.
Zone 4b (-25 to -20°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.
What to do before the first frost
About 3 weeks of growing season left in Colstrip before the first fall frost — a cold frame or row cover can stretch harvest weeks past that date.
Why 05/25 isn't a guarantee
05/25 is the middle of 30 years of NOAA records, not a promise. Plant by 05/12 and you're betting against frost in roughly 9 years out of 10. 06/07 is the safer bet, historically only a 1-in-10 chance of frost that late.
How Colstrip compares
At roughly 115 frost-free days, Colstrip places 39,316 of 40,283 ZIP codes in this dataset. Within MT, it ranks 248 of 392.
Learn more
local microclimates covers what one yard in Colstrip can differ from the 59003 normal. zone vs. frost date covers why Zone 4b and Colstrip's frost date answer different questions.