Chinook, MT 59547 Frost Dates
Frost dates for Chinook, MT, ZIP 59547
Typically, the last spring freeze lands around 05/14, roughly 259 days out. The first fall frost typically arrives around 09/23, about 130 frost-free days later.
The USDA Plant Hardiness Zone Map puts Chinook in Zone 3b (-35 to -30°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 3b around Chinook has about 4 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 05/14 isn't a guarantee
05/14 is the middle of 30 years of NOAA records, not a promise. Plant by 05/01 and you're betting against frost in roughly 9 years out of 10. 05/27 is the safer bet, historically only a 1-in-10 chance of frost that late.
How Chinook compares
At roughly 130 frost-free days, Chinook places 38,395 of 40,283 ZIP codes in this dataset. Within MT, it ranks 131 of 392.
Learn more
local microclimates explains what one yard in Chinook can differ from the 59547 normal, and zone vs. frost date explains why Zone 3b and Chinook's frost date answer different questions.