Boulder, MT 59631 Frost Dates
When can you plant in Boulder, MT?
The last spring frost here typically falls around 06/07, about 283 days from today. The first fall frost typically arrives around 09/08, about 90 frost-free days later.
The USDA Plant Hardiness Zone Map puts Boulder in Zone 5a (-20 to -15°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
About 2 weeks of growing season left in Boulder before the first fall frost — a cold frame or row cover can stretch harvest weeks past that date.
Why 06/07 isn't a guarantee
06/07 is the middle of 30 years of NOAA records, not a promise. Plant by 05/24 and you're betting against frost in roughly 9 years out of 10. 06/25 is the safer bet, historically only a 1-in-10 chance of frost that late.
How Boulder compares
Boulder's growing season runs about 90 days, ranking 39,914 of 40,283 US ZIP codes measured here, longest to shortest. Within MT, it ranks 337 of 392.
Learn more
the frost range covers how to read the range around Boulder's dates instead of trusting one. local microclimates covers what one yard in Boulder can differ from the 59631 normal.