Helena, MT 59633 Frost Dates
Frost dates for Helena, MT, ZIP 59633
The last spring frost here typically falls around 06/04, about 280 days from today. The first fall frost typically arrives around 09/14, about 100 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 Helena, not a forecast for this year.
What to do before the first frost
Zone 4b around Helena has about 2 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 06/04 isn't a guarantee
06/04 is the middle of 30 years of NOAA records, not a promise. Plant by 05/18 and you're betting against frost in roughly 9 years out of 10. 06/18 is the safer bet, historically only a 1-in-10 chance of frost that late.
How Helena compares
At roughly 100 frost-free days, Helena places 39,703 of 40,283 ZIP codes in this dataset. Within MT, it ranks 304 of 392.
Learn more
zone vs. frost date explains why Zone 4b and Helena's frost date answer different questions, and the frost range explains how to read the range around Helena's dates instead of trusting one.