Helena, MT 59624 Frost Dates
Frost dates for Helena, MT, ZIP 59624
Around 05/09 is the usual last spring frost, about 254 days away. The first fall frost typically arrives around 09/30, about 142 frost-free days later.
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 Helena, not a forecast for this year.
What to do before the first frost
Zone 5a around Helena has about 5 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.
The actual range around Helena's last frost
05/09 is the middle of 30 years of NOAA records, not a promise. Plant by 04/29 and you're betting against frost in roughly 9 years out of 10. 05/25 is the safer bet, historically only a 1-in-10 chance of frost that late.
How Helena compares
At roughly 142 frost-free days, Helena places 36,708 of 40,283 ZIP codes in this dataset. Within MT, it ranks 52 of 392.
Learn more
local microclimates explains what one yard in Helena can differ from the 59624 normal, and zone vs. frost date explains why Zone 5a and Helena's frost date answer different questions.