Helena, MT 59601 Frost Dates
Helena, MT: when does the frost end?
The last spring frost here typically falls around 05/09, about 254 days from today. Fall's first freeze usually shows up around 09/30, roughly 142 days after the last spring frost.
The USDA Plant Hardiness Zone Map puts Helena in Zone 4b (-25 to -20°F average annual minimum); frost dates are NOAA's 1991-2020 station normal, not this year's forecast.
Stretching the season past the first frost
With roughly 5 weeks until Helena's first fall frost, a row cover or cold frame is what stretches the last few weeks of harvest here.
Why 05/09 isn't a guarantee
05/09 is the middle of 30 years of NOAA records, not a promise. Getting tender plants in by 04/29 carries real risk, frost has hit that late in about 9 of 10 years here. Wait until 05/25 and only about 1 year in 10 sees frost after that.
Helena against the rest of the dataset
At roughly 142 frost-free days, Helena places 36,704 of 40,283 ZIP codes in this dataset. Within MT, it ranks 48 of 392.
Learn more
Before leaning on 59601's figures: zone vs. frost date explains why Zone 4b and Helena's frost date answer different questions, while the frost range explains how to read the range around Helena's dates instead of trusting one.