Fort Benton, MT 59420 Frost Dates
59420, near Fort Benton, MT: the frost window
The last spring frost here typically falls around 05/23, about 268 days from today. Fall's first freeze usually shows up around 09/23, roughly 120 days after the last spring frost.
Zone 4a (-30 to -25°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 Fort Benton, not a forecast for this year.
Stretching the season past the first frost
Fort Benton's growing season runs out in roughly 4 weeks — a cold frame or greenhouse kit is the usual way to keep cool-weather crops going past the first frost.
Why 05/23 isn't a guarantee
05/23 is the middle of 30 years of NOAA records, not a promise. Getting tender plants in by 05/10 carries real risk, frost has hit that late in about 9 of 10 years here. Wait until 06/09 and only about 1 year in 10 sees frost after that.
Fort Benton against the rest of the dataset
Fort Benton's growing season runs about 120 days, ranking 39,085 of 40,283 US ZIP codes measured here, longest to shortest. Within MT, it ranks 215 of 392.
Learn more
Two guides worth reading on 59420: local microclimates, on what one yard in Fort Benton can differ from the 59420 normal, and zone vs. frost date, on why Zone 4a and Fort Benton's frost date answer different questions.