Fort Benton, MT 59460 Frost Dates
59460, near Fort Benton, MT: the frost window
Around 05/20 is the usual last spring frost, about 265 days away. Fall's first freeze usually shows up around 09/19, 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 3 weeks — a cold frame or greenhouse kit is the usual way to keep cool-weather crops going past the first frost.
Why 05/20 isn't a guarantee
05/20 is the middle of 30 years of NOAA records, not a promise. Getting tender plants in by 05/08 carries real risk, frost has hit that late in about 9 of 10 years here. Wait until 06/03 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 39,086 of 40,283 US ZIP codes measured here, longest to shortest. Within MT, it ranks 216 of 392.
Learn more
Two guides worth reading on 59460: local microclimates, on what one yard in Fort Benton can differ from the 59460 normal, and zone vs. frost date, on why Zone 4a and Fort Benton's frost date answer different questions.