Hamilton, MT 59840 Frost Dates
59840, near Hamilton, MT: the frost window
Around 05/21 is the usual last spring frost, about 266 days away. Fall's first freeze usually shows up around 09/22, roughly 122 days after the last spring frost.
Hamilton sits in USDA Zone 5b (-15 to -10°F average annual minimum). The dates above are NOAA's 1991-2020 normal at the nearest reporting station, a 30-year average, not a forecast.
Stretching the season past the first frost
Hamilton'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 05/21 isn't a guarantee
05/21 is the middle of 30 years of NOAA records, not a promise. Getting tender plants in by 05/07 carries real risk, frost has hit that late in about 9 of 10 years here. Wait until 06/06 and only about 1 year in 10 sees frost after that.
Hamilton against the rest of the dataset
At roughly 122 frost-free days, Hamilton places 38,934 of 40,283 ZIP codes in this dataset. Within MT, it ranks 200 of 392.
Learn more
Two guides worth reading on 59840: local microclimates, on what one yard in Hamilton can differ from the 59840 normal, and zone vs. frost date, on why Zone 5b and Hamilton's frost date answer different questions.