Westby, MT 59275 Frost Dates
59275, near Westby, MT: the frost window
Typically, the last spring freeze lands around 05/18, roughly 263 days out. Fall's first freeze usually shows up around 09/21, roughly 125 days after the last spring frost.
Westby sits in USDA Zone 3b (-35 to -30°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
Westby'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.
How much risk comes with planting early here
05/18 is the middle of 30 years of NOAA records, not a promise. Getting tender plants in by 05/06 carries real risk, frost has hit that late in about 9 of 10 years here. Wait until 05/30 and only about 1 year in 10 sees frost after that.
Westby against the rest of the dataset
At roughly 125 frost-free days, Westby places 38,819 of 40,283 ZIP codes in this dataset. Within MT, it ranks 180 of 392.
Learn more
Two guides worth reading on 59275: local microclimates, on what one yard in Westby can differ from the 59275 normal, and zone vs. frost date, on why Zone 3b and Westby's frost date answer different questions.