Roundup, MT 59072 Frost Dates
59072, near Roundup, MT: the frost window
The last spring frost here typically falls around 05/20, about 265 days from today. Fall's first freeze usually shows up around 09/21, roughly 124 days after the last spring frost.
Zone 4b (-25 to -20°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 Roundup, not a forecast for this year.
Stretching the season past the first frost
Roundup'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 05/06 carries real risk, frost has hit that late in about 9 of 10 years here. Wait until 06/02 and only about 1 year in 10 sees frost after that.
Roundup against the rest of the dataset
Roundup's growing season runs about 124 days, ranking 38,842 of 40,283 US ZIP codes measured here, longest to shortest. Within MT, it ranks 184 of 392.
Learn more
Two guides worth reading on 59072: the frost range, on how to read the range around Roundup's dates instead of trusting one, and local microclimates, on what one yard in Roundup can differ from the 59072 normal.