Lewistown, MT 59418 Frost Dates
Frost dates for Lewistown, MT, ZIP 59418
Typically, the last spring freeze lands around 05/14, roughly 259 days out. The first fall frost typically arrives around 09/28, about 135 frost-free days later.
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 Lewistown, not a forecast for this year.
What to do before the first frost
Zone 4b around Lewistown has about 4 weeks left before the first fall frost — that's the usual window to get a cold frame up if you want to keep harvesting past it.
Why 05/14 isn't a guarantee
05/14 is the middle of 30 years of NOAA records, not a promise. Plant by 05/02 and you're betting against frost in roughly 9 years out of 10. 05/27 is the safer bet, historically only a 1-in-10 chance of frost that late.
How Lewistown compares
About 135 days separate Lewistown's last and first frost, putting it 37,844 of 40,283 ZIP codes nationally. Within MT, it ranks 94 of 392.
Learn more
zone vs. frost date explains why Zone 4b and Lewistown's frost date answer different questions, and the frost range explains how to read the range around Lewistown's dates instead of trusting one.