Longmont, CO 80503 Frost Dates
Frost dates for Longmont, CO, ZIP 80503
The last spring frost here typically falls around 05/06, about 251 days from today. The first fall frost typically arrives around 10/05, about 151 frost-free days later.
Zone 6a (-10 to -5°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 Longmont, not a forecast for this year.
What to do before the first frost
Zone 6a around Longmont has about 5 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 05/06 isn't a guarantee
05/06 is the middle of 30 years of NOAA records, not a promise. Plant by 04/24 and you're betting against frost in roughly 9 years out of 10. 05/18 is the safer bet, historically only a 1-in-10 chance of frost that late.
How Longmont compares
About 151 days separate Longmont's last and first frost, putting it 34,938 of 40,283 ZIP codes nationally. Within CO, it ranks 308 of 624.
Learn more
local microclimates explains what one yard in Longmont can differ from the 80503 normal, and zone vs. frost date explains why Zone 6a and Longmont's frost date answer different questions.