Thornton, CO 80233 Frost Dates
Frost dates for Thornton, CO, ZIP 80233
Around 05/02 is the usual last spring frost, about 247 days away. The first fall frost typically arrives around 10/11, about 160 frost-free days later.
The USDA Plant Hardiness Zone Map puts Thornton in Zone 6a (-10 to -5°F average annual minimum); frost dates are NOAA's 1991-2020 station normal, not this year's forecast.
What to do before the first frost
Zone 6a around Thornton has about 6 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 05/02 isn't a guarantee
05/02 is the middle of 30 years of NOAA records, not a promise. Plant by 04/18 and you're betting against frost in roughly 9 years out of 10. 05/14 is the safer bet, historically only a 1-in-10 chance of frost that late.
How Thornton compares
About 160 days separate Thornton's last and first frost, putting it 32,187 of 40,283 ZIP codes nationally. Within CO, it ranks 85 of 624.
Learn more
the frost range explains how to read the range around Thornton's dates instead of trusting one, and local microclimates explains what one yard in Thornton can differ from the 80233 normal.