Thornton, CO 80640 Frost Dates
When can you plant in Thornton, CO?
Typically, the last spring freeze lands around 05/03, roughly 248 days out. The first fall frost typically arrives around 10/07, about 155 frost-free days later.
Thornton sits in USDA Zone 5b (-15 to -10°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.
What to do before the first frost
About 6 weeks of growing season left in Thornton before the first fall frost — a cold frame or row cover can stretch harvest weeks past that date.
Why 05/03 isn't a guarantee
05/03 is the middle of 30 years of NOAA records, not a promise. Plant by 04/22 and you're betting against frost in roughly 9 years out of 10. 05/16 is the safer bet, historically only a 1-in-10 chance of frost that late.
How Thornton compares
About 155 days separate Thornton's last and first frost, putting it 33,901 of 40,283 ZIP codes nationally. Within CO, it ranks 241 of 624.
Learn more
local microclimates covers what one yard in Thornton can differ from the 80640 normal. zone vs. frost date covers why Zone 5b and Thornton's frost date answer different questions.