Boulder, CO 80305 Frost Dates
80305, near Boulder, CO: the frost window
The last spring frost here typically falls around 05/06, about 251 days from today. Fall's first freeze usually shows up around 10/05, roughly 151 days after the last spring frost.
The USDA Plant Hardiness Zone Map puts Boulder in Zone 6a (-10 to -5°F average annual minimum); frost dates are NOAA's 1991-2020 station normal, not this year's forecast.
Stretching the season past the first frost
Boulder's growing season runs out in roughly 5 weeks — a cold frame or greenhouse kit is the usual way to keep cool-weather crops going past the first frost.
The actual range around Boulder's last frost
05/06 is the middle of 30 years of NOAA records, not a promise. Getting tender plants in by 04/24 carries real risk, frost has hit that late in about 9 of 10 years here. Wait until 05/18 and only about 1 year in 10 sees frost after that.
Boulder against the rest of the dataset
Boulder's growing season runs about 151 days, ranking 34,936 of 40,283 US ZIP codes measured here, longest to shortest. Within CO, it ranks 306 of 624.
Learn more
Two guides worth reading on 80305: zone vs. frost date, on why Zone 6a and Boulder's frost date answer different questions, and the frost range, on how to read the range around Boulder's dates instead of trusting one.