Frost dates / Colorado / Boulder

Frost dates in Boulder, CO

The station on this page is BOULDER, CO US (USC00050848), 2.4 miles from the city point. Population in the Census 2024 estimate is 106,803.

Zone 6aUSDA average annual extreme minimum -10°F to -5°F, from ZIP 80310 (1.5 miles from the city point used on this page).

Median last 32°F is May 6. Median first 32°F is October 5. The 50% growing-season length is 151 days. The green band runs from the median last frost to the median first frost. Thin ticks mark the 10%, 50%, and 90% dates.

Outside the median season Growing seasonTicks: 10%, 50%, and 90%

Last 32°F in spring

NCEI’s probability is the chance the last spring minimum of 32°F or lower occurs on or after the date.

Probability	Date	How to read it
90%	April 24	Earlier date. A last 32°F on or after this day is common.
50%	May 6	Median.
10%	May 18	Later date. A last 32°F on or after this day is uncommon.

First 32°F in fall

NCEI’s probability is the chance the first autumn minimum of 32°F or lower occurs on or before the date.

Probability	Date	How to read it
10%	September 21	Earlier date. A first 32°F on or before this day is uncommon.
50%	October 5	Median.
90%	October 18	Later date. A first 32°F on or before this day is common.

32°F growing-season length

Days between the last spring 32°F and the first autumn 32°F. The figure is a length the season meets or exceeds at that probability, from NCEI, not a subtraction done on this site.

Probability the season is at least this long	Days
10%	170
50%	151
90%	131

28°F threshold

	10%	50%	90%
Last, on or after	May 10	April 27	April 10
First, on or before	September 30	October 15	October 30
Season at least	193 days	171 days	149 days

36°F threshold

	10%	50%	90%
Last, on or after	June 2	May 17	May 5
First, on or before	September 13	September 28	October 10
Season at least	150 days	130 days	109 days

Station BOULDER, CO US (USC00050848). Completeness flag S, 26 years used for the 32°F dates. NCEI’s annual probability of at least one day at or below 32°F is 100%. Freeze dates are conditional on that threshold occurring. Elevation 1671.5 m. The same probability language applies to the 28°F and 36°F rows when those dates passed the same quality checks.

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Boulder, CO

Zone 6aAverage annual extreme minimum -10°F to -5°F

Outside the median season Growing seasonTicks: 10%, 50%, and 90%

Colorado. Nearest qualified station: BOULDER, CO US (USC00050848), 2.4 miles. Median last 32°F May 6. Median first 32°F October 5. The 50% growing-season length is 151 days.

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32°F probabilities at this station

	10%	50%	90%
Last 32°F on or after	May 18	May 6	April 24
First 32°F on or before	September 21	October 5	October 18

Column order matches the NCEI file: for the last freeze, 10% is the later date and 90% is the earlier date. How to read the probabilities.

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Planting calendar

Frost dates for Colorado are above. Planting windows appear for states with a cited extension guide. The fall and seed tools use the numbers you enter and the station dates.

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