Frost dates / Colorado / Castle Rock

Frost dates in Castle Rock, CO

The station on this page is CASTLE ROCK, CO US (USC00051401), 3.7 miles from the city point. Population in the Census 2024 estimate is 83,213.

Zone 5bUSDA average annual extreme minimum -15°F to -10°F, from ZIP 80109 (3.2 miles from the city point used on this page).

Median last 32°F is May 9. Median first 32°F is October 2. The 50% growing-season length is 143 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 30	Earlier date. A last 32°F on or after this day is common.
50%	May 9	Median.
10%	May 24	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 17	Earlier date. A first 32°F on or before this day is uncommon.
50%	October 2	Median.
90%	October 16	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%	161
50%	143
90%	124

28°F threshold

	10%	50%	90%
Last, on or after	May 14	May 1	April 17
First, on or before	September 26	October 12	October 25
Season at least	181 days	161 days	141 days

36°F threshold

	10%	50%	90%
Last, on or after	June 6	May 21	May 9
First, on or before	September 11	September 25	October 7
Season at least	142 days	124 days	103 days

Station CASTLE ROCK, CO US (USC00051401). Completeness flag S, 25 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 1885.2 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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Castle Rock, CO

Zone 5bAverage annual extreme minimum -15°F to -10°F

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

Colorado. Nearest qualified station: CASTLE ROCK, CO US (USC00051401), 3.7 miles. Median last 32°F May 9. Median first 32°F October 2. The 50% growing-season length is 143 days.

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32°F probabilities at this station

	10%	50%	90%
Last 32°F on or after	May 24	May 9	April 30
First 32°F on or before	September 17	October 2	October 16

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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