Frost dates / Colorado / Longmont

Frost dates in Longmont, CO

The station on this page is LONGMONT 2 ESE, CO US (USC00055116), 1.2 miles from the city point. Population in the Census 2024 estimate is 99,818.

Zone 5bUSDA average annual extreme minimum -15°F to -10°F, from ZIP 80501 (0.2 miles from the city point used on this page).

Median last 32°F is May 7. 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 7	Median.
10%	May 22	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 23	Earlier date. A first 32°F on or before this day is uncommon.
50%	October 5	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%	169
50%	151
90%	131

28°F threshold

	10%	50%	90%
Last, on or after	May 12	April 28	April 11
First, on or before	September 30	October 12	October 25
Season at least	189 days	169 days	151 days

36°F threshold

	10%	50%	90%
Last, on or after	June 2	May 18	May 5
First, on or before	September 14	September 28	October 8
Season at least	151 days	132 days	111 days

Station LONGMONT 2 ESE, CO US (USC00055116). Completeness flag R, 17 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 1508.8 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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Longmont, 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: LONGMONT 2 ESE, CO US (USC00055116), 1.2 miles. Median last 32°F May 7. 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 22	May 7	April 24
First 32°F on or before	September 23	October 5	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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