Boulder climate

Boulder Frost Dates and Growing Season

NOAA 1991-2020 normals for Boulder, 2.6 km from Boulder, elevation 5484 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Boulder. A median date has an even chance of a later freeze in spring, or an earlier one in fall.

The last spring freeze typically comes around May 6 and the first fall freeze around October 5, a growing season of about 151 days.

Median at 36°F
last May 17, first September 28
Median at 32°F
last May 6, first October 5
Median at 28°F
last April 27, first October 15

Last freeze in spring

The chance of a later freeze than the date shown. At 10 percent, one spring in ten still freezes after the date; planting after the 10 percent date for 32°F is the cautious choice.

Threshold10%20%30%40%50%60%70%80%90%
36°FJun 2May 27May 22May 20May 17May 15May 12May 9May 5
32°FMay 18May 14May 11May 9May 6May 4May 1Apr 29Apr 24
28°FMay 10May 5May 2Apr 30Apr 27Apr 23Apr 20Apr 16Apr 10
24°FMay 2Apr 27Apr 22Apr 18Apr 14Apr 11Apr 7Apr 3Mar 29
20°FApr 22Apr 15Apr 10Apr 5Apr 2Mar 30Mar 25Mar 19Mar 10
16°FApr 12Apr 3Mar 31Mar 26Mar 21Mar 15Mar 9Mar 5Feb 27

First freeze in fall

The chance of an earlier freeze than the date shown. At 10 percent, one fall in ten freezes before the date.

Threshold10%20%30%40%50%60%70%80%90%
36°FSep 13Sep 19Sep 23Sep 26Sep 28Oct 1Oct 3Oct 5Oct 10
32°FSep 21Sep 27Sep 30Oct 3Oct 5Oct 8Oct 11Oct 14Oct 18
28°FSep 30Oct 5Oct 8Oct 12Oct 15Oct 18Oct 21Oct 25Oct 30
24°FOct 7Oct 13Oct 18Oct 21Oct 25Oct 28Oct 31Nov 3Nov 9
20°FOct 16Oct 22Oct 27Oct 31Nov 3Nov 6Nov 11Nov 14Nov 20
16°FOct 25Nov 1Nov 4Nov 8Nov 12Nov 16Nov 20Nov 23Nov 28

Growing season length

Days between the last spring and first fall freeze at each threshold, with the chance of a season at least that long.

Threshold10%20%30%40%50%60%70%80%90%
36°F150143138134130126123117109
32°F170164159155151146142137131
28°F193186180176171167162157149
24°F216209202198193188182176167
20°F240231224218214208203198188
16°F265256248241236230224217208

How often it gets that cold

The share of years with at least one night at or below each temperature.

36°F or colder
100% of years
32°F or colder
100% of years
28°F or colder
100% of years
24°F or colder
100% of years
20°F or colder
100% of years
16°F or colder
100% of years

Degree days

A degree day is one degree of the daily mean temperature beyond a base, for one day. Heating degree days count cold (the fuel a house burns), cooling degree days heat, growing degree days the warmth a crop needs; each crop has its own base.

Heating degree days, base 65°F
5,511
Cooling degree days, base 65°F
625
Growing degree days, base 40°F
5,140
Growing degree days, base 45°F
3,879
Growing degree days, base 50°F
2,812
Growing degree days, base 55°F
1,920
Growing degree days, base 60°F
1,191

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January95304
February83206
March682036
April491185
May26517250
June56110504
July6238697
August12190643
September11265410
October4215140
November711034
December97004

What this means

NOAA computes these from the daily lows at the station over 1991-2020, fitting the dates of each year's last and first freeze to a distribution, so a date can be stated at any level of risk. 36°F is a frost risk for tender plants on a clear, calm night, since the ground cools below the air above it; 32°F is a freeze; 28°F is a hard freeze that kills most annuals.

The station is 2.6 km from Boulder. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

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Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00050848 (Boulder). Public domain.