Last frost date in Thornton, CO

Average last frost
May 2

Expect the last frost in Thornton around May 2. Frost after May 14 happens in only one year out of ten. The first fall frost usually arrives about October 11, which leaves 160 frost-free days. An early freeze before September 26 happens one year in ten.

Safe after (10% risk)
May 14
Average first frost
Oct 11
Frost-free season
160 days
Hard freeze (28°F) ends
Apr 20
Next 7 nights in Thornton

No frost in the next 7 nights

Later than usual: the average first frost here is October 11.

National Weather Service forecast, updated Oct 22, 12:24 UTC

  1. ThuOct 2256°
  2. FriOct 2349°
  3. SatOct 2440°
  4. SunOct 2540°
  5. MonOct 2637°
  6. TueOct 2738°
  7. WedOct 2840°
JanFebMarAprMayJunJulAugSepOctNovDecLast frost May 2First frost Oct 11160 frost-free daysBlue: chance of 32°F frost. Green: average frost-free season.JFMAMJJASOND160 frost-free days
Frost risk through the year at Northglenn, 4.1 miles from Thornton.

Planting calendar for Thornton

Windows count forward or back from the average frost dates above. Click a crop for dates across Colorado.

Start indoorsPlant outSow outdoorsFall plantingAverage last and first frost
Warm-season crops
JanFebMarAprMayJunJulAugSepOctNovDec
TomatoesMay 9–May 23
PumpkinsMay 16–May 30
SunflowersMay 2–May 23
CucumbersMay 16–May 23
CornMay 9–May 23
Sweet potatoesMay 23–Jun 6
WatermelonMay 16–May 30
Green beansMay 9–May 30
ZucchiniMay 9–May 23
OkraMay 23–Jun 6
PeppersMay 16–May 30
BasilMay 9–May 23
Butternut squashMay 9–May 23
CantaloupeMay 16–May 30
EggplantMay 16–May 30
Cool-season crops
JanFebMarAprMayJunJulAugSepOctNovDec
PotatoesApr 4–Apr 18
OnionsMar 21–Apr 11
CarrotsApr 11–May 16
LettuceApr 18–May 9
BroccoliApr 4–Apr 18
BeetsApr 4–May 16
PeasMar 21–Apr 4
CabbageApr 4–Apr 18
RadishesMar 28–May 9
Brussels sproutsApr 18–May 2
SpinachMar 21–Apr 11
CilantroApr 18–May 9
KaleApr 4–Apr 18
Collard greensApr 4–Apr 18
TurnipsApr 4–Apr 18
DillMay 2–May 16
CeleryMay 9–May 16
CauliflowerApr 18–May 2
LeeksApr 18–May 2
Swiss chardApr 18–May 16
ParsleyApr 18–May 9
Fall bulbs and garlic
JanFebMarAprMayJunJulAugSepOctNovDec
Tulip bulbsSep 27–Nov 1
GarlicSep 20–Oct 25
Daffodil bulbsSep 13–Oct 25
Perennials
JanFebMarAprMayJunJulAugSepOctNovDec
StrawberriesApr 4–Apr 18
AsparagusApr 4–Apr 18

Frost probabilities

ThresholdSpring, 90% chance laterSpring, 50%Spring, 10%Fall, 10% chance earlierFall, 50%Fall, 90%
Light frost, 32°FApr 18May 2May 14Sep 26Oct 11Oct 24
Hard freeze, 28°FApr 4Apr 20May 5Oct 7Oct 21Nov 4

Read the spring 10% column as the date after which only one year in ten still gets frost. That is the safer date for tomatoes, peppers and basil.

Where these numbers come from

NOAA station Northglenn (USC00055984) sits 4.1 miles from the center of Thornton at 5,407 feet. Its averages cover 1991 to 2020. Low spots, valleys and open fields can keep getting frost a week or two after the station stops; city centers and lakeshores usually get less of it.

Questions

When is the last frost in Thornton, CO?

The average last frost date in Thornton is May 2, based on NOAA 1991–2020 climate normals. There is a 10% chance of frost after May 14.

When is the first frost in Thornton?

The first frost in fall usually comes around October 11. One year in ten it arrives before September 26.

How long is the growing season in Thornton?

About 160 days between the average last and first frost.

When should I plant tomatoes in Thornton?

Plant tomato seedlings outside from May 9 to May 23. Start seeds indoors six to eight weeks earlier.

Nearby in Colorado

All 34 cities in Colorado