Last frost date in Boulder, CO
Expect the last frost in Boulder around May 6. Frost after May 18 happens in only one year out of ten. The first fall frost usually arrives about October 5, which leaves 151 frost-free days. An early freeze before September 21 happens one year in ten.
- Safe after (10% risk)
- May 18
- Average first frost
- Oct 5
- Frost-free season
- 151 days
- Hard freeze (28°F) ends
- Apr 27
No frost in the next 7 nights
Later than usual: the average first frost here is October 5.
National Weather Service forecast, updated Oct 22, 12:57 UTC
- ThuOct 2253°
- FriOct 2347°
- SatOct 2439°
- SunOct 2539°
- MonOct 2637°
- TueOct 2739°
- WedOct 2840°
Planting calendar for Boulder
Windows count forward or back from the average frost dates above. Click a crop for dates across Colorado.
TomatoesMay 13–May 27
PumpkinsMay 20–Jun 3
SunflowersMay 6–May 27
CucumbersMay 20–May 27
CornMay 13–May 27
Sweet potatoesMay 27–Jun 10
WatermelonMay 20–Jun 3
Green beansMay 13–Jun 3
ZucchiniMay 13–May 27
OkraMay 27–Jun 10
PeppersMay 20–Jun 3
BasilMay 13–May 27
Butternut squashMay 13–May 27
CantaloupeMay 20–Jun 3
EggplantMay 20–Jun 3
PotatoesApr 8–Apr 22
OnionsMar 25–Apr 15
CarrotsApr 15–May 20
LettuceApr 22–May 13
BroccoliApr 8–Apr 22
BeetsApr 8–May 20
PeasMar 25–Apr 8
CabbageApr 8–Apr 22
RadishesApr 1–May 13
Brussels sproutsApr 22–May 6
SpinachMar 25–Apr 15
CilantroApr 22–May 13
KaleApr 8–Apr 22
Collard greensApr 8–Apr 22
TurnipsApr 8–Apr 22
DillMay 6–May 20
CeleryMay 13–May 20
CauliflowerApr 22–May 6
LeeksApr 22–May 6
Swiss chardApr 22–May 20
ParsleyApr 22–May 13
Tulip bulbsSep 21–Oct 26
GarlicSep 14–Oct 19
Daffodil bulbsSep 7–Oct 19
StrawberriesApr 8–Apr 22
AsparagusApr 8–Apr 22Frost probabilities
| Threshold | Spring, 90% chance later | Spring, 50% | Spring, 10% | Fall, 10% chance earlier | Fall, 50% | Fall, 90% |
|---|---|---|---|---|---|---|
| Light frost, 32°F | Apr 24 | May 6 | May 18 | Sep 21 | Oct 5 | Oct 18 |
| Hard freeze, 28°F | Apr 10 | Apr 27 | May 10 | Sep 30 | Oct 15 | Oct 30 |
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 Boulder (USC00050848) sits 2.4 miles from the center of Boulder at 5,484 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 Boulder, CO?
The average last frost date in Boulder is May 6, based on NOAA 1991–2020 climate normals. There is a 10% chance of frost after May 18.
When is the first frost in Boulder?
The first frost in fall usually comes around October 5. One year in ten it arrives before September 21.
How long is the growing season in Boulder?
About 151 days between the average last and first frost.
When should I plant tomatoes in Boulder?
Plant tomato seedlings outside from May 13 to May 27. Start seeds indoors six to eight weeks earlier.
Nearby in Colorado
- LouisvilleMay 6
- LafayetteMay 2
- ErieMay 7
- BroomfieldMay 2
- LongmontMay 7
- WestminsterMay 2
- ArvadaMay 5
- NorthglennMay 2
- ThorntonMay 2
- FirestoneMay 7
- Wheat RidgeMay 5
- LakewoodMay 3