Frost dates for Boulder, CO
At Boulder, the median last spring frost (32°F) is May 6 and the median first fall frost is Oct 5, for a typical growing season of 151 days. For a 90% chance that the last frost has passed, wait until May 18. Nearby ZIP codes are in USDA hardiness zone 6a.
- May 6Last spring frost (median)50% chance of 32°F after this date
- May 18Safer planting date10% chance of 32°F after this date
- Oct 5First fall frost (median)50% chance of 32°F by this date
- 151 daysGrowing season (median)between 32°F freezes
Last spring frost dates
The chance that the temperature drops to the threshold after each date.
| Temperature | 90% chance later | 50% chance later | 10% chance later |
|---|---|---|---|
| 36°F | May 5 | May 17 | Jun 2 |
| 32°F | Apr 24 | May 6 | May 18 |
| 28°F | Apr 10 | Apr 27 | May 10 |
| 24°F | Mar 29 | Apr 14 | May 2 |
First fall frost dates
The chance that the temperature has dropped to the threshold by each date.
| Temperature | 10% chance by | 50% chance by | 90% chance by |
|---|---|---|---|
| 36°F | Sep 13 | Sep 28 | Oct 10 |
| 32°F | Sep 21 | Oct 5 | Oct 18 |
| 28°F | Sep 30 | Oct 15 | Oct 30 |
| 24°F | Oct 7 | Oct 25 | Nov 9 |
Growing season length
Days between the last spring and first fall freeze, as the length reached in 9, 5 and 1 out of 10 years.
| Temperature | 9 in 10 years at least | 5 in 10 years at least | 1 in 10 years at least |
|---|---|---|---|
| 36°F | 109 days | 130 days | 150 days |
| 32°F | 131 days | 151 days | 170 days |
| 28°F | 149 days | 171 days | 193 days |
| 24°F | 167 days | 193 days | 216 days |
Chance of a freeze in any year
| Temperature | Chance per year |
|---|---|
| 36°F | 100% |
| 32°F | 100% |
| 28°F | 100% |
| 24°F | 100% |
ZIP codes whose closest station is this one
| ZIP | Hardiness zone | Avg. extreme low | Distance |
|---|---|---|---|
| 80310 | 6a | -10 to -5°F | 1 km |
| 80305 | 6a | -10 to -5°F | 2 km |
| 80303 | 6a | -10 to -5°F | 5 km |
| 80304 | 6a | -10 to -5°F | 6 km |
| 80302 | 6a | -10 to -5°F | 8 km |
| 80301 | 6a | -10 to -5°F | 8 km |
| 80027 | 6a | -10 to -5°F | 10 km |
| 80026 | 6a | -10 to -5°F | 14 km |
Not the official USDA Plant Hardiness Zone Map. Zones shown here are the values published in the USDA-ARS / Oregon State University PRISM Group 2023 ZIP code tables, shown unaltered. For the official interactive map, visit planthardiness.ars.usda.gov.
About this station
- NOAA station ID USC00050848, at 39.9919, -105.2667, elevation 1672 m (5,484 ft).
- Data quality: Standard (24+ years of data), 26 years of freeze data.
- Average days per year at or below 32°F: 138.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Gross Rsvr (9 km)
- Coal Creek Canyon (15 km)
- Ralston Rsvr (18 km)
- Boulder 14 W (24 km)
- Northglenn (24 km)
- Longmont 2 ESE (25 km)
- Wheat Ridge 2 (27 km)
- Lakewood (30 km)
Sources
- NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020, annual/seasonal normals by station (version 1.0.1), file dated June 27, 2023; documentation.
- USDA Agricultural Research Service and PRISM Group, Oregon State University, 2023 Plant Hardiness Zone Map ZIP code tables (CONUS, Alaska, Hawaii, Puerto Rico), files dated November 17, 2023. Data © PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed November 1, 2026. Official map: planthardiness.ars.usda.gov.
- U.S. Census Bureau, 2024 Gazetteer ZIP Code Tabulation Areas file, dated August 30, 2024 (ZIP centroids used to find the nearest station).
Freeze dates are 30-year probabilities, not forecasts. Check your local forecast before planting tender plants.