Frost dates for Portola, CA
At Portola, the median last spring frost (32°F) is Jun 11 and the median first fall frost is Sep 10, for a typical growing season of 91 days. For a 90% chance that the last frost has passed, wait until Jul 2. Nearby ZIP codes are in USDA hardiness zones 6b to 7a.
- Jun 11Last spring frost (median)50% chance of 32°F after this date
- Jul 2Safer planting date10% chance of 32°F after this date
- Sep 10First fall frost (median)50% chance of 32°F by this date
- 91 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 |
|---|---|---|---|
| 32°F | May 25 | Jun 11 | Jul 2 |
| 28°F | Apr 28 | May 17 | Jun 4 |
| 24°F | Apr 7 | Apr 24 | May 14 |
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 |
|---|---|---|---|
| 32°F | Aug 22 | Sep 10 | Sep 28 |
| 28°F | Sep 13 | Oct 1 | Oct 18 |
| 24°F | Oct 3 | Oct 20 | Nov 3 |
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 | 24 days | 45 days | 71 days |
| 32°F | 62 days | 91 days | 115 days |
| 28°F | 110 days | 136 days | 163 days |
| 24°F | 153 days | 179 days | 201 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 |
|---|---|---|---|
| 96122 | 6b | -5 to 0°F | 2 km |
| 96106 | 7a | 0 to 5°F | 10 km |
| 96129 | 6b | -5 to 0°F | 10 km |
| 96103 | 7a | 0 to 5°F | 17 km |
| 96124 | 6b | -5 to 0°F | 19 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 USC00047085, at 39.8053, -120.4719, elevation 1478 m (4,850 ft).
- Data quality: Standard (24+ years of data), 29 years of freeze data.
- Average days per year at or below 32°F: 205.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Doyle 4 SSE (38 km)
- Downieville (40 km)
- Bowman Dam (42 km)
- Quincy (43 km)
- Sagehen Creek (46 km)
- Verdi 2w (49 km)
- Lake Spaulding (56 km)
- Boca (57 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.