Last spring frost
Jun 11
Frost Dates for Portola, CA
First fall frost
Sep 10
Growing season
91 days
Hardiness zone
6b
In an average year around Portola, CA, the last spring frost (32°F) is about Jun 11 and the first fall frost is about Sep 10, giving roughly a 91-day growing season. To be safe most years, wait until May 25 before setting out tender plants.
Last spring frost
Dates the last frost of spring typically ends. Plant tender crops after the risk has passed for your comfort level.
| Frost intensity | Early (10%) | Average (50%) | Late (90%) |
|---|---|---|---|
| Freeze (32°F) | Jul 2 | Jun 11 | May 25 |
| Hard (28°F) | Jun 4 | May 17 | Apr 28 |
| Severe (24°F) | May 14 | Apr 24 | Apr 7 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 32°F | Jul 2 | Jun 23 | Jun 18 | Jun 14 | Jun 11 | Jun 7 | Jun 4 | Jun 1 | May 25 |
| 28°F | Jun 4 | May 31 | May 26 | May 21 | May 17 | May 12 | May 8 | May 3 | Apr 28 |
| 24°F | May 14 | May 6 | May 1 | Apr 27 | Apr 24 | Apr 20 | Apr 17 | Apr 12 | Apr 7 |
| 20°F | Apr 26 | Apr 19 | Apr 13 | Apr 9 | Apr 5 | Apr 2 | Mar 29 | Mar 21 | Mar 13 |
| 16°F | Apr 6 | Mar 31 | Mar 25 | Mar 19 | Mar 14 | Mar 10 | Mar 5 | Mar 1 | Feb 21 |
First fall frost
Dates the first frost of fall typically arrives. Count back by a crop's days-to-maturity to know your planting cutoff.
| Frost intensity | Early (10%) | Average (50%) | Late (90%) |
|---|---|---|---|
| Freeze (32°F) | Aug 22 | Sep 10 | Sep 28 |
| Hard (28°F) | Sep 13 | Oct 1 | Oct 18 |
| Severe (24°F) | Oct 3 | Oct 20 | Nov 3 |
Show full probability table (all thresholds & percentiles)
| °F | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 32°F | Aug 22 | Aug 29 | Sep 3 | Sep 7 | Sep 10 | Sep 13 | Sep 17 | Sep 21 | Sep 28 |
| 28°F | Sep 13 | Sep 20 | Sep 25 | Sep 29 | Oct 1 | Oct 5 | Oct 8 | Oct 13 | Oct 18 |
| 24°F | Oct 3 | Oct 8 | Oct 12 | Oct 16 | Oct 20 | Oct 23 | Oct 26 | Oct 29 | Nov 3 |
| 20°F | Oct 15 | Oct 23 | Oct 27 | Nov 1 | Nov 3 | Nov 6 | Nov 9 | Nov 13 | Nov 19 |
| 16°F | Oct 30 | Nov 6 | Nov 10 | Nov 13 | Nov 16 | Nov 20 | Nov 24 | Nov 27 | Dec 3 |
Growing season is measured at 32°F with about 50% probability. Hardiness zone from the 2023 USDA map; frost normals from NOAA 1991-2020.
Where this data comes from
@endif These dates are computed from the nearest official weather station with long-term records: Portola, Ca, about 1.1 miles away@if(!empty($station['elevation_ft'])), at 4,850 ft elevation. The further the station and the greater the elevation difference from your yard, the more your local frost timing can vary.
How we calculate these dates
Frost and freeze dates come from the NOAA 1991–2020 U.S. Climate Normals, the National Oceanic and Atmospheric Administration's 30-year averages for each weather station. A date shown at the 50% level is the median: in half of years the frost falls on or before it. The 10% and 90% columns show the early and late ends of the range, so you can plant on the odds you're comfortable with. These are long-term averages, not a forecast for this year - always check your local outlook before planting.
Source: NOAA National Centers for Environmental Information, 1991–2020 Climate Normals (public domain). Harman is not affiliated with or endorsed by NOAA.
Frequently asked questions
When is the last frost in Portola, CA?
On average, the last spring frost (32°F) around Portola, CA is about Jun 11. In cooler years it can run later, so for a safer bet many gardeners wait until May 25.
When is the first frost in the fall?
The first fall frost around Portola, CA typically arrives about Sep 10. Count back by a crop's days to maturity to find the latest date you can plant it.
When can I plant tomatoes and other tender crops?
Tender crops like tomatoes, peppers and basil are damaged by frost, so wait until the last spring frost has passed - around Jun 11 on average, or May 25 to be safe most years.
How long is the growing season here?
The frost-free growing season (between the last spring and first fall 32°F frost) is about 91 days in a typical year.
Are these dates a forecast for this year?
No. They are 30-year averages from the NOAA 1991–2020 Climate Normals, not a prediction for the current year. Use them for planning and always check your local weather forecast before planting or protecting crops.
Local weather for 96122 Planting calendar → California frost dates →
Data source: NOAA 1991–2020 Climate Normals · Page updated Aug 4, 2026