Frost dates for Mount Hamilton, CA
At Mount Hamilton, the median last spring frost (32°F) is May 3 and the median first fall frost is Nov 18, for a typical growing season of 197 days. For a 90% chance that the last frost has passed, wait until Jun 5. Nearby ZIP codes are in USDA hardiness zone 9b.
- May 3Last spring frost (median)50% chance of 32°F after this date
- Jun 5Safer planting date10% chance of 32°F after this date
- Nov 18First fall frost (median)50% chance of 32°F by this date
- 197 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 | Apr 28 | May 27 | Jun 18 |
| 32°F | Apr 7 | May 3 | Jun 5 |
| 28°F | Feb 23 | Apr 10 | May 6 |
| 24°F | Dec 31 | Mar 1 | Apr 11 |
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 | Oct 1 | Oct 29 | Nov 23 |
| 32°F | Oct 22 | Nov 18 | Dec 12 |
| 28°F | Nov 10 | Dec 7 | Jan 18 |
| 24°F | Nov 28 | Jan 1 | Mar 2 |
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 | 121 days | 155 days | 192 days |
| 32°F | 157 days | 197 days | 235 days |
| 28°F | 202 days | 241 days | 294 days |
| 24°F | 252 days | 305 days | 365 days |
Chance of a freeze in any year
| Temperature | Chance per year |
|---|---|
| 36°F | 100% |
| 32°F | 100% |
| 28°F | 100% |
| 24°F | 86% |
ZIP codes whose closest station is this one
| ZIP | Hardiness zone | Avg. extreme low | Distance |
|---|---|---|---|
| 95140 | 9b | 25 to 30°F | 1 km |
| 95138 | 9b | 25 to 30°F | 14 km |
| 95013 | 9b | 25 to 30°F | 17 km |
| 95135 | 9b | 25 to 30°F | 17 km |
| 95139 | 9b | 25 to 30°F | 17 km |
| 94550 | 9b | 25 to 30°F | 20 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 USC00045933, at 37.3436, -121.6425, elevation 1282 m (4,206 ft).
- Data quality: Standard (24+ years of data), 30 years of freeze data.
- Average days per year at or below 32°F: 45.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
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.