Frost dates for Napa State Hospital, CA
At Napa State Hospital, the median last spring frost (32°F) is Feb 12 and the median first fall frost is Dec 4, for a typical growing season of 299 days. For a 90% chance that the last frost has passed, wait until Mar 13. Nearby ZIP codes are in USDA hardiness zone 9b.
- Feb 12Last spring frost (median)50% chance of 32°F after this date
- Mar 13Safer planting date10% chance of 32°F after this date
- Dec 4First fall frost (median)50% chance of 32°F by this date
- 299 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 | Feb 26 | Mar 27 | Apr 19 |
| 32°F | Jan 3 | Feb 12 | Mar 13 |
| 28°F | Dec 16 | Jan 11 | Feb 13 |
| 24°F | Dec 7 | Dec 30 | Jan 22 |
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 31 | Nov 17 | Dec 4 |
| 32°F | Nov 15 | Dec 4 | Jan 4 |
| 28°F | Dec 1 | Dec 19 | Jan 23 |
| 24°F | Dec 5 | Dec 25 | Jan 19 |
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 | 206 days | 236 days | 272 days |
| 32°F | 257 days | 299 days | 338 days |
| 28°F | 316 days | 348 days | 365 days |
| 24°F | 351 days | 365 days | 365 days |
Chance of a freeze in any year
| Temperature | Chance per year |
|---|---|
| 36°F | 100% |
| 32°F | 98% |
| 28°F | 71% |
| 24°F | 22% |
ZIP codes whose closest station is this one
| ZIP | Hardiness zone | Avg. extreme low | Distance |
|---|---|---|---|
| 94559 | 9b | 25 to 30°F | 4 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 USC00046074, at 38.2778, -122.2647, elevation 11 m (35 ft).
- Data quality: Standard (24+ years of data), 28 years of freeze data.
- Average days per year at or below 32°F: 12.
- Frost risk changes over short distances. Valleys and low spots freeze earlier than hillsides and city centers.
Nearby weather stations
- Napa Co Airport (8 km)
- Fairfield (17 km)
- Sonoma (17 km)
- Vallejo (21 km)
- Markley Cove (27 km)
- Vacaville Nut Tree Airport (29 km)
- Vacaville (30 km)
- Petaluma Airport (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.