USDA Hardiness Zone 9b
Frost Dates for 94558 — Napa, California
NOAA 1991–2020 normals from Napa State Hospital, CA (35 ft), 5 miles southeast.
Updated
In Napa, the last spring frost (32°F) is around February 12 in half of years and has passed by March 13 in nine years out of ten. The first fall frost comes around December 4, as early as November 15 one year in ten, leaving a typical freeze-free season of 299 days (NOAA, Napa State Hospital, CA).
- Last spring frost
- February 12
- First fall frost
- December 4
- Growing season
- 299 days
- Hardiness zone
- 9b
50% probability
50% probability
freeze-free, 50%
25 to 30°F (-3.9 to -1.1°C) coldest night
- First snow
- Rare
- Last snow
- Rare
- Winters with snow
- 0%
- Snow per year
- 0"
median, 1991–2020
median, 1991–2020
0 of 20 observed
NOAA normal
Frost risk by date in Napa
Half of springs at Napa State Hospital, CA see their last 32°F night by February 12; only one in ten is past it by January 3, and one in ten still has one after March 13. Planting tender crops after February 27 leaves a 30% risk of a frost on them, after March 3 20%, and after March 13 10%. The 29 days between the even-odds date and the one-in-ten date is narrower than at 52% of the 220 California stations behind this site's ZIP pages, so a fixed calendar date is a gamble here.
In fall the first 32°F night comes by December 4 in half of years, as early as November 15 one year in ten, and holds off past January 4 one year in ten. A hard freeze of 28°F follows 15 days later, around December 19; in spring the one-in-ten date for 28°F is February 13, 28 days before the 32°F one. The freeze-free season lasts 299 days in a typical year, longer than at 64% of the 219 California stations; it reaches 338 days one year in ten and falls short of 257 one year in ten. On average 13 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 43 days longer, to about March 27, and return around November 17. A 24°F night is unusual: 22% of winters had one.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Feb 26 | Mar 6 | Mar 14 | Mar 21 | Mar 27 | Mar 31 | Apr 5 | Apr 11 | Apr 19 |
| 32°F98% of yrs | Jan 3 | Jan 18 | Jan 29 | Feb 5 | Feb 12 | Feb 20 | Feb 27 | Mar 3 | Mar 13 |
| 28°F71% of yrs | Dec 16 | Dec 24 | Dec 29 | Jan 3 | Jan 11 | Jan 17 | Jan 26 | Feb 1 | Feb 13 |
| 24°F22% of yrs | too rare for a date | ||||||||
| 20°F0% of yrs | too rare for a date | ||||||||
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Oct 31 | Nov 4 | Nov 10 | Nov 13 | Nov 17 | Nov 22 | Nov 26 | Nov 30 | Dec 4 |
| 32°F98% of yrs | Nov 15 | Nov 23 | Nov 28 | Dec 1 | Dec 4 | Dec 7 | Dec 13 | Dec 20 | Jan 4 |
| 28°F71% of yrs | Dec 1 | Dec 5 | Dec 9 | Dec 14 | Dec 19 | Dec 26 | Jan 1 | Jan 8 | Jan 23 |
| 24°F22% of yrs | too rare for a date | ||||||||
| 20°F0% of yrs | too rare for a date | ||||||||
Freeze probabilities 1991–2020 at Napa State Hospital, CA, from NOAA NCEI U.S. Climate Normals. Where a temperature is reached in fewer than 99.5% of years, the dates describe only the years that reach it. Not a forecast.
Frost risk on any date in 94558 →Compare Napa with another place →
Weather by month in Napa
August is the warmest month at Napa State Hospital, CA, averaging 84°F by day and 56°F at night; December is the coldest at 59°F and 40°F. From April to October the chance of a 32°F night stays under 10% each month. In spring, February still brings one in 55% of years and March still brings one in 21% of years.
Warmth for growing adds up to 3,959 growing degree days a year (base 50°F), fewer than at 61% of the 283 California stations. 39% of them (1,549) have built up by the end of June, and August adds the most, 608. Precipitation totals 26.7 inches a year, most in December (5.5") and least in July (0.0"); May to September bring 6% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 59 | 40 | 5.0 | 72% | 5 | 50 |
| Feb | 63 | 42 | 5.1 | 55% | 2 | 98 |
| Mar | 68 | 44 | 3.7 | 21% | <1 | 183 |
| Apr | 72 | 46 | 1.7 | 3% | <1 | 267 |
| May | 77 | 50 | 1.1 | 0% | 0 | 411 |
| Jun | 82 | 54 | 0.3 | 0% | 0 | 540 |
| Jul | 83 | 56 | 0.0 | 0% | 0 | 601 |
| Aug | 84 | 56 | 0.1 | 0% | 0 | 608 |
| Sep | 84 | 53 | 0.1 | 0% | 0 | 567 |
| Oct | 78 | 49 | 1.3 | 1% | 0 | 425 |
| Nov | 66 | 44 | 2.8 | 34% | 1 | 163 |
| Dec | 59 | 40 | 5.5 | 78% | 5 | 47 |
Monthly normals 1991–2020 at Napa State Hospital, CA (35 ft), NOAA NCEI. "32°F night": chance of at least one in the month. GDD: growing degree days, base 50°F.
Compared with the next station
The nearest other station, Napa Co Airport, CA, 9 miles south of Napa, has a last frost 21 days later (March 5), a first frost 13 days earlier (November 21), a season 43 days shorter.
When it snows in Napa, California
Measurable snow is rare here: only 0 of 20 winters from 1991 to 2020 recorded 0.1 inch or more at Napa State Hospital, CA, 4.5 miles away, so there is no typical first-snow date to plan around.
Planting calendar for 94558
| When | What to do |
|---|---|
| December 4 – December 18 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| December 18 – January 1 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| January 15 – January 29 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| February 12 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by September 11 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by November 20 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| December 4 onward | Plant garlic, mulch beds, lift tender bulbsGarlic wants a cold spell; bare soil loses structure over winter. |
The weeks before and after the 50% dates are general rules of thumb, not NOAA figures; a seed packet gives the timing for each variety.
Where these figures come from
ZIP 94558 is 5 miles from Napa State Hospital, CA, which stands at 35 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 1 other ZIP code shares the station. The 2023 USDA Plant Hardiness Zone Map lists ZIP 94558 in zone 9b, the band where the coldest night of the year averaged 25 to 30°F (-3.9 to -1.1°C) over 1991–2020; a zone describes winter lows, not frost dates.
Frost dates across California →
Hardiness zone and temperature range: USDA Plant Hardiness Zone Map 2023 (USDA-ARS, Oregon State University), ZIP-code listing, unchanged. °C converted from °F by this site.
Thirty-year averages, not a forecast: check the forecast before planting out. How stations are matched and which pages are offered to search engines: methodology.