USDA Hardiness Zone 9b
Frost Dates for 94574 — Saint Helena, California
NOAA 1991–2020 normals from Saint Helena, CA (225 ft), under a mile west.
Updated
In Saint Helena, the last spring frost (32°F) is around February 3 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 14 one year in ten, leaving a typical freeze-free season of 306 days (NOAA, Saint Helena, CA).
- Last spring frost
- February 3
- First fall frost
- December 4
- Growing season
- 306 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 Saint Helena
Half of springs at Saint Helena, CA see their last 32°F night by February 3; only one in ten is past it by December 28, and one in ten still has one after March 13. Planting tender crops after February 22 leaves a 30% risk of a frost on them, after February 28 20%, and after March 13 10%. The 38 days between the even-odds date and the one-in-ten date is wider than at 81% 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 14 one year in ten, and holds off past January 1 one year in ten. A hard freeze of 28°F follows 13 days later, around December 17; in spring the one-in-ten date for 28°F is February 9, 32 days before the 32°F one. The freeze-free season lasts 306 days in a typical year, longer than at 67% of the 219 California stations; it reaches 341 days one year in ten and falls short of 260 one year in ten. On average 14 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 44 days longer, to about March 19, and return around November 18. A 24°F night is unusual: 15% of winters had one.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Feb 14 | Feb 27 | Mar 7 | Mar 13 | Mar 19 | Mar 27 | Apr 1 | Apr 9 | Apr 17 |
| 32°F97% of yrs | Dec 28 | Jan 7 | Jan 19 | Jan 28 | Feb 3 | Feb 11 | Feb 22 | Feb 28 | Mar 13 |
| 28°F65% of yrs | Dec 11 | Dec 19 | Dec 25 | Dec 31 | Jan 5 | Jan 11 | Jan 19 | Jan 30 | Feb 9 |
| 24°F15% of yrs | too rare for a date | ||||||||
| 20°F1% of yrs | too rare for a date | ||||||||
| 16°F1% of yrs | too rare for a date | ||||||||
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Oct 31 | Nov 5 | Nov 9 | Nov 13 | Nov 18 | Nov 21 | Nov 25 | Nov 30 | Dec 5 |
| 32°F97% of yrs | Nov 14 | Nov 21 | Nov 27 | Dec 1 | Dec 4 | Dec 8 | Dec 12 | Dec 18 | Jan 1 |
| 28°F65% of yrs | Nov 27 | Dec 3 | Dec 7 | Dec 12 | Dec 17 | Dec 22 | Dec 30 | Jan 5 | Jan 18 |
| 24°F15% of yrs | too rare for a date | ||||||||
| 20°F1% of yrs | too rare for a date | ||||||||
| 16°F1% of yrs | too rare for a date | ||||||||
Freeze probabilities 1991–2020 at Saint Helena, 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 94574 →Compare Saint Helena with another place →
Weather by month in Saint Helena
July is the warmest month at Saint Helena, CA, averaging 89°F by day and 56°F at night; December is the coldest at 58°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 44% of years and March still brings one in 14% of years.
Warmth for growing adds up to 4,262 growing degree days a year (base 50°F), fewer than at 55% of the 283 California stations. 38% of them (1,639) have built up by the end of June, and July adds the most, 693. Precipitation totals 34.1 inches a year, most in December (7.2") and least in July (0.0"); May to September bring 5% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 59 | 40 | 6.5 | 68% | 5 | 51 |
| Feb | 63 | 42 | 6.7 | 44% | 2 | 94 |
| Mar | 67 | 44 | 5.0 | 14% | <1 | 182 |
| Apr | 72 | 46 | 2.0 | 5% | <1 | 275 |
| May | 78 | 51 | 1.3 | 0% | 0 | 443 |
| Jun | 85 | 54 | 0.4 | 0% | 0 | 594 |
| Jul | 89 | 56 | 0.0 | 0% | 0 | 693 |
| Aug | 89 | 56 | 0.0 | 0% | 0 | 690 |
| Sep | 87 | 54 | 0.1 | 0% | 0 | 606 |
| Oct | 78 | 49 | 1.7 | 2% | <1 | 427 |
| Nov | 66 | 44 | 3.2 | 34% | 1 | 161 |
| Dec | 58 | 40 | 7.2 | 79% | 5 | 46 |
Monthly normals 1991–2020 at Saint Helena, CA (225 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, Angwin Pacific Union College, CA, 4 miles north of Saint Helena and 1,490 feet higher, has a last frost 63 days later (April 7), a first frost 1 day later (December 5), a season 61 days shorter.
When it snows in Saint Helena, California
Measurable snow is rare here: only 0 of 20 winters from 1991 to 2020 recorded 0.1 inch or more at Saint Helena, CA, 0.8 miles away, so there is no typical first-snow date to plan around.
Planting calendar for 94574
| When | What to do |
|---|---|
| November 25 – December 9 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| December 9 – December 23 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| January 6 – January 20 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| February 3 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 94574 is under a mile from Saint Helena, CA, which stands at 225 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 94574 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.