USDA Hardiness Zone 9b
Frost Dates for 91901 — Alpine, California
NOAA 1991–2020 normals from Alpine, CA (1,695 ft), 2 miles west.
Updated
In Alpine, the last spring frost (32°F) is around February 12 in half of years and has passed by March 20 in nine years out of ten. The first fall frost comes around December 24, as early as December 2 one year in ten, leaving a typical freeze-free season of 317 days (NOAA, Alpine, CA).
- Last spring frost
- February 12
- First fall frost
- December 24
- Growing season
- 317 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
- 5%
- Snow per year
- —
median, 1991–2020
median, 1991–2020
1 of 21 observed
NOAA normal
Frost risk by date in Alpine
Half of springs at Alpine, CA see their last 32°F night by February 12; only one in ten is past it by December 29, and one in ten still has one after March 20. Planting tender crops after February 26 leaves a 30% risk of a frost on them, after March 4 20%, and after March 20 10%. The 36 days between the even-odds date and the one-in-ten date is wider than at 75% 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 24 in half of years, as early as December 2 one year in ten, and holds off past February 3 one year in ten. A hard freeze of 28°F is less common: it came in 38% of winters. The freeze-free season lasts 317 days in a typical year, longer than at 73% of the 219 California stations; it reaches 364 days one year in ten and falls short of 272 one year in ten. On average 3 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 33 days longer, to about March 17, and return around December 7. A 24°F night is unusual: 6% of winters had one.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Feb 15 | Feb 25 | Mar 3 | Mar 11 | Mar 17 | Mar 24 | Mar 31 | Apr 7 | Apr 16 |
| 32°F90% of yrs | Dec 29 | Jan 10 | Jan 27 | Feb 4 | Feb 12 | Feb 19 | Feb 26 | Mar 4 | Mar 20 |
| 28°F38% of yrs | too rare for a date | ||||||||
| 24°F6% of yrs | too rare for a date | ||||||||
| 20°F1% of yrs | too rare for a date | ||||||||
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Nov 15 | Nov 24 | Nov 29 | Dec 2 | Dec 7 | Dec 10 | Dec 13 | Dec 17 | Dec 23 |
| 32°F90% of yrs | Dec 2 | Dec 10 | Dec 14 | Dec 19 | Dec 24 | Dec 28 | Jan 1 | Jan 19 | Feb 3 |
| 28°F38% of yrs | too rare for a date | ||||||||
| 24°F6% of yrs | too rare for a date | ||||||||
| 20°F1% of yrs | too rare for a date | ||||||||
Freeze probabilities 1991–2020 at Alpine, 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 91901 →Compare Alpine with another place →
Weather by month in Alpine
August is the warmest month at Alpine, CA, averaging 89°F by day and 62°F at night; December is the coldest at 63°F and 42°F. From April to November the chance of a 32°F night stays under 10% each month. In spring, February still brings one in 46% of years and March still brings one in 16% of years.
Warmth for growing adds up to 4,757 growing degree days a year (base 50°F), more than at 57% of the 283 California stations. 35% of them (1,650) have built up by the end of June, and August adds the most, 792. Precipitation totals 16.1 inches a year, most in February (3.7") and least in June (0.1"); May to September bring 7% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 64 | 43 | 3.2 | 51% | 1 | 135 |
| Feb | 64 | 42 | 3.7 | 46% | 1 | 123 |
| Mar | 67 | 44 | 2.7 | 16% | <1 | 197 |
| Apr | 71 | 47 | 1.2 | 6% | <1 | 271 |
| May | 74 | 51 | 0.6 | 0% | 0 | 391 |
| Jun | 81 | 54 | 0.1 | 0% | 0 | 533 |
| Jul | 87 | 60 | 0.1 | 0% | 0 | 727 |
| Aug | 89 | 62 | 0.1 | 0% | 0 | 792 |
| Sep | 86 | 60 | 0.3 | 0% | 0 | 693 |
| Oct | 78 | 54 | 0.6 | 0% | 0 | 501 |
| Nov | 71 | 48 | 1.5 | 7% | <1 | 280 |
| Dec | 63 | 42 | 2.0 | 59% | 1 | 114 |
Monthly normals 1991–2020 at Alpine, CA (1,695 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, El Cajon, CA, 11 miles west of Alpine and 1,200 feet lower, has a last frost 37 days earlier (January 6), a first frost 3 days earlier (December 21), a season 33 days longer.
When it snows in Alpine, California
Measurable snow is rare here: only 1 of 21 winters from 1991 to 2020 recorded 0.1 inch or more at Alpine, CA, 2.4 miles away, so there is no typical first-snow date to plan around.
Planting calendar for 91901
| 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 October 1 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by December 10 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| December 24 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 91901 is 2 miles from Alpine, CA, which stands at 1,695 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. The 2023 USDA Plant Hardiness Zone Map lists ZIP 91901 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.