USDA Hardiness Zone 9b
Frost Dates for 94507 — Alamo, California
NOAA 1991–2020 normals from Mt Diablo Junction, CA (2,170 ft), 5 miles east.
Updated
In Alamo, the last spring frost (32°F) is around March 8 in half of years and has passed by April 15 in nine years out of ten. The first fall frost comes around December 22, as early as November 23 one year in ten, leaving a typical freeze-free season of 282 days (NOAA, Mt Diablo Junction, CA).
- Last spring frost
- March 8
- First fall frost
- December 22
- Growing season
- 282 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
- 36%
- Snow per year
- 0.7"
median, 1991–2020
median, 1991–2020
8 of 22 observed
NOAA normal
Frost risk by date in Alamo
Half of springs at Mt Diablo Junction, CA see their last 32°F night by March 8; only one in ten is past it by January 12, and one in ten still has one after April 15. Planting tender crops after March 28 leaves a 30% risk of a frost on them, after April 5 20%, and after April 15 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 22 in half of years, as early as November 23 one year in ten, and holds off past February 11 one year in ten. A hard freeze of 28°F is less common: it came in 50% of winters. The freeze-free season lasts 282 days in a typical year, longer than at 54% of the 219 California stations; it reaches 343 days one year in ten and falls short of 238 one year in ten. On average 19 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 42 days longer, to about April 19, and return around November 29. A 24°F night is unusual: 7% of winters had one.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Mar 16 | Mar 30 | Apr 7 | Apr 13 | Apr 19 | Apr 25 | May 1 | May 10 | May 20 |
| 32°F96% of yrs | Jan 12 | Jan 31 | Feb 18 | Feb 27 | Mar 8 | Mar 19 | Mar 28 | Apr 5 | Apr 15 |
| 28°F50% of yrs | too rare for a date | ||||||||
| 24°F7% of yrs | too rare for a date | ||||||||
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Nov 6 | Nov 14 | Nov 19 | Nov 25 | Nov 29 | Dec 2 | Dec 6 | Dec 12 | Dec 20 |
| 32°F96% of yrs | Nov 23 | Dec 1 | Dec 9 | Dec 15 | Dec 22 | Jan 1 | Jan 8 | Jan 22 | Feb 11 |
| 28°F50% of yrs | too rare for a date | ||||||||
| 24°F7% of yrs | too rare for a date | ||||||||
Freeze probabilities 1991–2020 at Mt Diablo Junction, 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 94507 →Compare Alamo with another place →
Weather by month in Alamo
July is the warmest month at Mt Diablo Junction, CA, averaging 83°F by day and 62°F at night; December is the coldest at 53°F and 41°F. From May to October the chance of a 32°F night stays under 10% each month. In spring, February still brings one in 51% of years and March still brings one in 41% of years and April still brings one in 23% of years.
Warmth for growing adds up to 3,645 growing degree days a year (base 50°F), fewer than at 69% of the 283 California stations. 30% of them (1,104) have built up by the end of June, and July adds the most, 682. Precipitation totals 26.1 inches a year, most in January (5.1") 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 | 54 | 42 | 5.1 | 60% | 4 | 54 |
| Feb | 55 | 42 | 4.9 | 51% | 4 | 57 |
| Mar | 58 | 43 | 3.7 | 41% | 3 | 97 |
| Apr | 62 | 44 | 1.9 | 23% | 2 | 145 |
| May | 68 | 49 | 1.1 | 1% | 0 | 280 |
| Jun | 76 | 55 | 0.3 | 0% | 0 | 471 |
| Jul | 83 | 62 | 0.0 | 0% | 0 | 682 |
| Aug | 83 | 61 | 0.0 | 0% | 0 | 679 |
| Sep | 80 | 60 | 0.1 | 0% | 0 | 596 |
| Oct | 72 | 54 | 1.2 | 0% | 0 | 403 |
| Nov | 60 | 46 | 2.7 | 17% | 2 | 143 |
| Dec | 53 | 41 | 5.1 | 49% | 5 | 39 |
Monthly normals 1991–2020 at Mt Diablo Junction, CA (2,170 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, Concord Wwtp, CA, 9 miles north of Alamo and 2,130 feet lower, has a last frost 60 days earlier (January 7), a first frost 3 days earlier (December 19), a season 64 days longer.
When it snows in Alamo, California
Measurable snow is rare here: only 8 of 22 winters from 1991 to 2020 recorded 0.1 inch or more at Mt Diablo Junction, CA, 5.4 miles away, so there is no typical first-snow date to plan around.
Planting calendar for 94507
| When | What to do |
|---|---|
| December 28 – January 11 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| January 11 – January 25 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| February 8 – February 22 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| March 8 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by September 29 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by December 8 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| December 22 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 94507 is 5 miles from Mt Diablo Junction, CA, which stands at 2,170 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 5 other ZIP codes share the station; its figures are presented in full for 94583 (San Ramon). The 2023 USDA Plant Hardiness Zone Map lists ZIP 94507 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.