USDA Hardiness Zone 9b
Frost Dates for 93401 — San Luis Obispo, California
NOAA 1991–2020 normals from San Luis Obispo Airport, CA (200 ft), 2 miles south.
Updated
In San Luis Obispo, the last spring frost (32°F) is around January 31 in half of years and has passed by March 7 in nine years out of ten. The first fall frost comes around December 11, as early as November 20 one year in ten, leaving a typical freeze-free season of 319 days (NOAA, San Luis Obispo Airport, CA).
- Last spring frost
- January 31
- First fall frost
- December 11
- Growing season
- 319 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 19 observed
NOAA normal
Frost risk by date in San Luis Obispo
Half of springs at San Luis Obispo Airport, CA see their last 32°F night by January 31; only one in ten is past it by December 23, and one in ten still has one after March 7. Planting tender crops after February 17 leaves a 30% risk of a frost on them, after February 25 20%, and after March 7 10%. The 35 days between the even-odds date and the one-in-ten date is wider than at 71% 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 11 in half of years, as early as November 20 one year in ten, and holds off past January 13 one year in ten. A hard freeze of 28°F is less common: it came in 44% of winters. The freeze-free season lasts 319 days in a typical year, longer than at 74% of the 219 California stations; it reaches 357 days one year in ten and falls short of 273 one year in ten. On average 8 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 45 days longer, to about March 17, and return around November 24. A 24°F night is unusual: 6% of winters had one.
| Low | 90% | 80% | 70% | 60% | 50% | 40% | 30% | 20% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Feb 6 | Feb 20 | Feb 28 | Mar 8 | Mar 17 | Mar 25 | Mar 31 | Apr 7 | Apr 16 |
| 32°F95% of yrs | Dec 23 | Jan 3 | Jan 13 | Jan 23 | Jan 31 | Feb 7 | Feb 17 | Feb 25 | Mar 7 |
| 28°F44% of yrs | too rare for a date | ||||||||
| 24°F6% of yrs | too rare for a date | ||||||||
| Low | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|---|
| 36°F | Nov 3 | Nov 11 | Nov 16 | Nov 20 | Nov 24 | Nov 28 | Dec 1 | Dec 6 | Dec 11 |
| 32°F95% of yrs | Nov 20 | Nov 28 | Dec 2 | Dec 6 | Dec 11 | Dec 15 | Dec 23 | Jan 1 | Jan 13 |
| 28°F44% of yrs | too rare for a date | ||||||||
| 24°F6% of yrs | too rare for a date | ||||||||
Freeze probabilities 1991–2020 at San Luis Obispo Airport, 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 93401 →Compare San Luis Obispo with another place →
Weather by month in San Luis Obispo
August is the warmest month at San Luis Obispo Airport, CA, averaging 79°F by day and 55°F at night; December is the coldest at 65°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 40% of years and March still brings one in 12% of years.
Warmth for growing adds up to 3,646 growing degree days a year (base 50°F), fewer than at 68% of the 283 California stations. 37% of them (1,354) have built up by the end of June, and August adds the most, 530. Precipitation totals 18.0 inches a year, most in January (3.9") and least in August (0.0"); May to September bring 4% of it.
| Month | High °F | Low °F | Precip. in | 32°F night | Nights ≤ 32°F | GDD 50°F |
|---|---|---|---|---|---|---|
| Jan | 65 | 41 | 3.9 | 61% | 3 | 116 |
| Feb | 65 | 42 | 3.5 | 40% | 2 | 127 |
| Mar | 67 | 45 | 3.2 | 12% | <1 | 185 |
| Apr | 69 | 46 | 1.1 | 2% | <1 | 224 |
| May | 72 | 48 | 0.5 | 0% | 0 | 307 |
| Jun | 75 | 51 | 0.1 | 0% | 0 | 395 |
| Jul | 78 | 54 | 0.1 | 0% | 0 | 501 |
| Aug | 79 | 55 | 0.0 | 0% | 0 | 530 |
| Sep | 80 | 53 | 0.0 | 0% | 0 | 494 |
| Oct | 77 | 50 | 0.9 | 0% | 0 | 417 |
| Nov | 71 | 45 | 1.4 | 23% | 1 | 243 |
| Dec | 65 | 40 | 3.3 | 68% | 3 | 108 |
Monthly normals 1991–2020 at San Luis Obispo Airport, CA (200 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, San Luis Obispo Poly, CA, 3 miles north of San Luis Obispo, has a last frost 15 days earlier (January 16), a first frost 14 days later (December 25), a season 30 days longer.
When it snows in San Luis Obispo, California
Measurable snow is rare here: only 0 of 19 winters from 1991 to 2020 recorded 0.1 inch or more at San Luis Obispo Poly, CA, 3 miles away, so there is no typical first-snow date to plan around.
Planting calendar for 93401
| When | What to do |
|---|---|
| November 22 – December 6 | Start peppers and aubergines indoorsThey need eight to ten weeks under lights before they can go out. |
| December 6 – December 20 | Start tomatoes indoorsSix to eight weeks gives a transplant big enough to set fruit early. |
| January 3 – January 17 | Sow peas, spinach, lettuce, radish outdoorsThese tolerate light frost and prefer to germinate in cool soil. |
| January 31 onward | Transplant tomatoes, peppers, basil, squash, cucumbersThe average last freeze has passed — still watch the actual forecast. |
| by September 18 | Sow the autumn crop of carrots, beets and brassicasThey need about twelve weeks to size up before growth stops. |
| by November 27 | Harvest tender crops, or be ready to cover themA first freeze two weeks early is well within normal variation. |
| December 11 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 93401 is 2 miles from San Luis Obispo Airport, CA, which stands at 200 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 93401 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.