Frost dates for 91023Mount Wilson, CA
The average last spring frost in Mount Wilson is April 28 and the average first fall frost is November 15, giving a typical growing season of about 199 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- Apr 28
- Average first fall frost
- Nov 15
- Growing season
- 199 days
Safer after May 27 (1-in-10 risk)
Possible by Oct 18 (1-in-10 risk)
First frost typically in 14 days
Frost risk by date
NOAA reports frost dates as probabilities. The middle date is the average (a 50% chance). For spring, the 10% date is a safer planting date: frost after it happens in about 1 year in 10. Light frost (36°F) can damage tender plants; a hard freeze (28°F) kills most annuals.
| Temperature | Last spring: 90% / 50% / 10% chance | First fall: 10% / 50% / 90% chance | Growing season (50%) | Years with at least one |
|---|---|---|---|---|
| Light frost (36°F) | Apr 21 · May 15 · Jun 4 | Oct 3 · Oct 30 · Nov 19 | 167 days | 100% |
| Frost / freeze (32°F) | Mar 31 · Apr 28 · May 27 | Oct 18 · Nov 15 · Dec 9 | 199 days | 100% |
| Hard freeze (28°F) | Feb 28 · Apr 8 · May 10 | Nov 5 · Dec 4 · Dec 29 | 240 days | 100% |
Planting calendar for Mount Wilson
Planting windows from the University of Maryland Extension calendar, shifted to your average frost dates. Use them as a starting point and adjust for seed packet instructions and local conditions.
| Crop | Task | Window |
|---|---|---|
| Tomatoes | Start seeds indoors | Mar 28 – Jun 3 |
| Tomatoes | Transplant outdoors | May 14 – Jul 26 |
| Peppers | Start seeds indoors | Mar 28 – Apr 28 |
| Peppers | Transplant outdoors | May 28 – Jun 28 |
| Eggplant | Start seeds indoors | Apr 14 – May 13 |
| Eggplant | Transplant outdoors | May 28 – Jun 28 |
| Cucumbers | Sow outdoors | May 28 – Aug 5 |
| Summer squash | Sow outdoors | May 14 – Jun 28 |
| Winter squash & pumpkins | Sow outdoors | May 28 – Jun 28 |
| Snap beans | Sow outdoors | May 14 – Aug 28 |
| Sweet corn | Sow outdoors | May 14 – Jul 26 |
| Watermelon & cantaloupe | Sow outdoors | May 28 – Jun 28 |
| Okra | Sow outdoors | May 21 – Jun 13 |
| Peas | Sow outdoors (spring) | Mar 28 – May 13 |
| Potatoes | Plant outdoors (spring) | Mar 28 – May 28 |
| Onions | Start seeds indoors (spring) | Feb 14 – Mar 13 |
| Onions | Transplant outdoors (spring) | Apr 14 – May 13 |
| Broccoli | Start seeds indoors (spring) | Mar 10 – Apr 13 |
| Broccoli | Transplant outdoors (spring) | Apr 14 – May 13 |
| Broccoli | Start seeds indoors (fall) | Jul 16 – Jul 26 |
| Broccoli | Transplant outdoors (fall) | Aug 15 – Aug 26 |
| Cabbage | Start seeds indoors (spring) | Feb 28 – Mar 28 |
| Cabbage | Transplant outdoors (spring) | Mar 28 – Apr 28 |
| Cabbage | Start seeds indoors (fall) | Jul 6 – Aug 5 |
| Cabbage | Transplant outdoors (fall) | Aug 5 – Sep 5 |
| Leaf lettuce | Sow outdoors (spring) | Mar 28 – Jun 13 |
| Leaf lettuce | Sow outdoors (fall) | Aug 5 – Sep 21 |
| Spinach | Sow outdoors (spring) | Mar 21 – May 4 |
| Spinach | Sow outdoors (fall) | Aug 22 – Sep 26 |
| Radishes | Sow outdoors (spring) | Mar 28 – May 18 |
| Radishes | Sow outdoors (fall) | Aug 5 – Oct 6 |
| Kale | Sow outdoors (spring) | Apr 14 – May 4 |
| Kale | Sow outdoors (fall) | Jul 29 – Sep 5 |
| Carrots | Sow outdoors (spring) | Apr 28 – Jun 13 |
| Carrots | Sow outdoors (fall) | Jul 6 – Aug 21 |
| Garlic | Plant outdoors (fall) | Nov 5 – Dec 6 |
Where these dates come from
ZIP code 91023 is matched to the NOAA station Mt Wilson Cbs, CA, 0.7 km away, at 5,710 ft elevation (5 ft higher than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Mt Wilson Cbs, CA (used) | 5,710 ft | Apr 28 | Nov 15 |
| Pasadena, CA | 864 ft | Jan 8 | Jan 7 |
| San Gabriel Fire Dept, CA | 363 ft | Jan 4 | Jan 2 |
| Los Angeles Dwtn Usc Campus, CA | 230 ft | — | — |
Frost dates near Mount Wilson
- Sierra Madre, CA 91024 · last Jan 8, first Jan 7
- Altadena, CA 91003 · last Jan 8, first Jan 7
- Pasadena, CA 91104 · last Jan 8, first Jan 7
- Arcadia, CA 91066 · last Jan 4, first Jan 2
- Monrovia, CA 91017 · last Jan 4, first Jan 2
- La Canada Flintridge, CA 91012 · last Jan 8, first Jan 7
- San Marino, CA 91118 · last Jan 4, first Jan 2
- San Gabriel, CA 91775 · last Jan 4, first Jan 2
Frequently asked questions
- When is the last frost in Mount Wilson, CA?
- For ZIP code 91023, the average last spring frost (32°F) is April 28, based on NOAA 1991-2020 climate normals for the Mt Wilson Cbs, CA weather station. There is a 1-in-10 chance of a frost after May 27.
- When is the first frost in Mount Wilson, CA?
- The average first fall frost (32°F) is November 15. There is a 1-in-10 chance of frost by October 18, so tender plants are at risk from that point on.
- How long is the growing season in 91023?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 199 days long.
- Are these frost dates a forecast?
- No. They are 30-year averages and probabilities from NOAA weather station records. Use them to plan, then watch your local forecast before planting tender crops or leaving them uncovered.