When to plant in Altadena, CA
USDA Zone 10bEverything below — frost dates, hardiness zone, and what to plant when in Altadena, California — is derived from the closest NOAA station with complete climate normals.
A ~-1-day frost-free window makes Altadena a short-season garden: choose early varieties and start long-season crops indoors well ahead of the last frost. In zone 10b, frost is a minor factor for Altadena — most perennials thrive, and annual vegetables can go out early and stay late.
Frost probability
PASADENA · 1991–2020The date the last spring and first fall frost occur, by threshold and probability. A 90% date is later in spring — and earlier in fall — than a 10% date; the 50% · 32°F row is what most gardeners plan around. These are Altadena’s own odds, recorded at PASADENA.
| Threshold | SPRING 10% | SPRING 50% | SPRING 90% | FALL 10% | FALL 50% | FALL 90% |
|---|---|---|---|---|---|---|
| 36°F | Feb 27 | Jan 17 | Dec 18 | Dec 5 | Dec 28 | Feb 7 |
| 32°F | Feb 17 | Jan 8 | Dec 20 | Dec 20 | Jan 7 | Feb 14 |
Download this table as CSV ↓ — every threshold and probability, plus this city’s planting-window dates.
What to plant now
TODAY · JULY 19Nothing new to sow or transplant outdoors in the next few weeks — a seasonal lull. Check the full-year calendar below for the next window.
Full-year planting calendar
Each bar is the exact window to take a planting action in Altadena, drawn to the day from the local frost dates. The dashed line is today.
Nearby weather stations
3 within 12 km · complete 32°F normalsWhen stations disagree by more than a few days, that spread is real microclimate variation — elevation, water, urban heat. Judge which station best matches your own yard.
Altadena planting FAQ
When is the last spring frost in Altadena, CA?
Altadena's average last spring frost falls near January 8 — the 50% mark at 32°F in the 1991–2020 normals. Hold tender transplants until the risk has passed, then plant out.
When is the first fall frost in Altadena, CA?
The first fall frost in Altadena typically arrives around January 7 (50% probability at 32°F). Harvest or protect frost-sensitive crops before then.
What hardiness zone is Altadena in?
Altadena is in USDA hardiness zone 10b. In zone 10b, winters are mild — many tender perennials overwinter here.
How long is the growing season in Altadena?
Altadena has about -1 frost-free days — a short growing season — between the average last spring frost (January 8) and first fall frost (January 7).
When should I plant tomatoes in Altadena?
For Altadena, sow tomatoes indoors about November 13–November 27 and move the seedlings out around January 15, after the last spring frost.
Never miss a window in Altadena
An email when it’s time to start seeds, transplant, and sow — timed to Altadena’s frost dates. Double opt-in, one-click unsubscribe, no spam.
Nearby cities
8 within reach- Pasadena · 4 km
- La Cañada Flintridge · 6 km
- San Marino · 8 km
- Sierra Madre · 8 km
- South Pasadena · 9 km
- East San Gabriel · 10 km
- Glendale · 10 km
- La Crescenta-Montrose · 10 km
Frost dates recorded at PASADENA, 5 km from the city center · 1991–2020 NOAA climate normals · zone from the USDA/PRISM 2023 map. How we compute this.
BlissGarden. "When to Plant in Altadena, CA — Frost Dates & Zone 10b." Frost normals: NOAA NCEI U.S. Climate Normals 1991–2020, station USC00046719. Retrieved from https://blissgarden.com/california/altadena.