When to plant in Ontario, CA
USDA Zone 10aEverything below — frost dates, hardiness zone, and what to plant when in Ontario, California — is derived from the closest NOAA station with complete climate normals.
With about 360 frost-free days, Ontario supports back-to-back plantings; stagger sowings every few weeks to keep beds productive spring through fall. Zone 10a is warm enough that Ontario can grow subtropical perennials, and the short (or absent) frost period barely limits the annual calendar.
Frost probability
ONTARIO INTL AP · 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 Ontario’s own odds, recorded at ONTARIO INTL AP.
| Threshold | SPRING 10% | SPRING 50% | SPRING 90% | FALL 10% | FALL 50% | FALL 90% |
|---|---|---|---|---|---|---|
| 36°F | Feb 28 | Jan 27 | Dec 24 | Nov 27 | Dec 17 | Jan 17 |
| 32°F | Feb 7 | Jan 4 | Dec 14 | Dec 11 | Dec 30 | Feb 1 |
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 Ontario, drawn to the day from the local frost dates. The dashed line is today.
Nearby weather stations
3 within 16 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.
Ontario planting FAQ
When is the last spring frost in Ontario, CA?
Ontario's average last spring frost falls near January 4 — 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 Ontario, CA?
The first fall frost in Ontario typically arrives around December 30 (50% probability at 32°F). Harvest or protect frost-sensitive crops before then.
What hardiness zone is Ontario in?
Ontario is in USDA hardiness zone 10a. In zone 10a, winters are mild — many tender perennials overwinter here.
How long is the growing season in Ontario?
There are roughly 360 frost-free days in Ontario (a long growing season), running from the average last frost around January 4 to the first fall frost near December 30.
When should I plant tomatoes in Ontario?
In Ontario, start tomato seeds indoors around November 9–November 23, then transplant seedlings outdoors around January 11 once the danger of frost has passed.
Never miss a window in Ontario
An email when it’s time to start seeds, transplant, and sow — timed to Ontario’s frost dates. Double opt-in, one-click unsubscribe, no spam.
Nearby cities
8 within reach- Eastvale · 8 km
- Chino · 9 km
- Montclair · 9 km
- Upland · 10 km
- Rancho Cucamonga · 11 km
- Jurupa Valley · 13 km
- Norco · 13 km
- Claremont · 14 km
Frost dates recorded at ONTARIO INTL AP, 2 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 Ontario, CA — Frost Dates & Zone 10a." Frost normals: NOAA NCEI U.S. Climate Normals 1991–2020, station USW00003102. Retrieved from https://blissgarden.com/california/ontario.