Ontario, CA planting calendar
- USDA zone
- 10a30 to 35 °F
- Last frost
- Jan 17avg, 32°F
- First frost
- Dec 19avg, 32°F
- Growing season
- 335days
Ontario, California is in USDA plant hardiness zone 10a. Its average last spring frost is around January 17 and the first fall frost around December 19, giving a growing season of about 335 days (NOAA 1991–2020 normals, 32°F, median). Start tender crops like tomatoes and peppers indoors weeks before the last frost and set them out afterward; sow hardy crops such as peas, spinach, and lettuce before it. The planner below turns those frost dates into a printable per-crop planting calendar.
Ontario planting calendar
Each crop's windows are counted from Ontario's average frost dates. hatched = start seeds indoors, solid green = plant out, teal = a fall sowing, and the terracotta dot marks the estimated first harvest. Ranges are extension-guide planning guidance, not guarantees.
- Start indoors
- Plant out
- Fall sowing
- First harvest
| Crop | Frost tolerance | Start indoors | Plant out | First harvest | Fall planting |
|---|---|---|---|---|---|
| Tomato | Tender | Jan 1 | Jan 24 – Jan 31 | Mar 25 – Apr 14 | — |
| Pepper | Very tender | Jan 1 | Jan 31 – Feb 7 | Apr 1 – May 1 | — |
| Cucumber | Tender | Jan 1 | Jan 24 – Jan 31 | Mar 15 – Apr 4 | — |
| Summer squash / zucchini | Tender | — | Jan 24 – Jan 31 | Mar 10 – Mar 25 | — |
| Bush bean | Tender | — | Jan 24 – Jan 31 | Mar 15 – Mar 25 | Oct 20 – Oct 30 |
| Sweet corn | Tender | — | Jan 17 – Jan 31 | Mar 18 – Apr 17 | — |
| Basil | Very tender | Jan 1 | Jan 24 – Jan 31 | Feb 23 – Mar 10 | — |
| Lettuce | Half-hardy | Jan 1 | Jan 1 – Jan 3 | Feb 15 – Mar 2 | Oct 6 – Oct 21 |
| Pea | Hardy | — | Jan 1 | Feb 25 – Mar 12 | Sep 26 – Oct 11 |
| Spinach | Hardy | — | Jan 1 | Feb 10 – Feb 20 | Oct 16 – Oct 26 |
| Carrot | Half-hardy | — | Jan 1 – Jan 3 | Mar 2 – Mar 22 | Sep 16 – Oct 6 |
| Broccoli | Half-hardy | Jan 1 | Jan 1 – Jan 3 | Feb 25 – Mar 17 | Sep 21 – Oct 11 |
Data: NOAA NCEI U.S. Climate Normals 1991–2020 (public domain) and USDA Plant Hardiness Zone Map 2023. Planting windows synthesized from U.S. Cooperative Extension guides.
Frost & freeze dates
From NOAA's 1991–2020 Climate Normals at station USW00003179. The median (p50) is the average date; the 90%-safe column is the date the freeze has passed in about 9 years out of 10 (p10 for spring, p90 for fall) — the conservative date to plant after or harvest before.
| Threshold | Last spring — avg | Last spring — 90%-safe | First fall — avg | First fall — 90%-safe | Season (days) |
|---|---|---|---|---|---|
| 36°F | Feb 24 | Mar 29 | Dec 1 | Dec 18 | 278 |
| 32°F (freeze) | Jan 17 | Feb 22 | Dec 19 | Jan 17 | 335 |
| 28°F | Jan 3 | Jan 28 | Dec 30 | Jan 23 | 365 |
| 24°F | — | — | — | — | 365 |
32°F is the standard "freeze" line that damages tender crops; lighter 36°F frost can nip the most cold-sensitive plants, while hardy crops shrug off light frost down toward 28°F. Use the threshold that matches what you are protecting.
Growing degree days
Growing degree days (GDD) accumulate warmth above a base temperature over the year — a better predictor of crop development than the calendar alone. Warm-season crops need a long, warm GDD total; a short, cool GDD total favors greens and brassicas.
| Model | °F·days | Used for |
|---|---|---|
| Base 50°F (warm-season) | 5,949 | standard warm-season base (tomato, corn, beans) |
| Base 40°F (cool-season) | 9,555 | cool-season crops (brassicas, greens) |
Hardiness zone 10a
Ontario sits in USDA plant hardiness zone 10a on the 2023 map — meaning its average annual extreme minimum winter temperature is about 30 to 35 °F. That number tells you which perennials, shrubs, and trees reliably survive an average winter here; it does not set your planting dates, which come from the frost calendar above.
Explore more places in zone 10a, or see all USDA hardiness zones.
Frequently asked questions
- What USDA hardiness zone is Ontario?
- Ontario, California is in USDA plant hardiness zone 10a on the 2023 map (average annual extreme minimum temperature 30 to 35 °F) — from the USDA Plant Hardiness Zone Map 2023, matched to this location's ZIP. See the methodology page for sources.
- When is the last frost in Ontario?
- The average (median) last spring frost at 32°F is around January 17, from NOAA's 1991–2020 climate normals at the nearest reporting station. Roughly one year in ten the last frost is as late as February 22, so wait until then before setting out frost-tender plants if you want to be safe.
- When is the first fall frost in Ontario?
- The average first fall frost at 32°F is around December 19. That leaves a growing season of about 335 days between the average last spring and first fall frosts.
- When should I start tomatoes in Ontario?
- Start tomato seeds indoors about Jan 1 and transplant them outside about Jan 24 – Jan 31, once the danger of frost has passed. Estimated first harvest is around Mar 25 – Apr 14.
- How long is the growing season in Ontario?
- About 335 days at the 32°F threshold (NOAA 1991–2020, median) — the span between the average last spring frost (~January 17) and the average first fall frost (~December 19). Cold-hardy crops extend usable time at both ends; frost-tender crops fit inside it.
Sources & method
Frost, freeze, growing-season, and growing-degree-day figures are NOAA NCEI U.S. Climate Normals 1991–2020 for station USW00003179 (Chino Ap, 5.4 km away). The hardiness zone is the USDA Plant Hardiness Zone Map 2023, matched to this location's ZIP. Planting windows are computed by counting from the average last and first frost using per-crop offsets synthesized from U.S. Cooperative Extension guides — the full method and citations are on the methodology page.