Frost Dates for Ontario, CA 91761
Last spring frost January 19 · First fall frost December 23 · 338 frost-free days · USDA Zone 10a
Frost probability for 91761
| Frost | 10% | 50% | 90% |
|---|---|---|---|
| Spring (last frost) | March 5 | January 19 | — |
| Fall (first frost) | November 21 | December 23 | — |
Dates come from NOAA 1971–2000 freeze probabilities at the nearest reporting station (16.9 km away); the 50% date is the median year.
Month by month in Ontario
| Month | What to do |
|---|---|
| January | Last frost around January 19. Transplant cold-hardy crops (broccoli, kale, cabbage). Harden off tomato and pepper seedlings. |
| February | Frost has passed. Transplant tomatoes, peppers, and basil outdoors. Direct sow beans, cucumbers, squash, and zucchini. |
| March | Succession plant lettuce and greens. Keep beds weeded and watered consistently. |
| April | Harvest regularly to encourage production. Monitor for pests and disease. |
| May | Midsummer: harvest beans, zucchini, cucumbers. Start brassica seeds indoors for fall. |
| June | Direct sow carrots and beets for fall harvest. Start fall broccoli and kale indoors. |
| July | Direct sow carrots and beets for fall harvest. Start fall broccoli and kale indoors. |
| August | Direct sow carrots and beets for fall harvest. Start fall broccoli and kale indoors. |
| September | Direct sow carrots and beets for fall harvest. Start fall broccoli and kale indoors. |
| October | Direct sow carrots and beets for fall harvest. Start fall broccoli and kale indoors. |
| November | First fall frost approaching (around December 23). Transplant fall brassicas. Sow spinach, arugula, and radishes. |
| December | First frost around December 23. Harvest tomatoes and peppers before frost. Cover cold-hardy crops with row covers to extend the season. |
What grows in a 338-day season
Frequently Asked Questions
What is the last frost date for Ontario, CA (ZIP 91761)?
The median last spring frost date for Ontario, CA (ZIP code 91761) is around January 19. This is the 50th-percentile date — meaning half of all years see frost end before this date. Use the 10% and 90% dates to understand the full range of risk in your area.
What is the first fall frost date in Ontario, CA?
The median first fall frost date for Ontario, CA is around December 23. Plan to harvest or protect frost-sensitive crops like tomatoes and peppers at least a week before this date to avoid losses.
How long is the growing season in Ontario, CA?
The frost-free growing season in Ontario is about 338 days. This window — from your last spring frost to your first fall frost — is when warm-season vegetables thrive. Cold-hardy crops like kale, spinach, and peas can extend your season by weeks on either side.
What USDA hardiness zone is Ontario, CA?
Based on frost date patterns, Ontario falls approximately in 10a. USDA hardiness zones are determined by average annual minimum winter temperatures and guide both plant selection and planting timing for your location.
When should I start tomatoes indoors in Ontario, CA?
Start tomato seeds indoors 6–8 weeks before your last frost date (around January 19). For Ontario, that's roughly November 24 to December 8. Transplant outdoors 1–2 weeks after your last frost.
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