When is the last frost in Ontario, CA?

For ZIP code 91758, the average last spring frost (32°F) is January 8, based on NOAA 1991-2020 climate normals for the Pomona/Fairplex, CA weather station. There is a 1-in-10 chance of a frost after February 18.

When is the first frost in Ontario, CA?

The average first fall frost (32°F) is December 28. There is a 1-in-10 chance of frost by December 5, so tender plants are at risk from that point on.

How long is the growing season in 91758?

In half of years, the frost-free season (between the last and first 32°F frost) is at least 354 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.

Frost dates for 91758Ontario, CA

The average last spring frost in Ontario is January 8 and the average first fall frost is December 28, giving a typical growing season of about 354 days. Dates are based on NOAA 1991-2020 climate normals.

Average last spring frost: Jan 8
Average first fall frost: Dec 28
Growing season: 354 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.

Spring dates: chance of a frost AFTER the date. Fall dates: chance of a frost BEFORE the date.
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)	Dec 29 · Feb 8 · Mar 9	Nov 21 · Dec 10 · Dec 26	303 days	99%
Frost / freeze (32°F)	Dec 14 · Jan 8 · Feb 18	Dec 5 · Dec 28 · Feb 2	354 days	65%
Hard freeze (28°F)	—	—	365 days	7%

Where these dates come from

ZIP code 91758 is matched to the NOAA station Pomona/Fairplex, CA, 10.8 km away, at 999 ft elevation (2 ft lower than the ZIP code's center). Nearby stations for comparison:

Station	Elevation	Avg last frost	Avg first frost
Pomona/Fairplex, CA (used)	999 ft	Jan 8	Dec 28
Ontario Intl Ap, CA	949 ft	Jan 4	Dec 30
Chino Ap, CA	650 ft	Jan 17	Dec 19
Riverside Muni Ap, CA	805 ft	Jan 9	Dec 30

Frost dates near Ontario

Ontario, CA 91762 · last Jan 8, first Dec 28
Montclair, CA 91763 · last Jan 8, first Dec 28
Upland, CA 91785 · last Jan 8, first Dec 28
Guasti, CA 91743 · last Jan 4, first Dec 30
Chino, CA 91710 · last Jan 17, first Dec 19
Rancho Cucamonga, CA 91730 · last Jan 8, first Dec 28
Claremont, CA 91711 · last Jan 8, first Dec 28
Pomona, CA 91767 · last Jan 8, first Dec 28

Frequently asked questions

When is the last frost in Ontario, CA?: For ZIP code 91758, the average last spring frost (32°F) is January 8, based on NOAA 1991-2020 climate normals for the Pomona/Fairplex, CA weather station. There is a 1-in-10 chance of a frost after February 18.
When is the first frost in Ontario, CA?: The average first fall frost (32°F) is December 28. There is a 1-in-10 chance of frost by December 5, so tender plants are at risk from that point on.
How long is the growing season in 91758?: In half of years, the frost-free season (between the last and first 32°F frost) is at least 354 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.