When is the last frost in Ontario, CA?

The average last spring frost (50% probability of 32°F) in Ontario is around March 6, based on NOAA/NCEI 1991–2020 normals at the Mitchell Caverns, CA station (24.9 mi away). Half of years it falls outside that date, so wait 1–2 weeks past it before setting out frost-tender plants.

When is the first fall frost in Ontario?

The average first fall frost in Ontario, CA is around December 8, giving roughly a 272-day frost-free growing season.

What planting zone is Ontario, CA?

Ontario is in USDA plant hardiness zone 10a (average annual extreme minimum temperature 30 to 35°F), from the 2023 USDA Plant Hardiness Zone Map.

What is the weather like in Ontario?

In Ontario, CA the warmest month is July (avg high 92.2°F) and the coldest is December (avg low 39.6°F). It averages about 10.6″ of precipitation a year and 0.8″ of snow.

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Ontario, CA Planting Calendar & Frost Dates

USDA hardiness zone 10a · ZIP 91758 · elevation 1326′ · NOAA & USDA data

Ontario is in USDA hardiness zone 10a (winters can dip to 30 to 35°F), with a growing season of about 272 days between the average last spring frost (March 6) and first fall frost (December 8). The warmest month is July (avg high 92.2°F); the coldest is December (avg low 39.6°F). It averages 10.6″ of precipitation a year and 0.8″ of snow, mostly mid-winter. Start cool-season crops several weeks before the last frost; wait until after it for tomatoes, peppers and other tender crops.

Hardiness zone

10a

min temp 30 to 35°F

Last spring frost

March 6

avg (50% prob, 32°F)

First fall frost

December 8

avg (50% prob, 32°F)

Growing season

272 days

frost-free window

Warmest month

July

avg high 92.2°F

Month-by-month typical weather in Ontario

Average conditions from the NOAA/NCEI 1991–2020 U.S. Climate Normals at Mitchell Caverns, CA (24.9 mi away). Temperatures °F, precipitation & snow in inches.

Month	Avg high	Avg low	Mean	Precip (in)	Snow (in)
January	53.9	40.1	47	1.3	0.2
February	55.4	41.2	48.3	1.8	0.1
March	61.9	44.7	53.3	1.6	0.4
April	67.8	49.7	58.8	0.6	0
May	77	58.3	67.7	0.2	0
June	87.3	68.6	78	0.1	0
July	92.2	73.1	82.6	1.1	0
August	90.8	72.4	81.6	0.6	0
September	85	67.8	76.4	0.7	0
October	73.7	56.6	65.1	0.6	0
November	62.4	46.7	54.6	0.5	0
December	53.3	39.6	46.4	1.6	0.1

What to plant when in Ontario

Crop	Season	What to do & when
Tomatoes	warm	Start seeds indoors: January 23 · Transplant out: March 13
Peppers	warm	Start seeds indoors: January 9 · Transplant out: March 20
Cucumbers	warm	Direct sow: March 13
Zucchini / squash	warm	Direct sow: March 13
Bush beans	warm	Direct sow: March 13
Sweet corn	warm	Direct sow: March 13
Basil	warm	Transplant out: March 13
Peas	cool	Sow (spring): January 30 · Sow (fall): October 13
Spinach	cool	Sow (spring): January 23 · Sow (fall): October 13
Lettuce	cool	Sow (spring): February 6 · Sow (fall): October 13
Kale	cool	Transplant (spring): February 13 · Sow (fall): September 29
Broccoli	cool	Transplant (spring): February 20
Carrots	cool	Sow (spring): February 13 · Sow (fall): September 29
Onions	cool	Transplant (spring): February 6
Potatoes	cool	Plant seed potatoes: February 20
Radishes	cool	Sow (spring): February 6 · Sow (fall): October 27

Dates are computed from Ontario's average frost dates using standard cool-/warm-season timing. They're a starting point — a cold spring, a warm microclimate, or a fast-maturing variety can shift them by a week or two.

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Gear that helps you plant on time

Some links below are Amazon affiliate links (tag theboringthin-20). If you buy through them we may earn a small commission at no extra cost to you — it keeps this almanac free. We only list generic, genuinely-useful categories.

Soil thermometer →Know when the ground is warm enough to sow — the honest signal, not the calendar.Seed-starting trays + dome →Start tomatoes & peppers indoors on the schedule below.Seedling heat mat →Warms the soil for faster, more even germination indoors.Floating row cover / frost cloth →Buys you 4–8°F to beat an early or late frost.Raised garden bed kit →Warms and drains faster in spring than in-ground soil.Garden gloves →Basic kit for transplant day.

Nearby cities in California

Apple Valley, CA Cathedral City, CA Indio, CA Coachella, CA Yucaipa, CA Palm Desert, CA Hesperia, CA Palm Springs, CA

Sources & method. Hardiness zone: 2023 USDA Plant Hardiness Zone Map (PRISM Climate Group, Oregon State University). Frost dates & growing season: NOAA/NCEI 1991–2020 U.S. Climate Normals, nearest station with published freeze normals — Mitchell Caverns, CA, 24.9 mi (confidence: medium). Monthly weather normals: Mitchell Caverns, CA, 24.9 mi (confidence: medium). ZIP centroid from U.S. Census 2023 ZCTA data. Frost dates are the 50%-probability date — half of years frost occurs outside the stated date. Nothing here is invented; where the nearest station is far, we say so. Last updated 2026-07-22.