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FIMBY: Why won’t my tomatoes turn red?

Temperature plays a role in ripening – especially at night

Sacramento Digs Gardening
BY Debbie Arrington PUBLISHED JUL 29, 2025

Frustrated by tomatoes that won't ripen? Blame the weather, especially the cooler than usual nights we've had this month. Kathy Morrison

This is another installment in our Food in My Back Yard series, devoted to edible gardening.

“Why won’t my tomatoes turn red?” Local master gardeners have heard that question repeatedly this summer.

By the end of July, most Sacramento gardeners should be harvesting ripe tomatoes. That’s assuming those transplants went into the ground in late April or early May.

But for Sacramento gardeners, this has been an unusually mild summer. In July, Sacramento recorded only one triple-digit day (102 on July 11). Through Monday (July 28), 19 days in July saw lower than normal temperatures, according to the National Weather Service.

Daytime highs in July have averaged 2.5 degrees lower than average (89.1 compared to 92.6). Perhaps more consequentially, overnight lows have also dipped about 2 degrees below normal, averaging 57.5 degrees. July 5 set a record low for that date in Sacramento: 54 degrees.

As much as they need abundant sun, tomatoes need warm nights to produce ripe fruit. According to UC research, California tomatoes struggle to ripen when nighttime temperatures dip into the mid to low 50s. Ripening may stop altogether when nights go below 50 degrees.

Usually, it’s too much heat that interferes with ripening. Red pigments may not form properly during triple-digit weather. (This phenomenon has plagued gardeners coast to coast this summer – just not here.)

Colder nights may add days or even weeks to the ripening process. (And it may seem like forever when you’re waiting for that first ripe beefsteak.) But other factors may also be on play, such as too much nitrogen fertilizer, too much shade or too much direct sun.

So, what’s the ideal temperature for tomato ripening? According to research at San Diego State University, the sweet spot for tomatoes are temperatures between 68 and 77 degrees. That’s not the afternoon high but the actual air temperature during day or night. Above 85 degrees or below 55, tomato ripening may be negatively impacted.

Research also shows that tomatoes – once they reach their full size and firmness – don’t need sunlight to turn red. They develop more sugar (and sweeter flavor) if allowed to ripen on the vine. But as for color, they’ll turn red on the kitchen counter or tucked inside a paper bag (preferably with an apple or banana).

Tired of waiting? Pick the full-sized but still green tomatoes once they show a blush of color and let them ripen indoors. Meanwhile, the tomato plant can concentrate on setting new fruit – and, we hope, enjoying warmer nights in August.

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FIMBY Food in My Back Yard edible gardening tomatoes summer vegetables weather overnight temperatures summer July Sacramento gardening

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Flowers in My Back Yard Series

June 9: Grow coneflowers for pollinators -- and yourself

June 2: Sunflowers capture Sacramento's summer attitude

May 29: Are your roses going 'blind'?

May 26: Zinnias are the summer flowers every garden needs

May 19: Plant dahlias now for late-summer flower power

May 12: Know your coreopsis from your bidens

May 5: Mums the word on Mother's Day weekend

April 28: Majestic Matilija poppy is worth a look

April 21: Celebrate roses, America's favorite flower

April 14: Small flowers with outsized impact

April 7: Calendulas do double duty

April 3: Make Easter lilies last for years to come

March 31: In praise of a pollinator magnet (small-leaf salvias)

March 24: Azaleas brighten shady spots

March 17: The perfect flower for beginners? Try zonal geraniums

March 10: Keep camellias happy for years to come

March 3: Fruit tree blossoms are a fleeting joy

Feb. 27: Are your roses looking rusty?

Feb. 24: Treasure spring daffodils now and for years to come

Feb. 17: How and why to grow wildflowers

Feb. 10: Let's talk Valentine's Day roses

Feb. 3: Why grow flowers?

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Garden checklist for week of June 7

Afternoon highs are expected to be back in the mid 90s by midweek, then edging towards triple digits. Plan your planting and garden activities accordingly.

* Remember to water early.

* It’s not too late to transplant tomatoes, peppers, eggplant or other summer favorites. Make sure they stay hydrated.

* From seed, plant corn, melons, pumpkins, radishes, squash and sunflowers.

* Plant basil to go with your tomatoes.

* Transplant summer annuals such as petunias, marigolds and zinnias.

* It’s also a good time to transplant perennial flowers including astilbe, columbine, coneflowers, coreopsis, dahlias, rudbeckia, salvia and verbena.

* Let the grass grow longer. Set the mower blades high to reduce stress on your lawn during summer heat. To cut down on evaporation, water your lawn deeply during the wee hours of the morning, between 2 and 8 a.m.

* Tie up vines and stake tall plants such as gladiolus and lilies. That gives their heavy flowers some support.

* Dig and divide crowded bulbs after the tops have died down.

* Feed summer flowers with a slow-release fertilizer.

* Mulch, mulch, mulch! This “blanket” keeps moisture in the soil longer and helps your plants cope during hot weather.

* Cut back fruit-bearing canes on berries.

* Feed camellias, azaleas and other acid-loving plants. Mulch to conserve moisture and reduce heat stress.

* Cut back Shasta daisies after flowering to encourage a second bloom in the fall.

* Trim off dead flowers from rose bushes to keep them blooming through the summer. Roses also benefit from deep watering and feeding now. A top dressing of aged compost will keep them happy. It feeds as well as keeps roots moist.

* Pinch back chrysanthemums for bushier plants with many more flowers in September.

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