FIMBY: Why won’t my tomatoes turn red?

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.