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Bumper or bummer? Tomato crops affected by weather swings

Heat spikes dried out flowers or caused plants to abort fruit

On this Chef's Choice Orange tomato plant, one blossom didn't pollinate but others fortunately did. Heat and low humidity affect tomato pollen.

On this Chef's Choice Orange tomato plant, one blossom didn't pollinate but others fortunately did. Heat and low humidity affect tomato pollen. Debbie Arrington

It’s a tale of two harvests. Some Sacramento-area gardeners are having bumper tomato crops. Others, not so much.

Here at Sacramento Digs Gardening, the kitchen counters are covered with big fat slicers. Both Kathy and I have had excellent tomato seasons.

But I’ve heard from several other gardeners who saw their vines struggle.

“My tomato crop is the worst ever this year,” wrote reader Robin Wham. “Are others having this challenge? Was it the long spring?”

Maybe – if tomato vines were planted in March or early April. That long, cool spring with cold soil temperatures did those plants no favors.

Timing and temperature are key to tomato success. When they transplant their tomato seedlings outdoors, gardeners gamble on weather reliability.

This season, later may have been better. Tomatoes planted in mid- to late April and mid-May seemed to produce the most fruit. Their growth spurts and bloom cycles seemed in sync with temperature ups and downs.

April 21 through 29 saw a streak of afternoon days in the 80s – perfect tomato weather. That sweet spot got transplants off to a strong start. (That's when my vines went in the ground.)

But Sacramento started May with another cold streak; overnight lows dipped into the 40s on the first eight out of 11 nights. That challenged plants set out during early May. Tomatoes won’t set if nighttime temperatures are below 55 degrees.

It wasn’t until May 12 when Sacramento finally saw an upswing in temperatures and more typical growing conditions. Tomatoes planted after that date seemed to flourish, too.

June also featured good tomato-growing weather with most afternoons in the 80s. Tomato plants that were mature enough to produce flowers had good fruit set and started producing ripe tomatoes in July.

When it comes to setting fruit, tomatoes are fickle about temperature. Flowers are usually pollinated with the help of a light breeze or the vibration of bumblebees.

Tomato pollen dries out when temperatures are above 95 degrees; it won’t stick. Unpollinated flowers dry up.

Even if pollinated, flowers may drop off in high heat. University of Nevada research showed tomato plants will abort new fruit if exposed to more than four consecutive hours over 100 degrees.

July saw plenty of triple-digit hours. Sacramento reached at least 100 degrees on eight days. Those high temperatures prevented new tomatoes from setting and some vines just shut down completely.

So far in August, we’ve had eight days over 100 degrees. That fried new flowers on the vine. Instead of forming new fruit, they shriveled up and dropped off.

One more factor in tomato set: Fertilizer. Tomatoes that get too much nitrogen produce lots of vine but little fruit. During fruit formation, hold back on fertilizer except for a little bone meal.

As long as vines produce flowers, there’s hope. “Average” days in the 90s are forecast for the remainder of August and through Labor Day weekend, and soil remains warm. Those late summer blooms could produce autumn tomatoes.

Share your tomato updates

How are your tomatoes producing this season? Tell us about your 2023 crop including what and when you planted. We’ll follow up with another report.

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Garden Checklist for week of April 14

It's still not warm enough to transplant tomatoes directly in the ground, but we’re getting there.

* April is the last chance to plant citrus trees such as dwarf orange, lemon and kumquat. These trees also look good in landscaping and provide fresh fruit in winter.

* Smell orange blossoms? Feed citrus trees with a low dose of balanced fertilizer (such as 10-10-10) during bloom to help set fruit. Keep an eye out for ants.

* Apply slow-release fertilizer to the lawn.

* Thoroughly clean debris from the bottom of outdoor ponds or fountains.

* Spring brings a flush of rapid growth, and that means your garden needs nutrients. Fertilize shrubs and trees with a slow-release fertilizer. Or mulch with a 1-inch layer of compost.

* Azaleas and camellias looking a little yellow? If leaves are turning yellow between the veins, give them a boost with chelated iron.

* Trim dead flowers but not leaves from spring-flowering bulbs such as daffodils and tulips. Those leaves gather energy to create next year's flowers. Also, give the bulbs a fertilizer boost after bloom.

* Pinch chrysanthemums back to 12 inches for fall flowers. Cut old stems to the ground.

* Mulch around plants to conserve moisture and control weeds.

* From seed, plant beans, beets, cantaloupes, carrots, corn, cucumbers, melons, radishes and squash.

* Plant onion sets.

* In the flower garden, plant seeds for asters, cosmos, celosia, marigolds, salvia, sunflowers and zinnias.

* Transplant petunias, zinnias, geraniums and other summer bloomers.

* Plant perennials and dahlia tubers for summer bloom.

* Mid to late April is about the last chance to plant summer bulbs, such as gladiolus and tuberous begonias.

* Transplant lettuce seedlings. Choose varieties that mature quickly such as loose leaf.

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