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What’s wrong with my tomatoes? Probably the heat

Triple-digit temperatures can affect pollination, tomato development

Extreme heat shuts down production in tomato plants: pollen dries up and the flowers won't set fruit.

Extreme heat shuts down production in tomato plants: pollen dries up and the flowers won't set fruit. Kathy Morrison

High heat is on its way – and that’s bound to affect our tomatoes.

First this update from the National Weather Service: Saturday is going to be hot!

A heat advisory is in effect from 11 a.m. to 8 p.m. Saturday, June 22, says the weather service.

“High temperatures 100 to 107 expected. Limited overnight relief with low temperatures in the mid 60s to mid 70s.”

The areas expected to be hardest hit include central Sacramento Valley, Mother Lode, northern Sacramento Valley, northern San Joaquin Valley and southern Sacramento Valley, says the advisory. The warmest overnight temperatures will be in the northern and central Sacramento Valley and foothills.

Saturday’s high temperature in downtown Sacramento is expected to reach 103, says the weather service. That’s 15 degrees above normal.

Be prepared to keep the A.C. running all night. Stay hydrated. Bring pets indoors.

But we can’t bring our vegetable gardens indoors. They have to cope where they’re planted.

So concentrate on keeping them comfortable. Water deeply in the morning before the heat arrives. Make sure there’s mulch (straw, leaves, wood chips, etc.) to help retain that moisture and keep roots cool. Erect some temporary shade (such as an umbrella or shade cloth) over peppers and tender transplants.

Tomatoes and large-leaved vegetables such as squash tend to wilt in the afternoon; that’s a normal coping mechanism. If they’re still wilted in the morning, they need a drink.

Speaking of tomatoes, the top issue right now is lack of pollination. Plants have blooms, but they drop off before they form fruit.

That’s related to heat, too. Tomato flowers won’t set when temperatures stay high – over 90 degrees – for prolonged periods. The pollen dries out. But indeterminate varieties will produce more flowers with new growth.

Tomatoes like some heat; days between 80 and 90 degrees produce optimum growth, according to UC research. But flowers will not set on 100-degree days.

Heat isn’t the only weather factor that can affect tomato pollination. Lack of wind or breeze also can affect tomato set. Tomatoes generally are wind-pollinated. To help with pollination, gently wiggle the tomato’s cage or trellis to shake up and spread the pollen.

For more advice on tomatoes and other summer crops, check out the UC IPM website:


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Garden Checklist for week of July 7

Take care of garden chores early in the morning, concentrating on watering. We’re still in survival mode until this heat wave breaks.

* Keep your vegetable garden watered, mulched and weeded. Water before 8 a.m. to conserve moisture.

* Prevent sunburn; provide temporary shade for tomatoes, peppers, eggplant, melons, squash and other crops with “sensitive” skin.

* Hold off on feeding plants until temperatures cool back down to “normal” range. That means daytime highs in the low to mid 90s.

* Don’t let tomatoes wilt or dry out completely. Give tomatoes a deep watering two to three times a week. Harvest vegetables promptly to encourage plants to produce more.

* Squash especially tends to grow rapidly in hot weather. Keep an eye on zucchini.

* Some weeds thrive in hot weather. Whack them before they go to seed.

* Pinch back chrysanthemums for bushy plants and more flowers in September.

* Harvest tomatoes, squash, peppers and eggplant. Prompt picking will help keep plants producing.

* Remove spent flowers from roses, daylilies and other bloomers as they finish flowering.

* Pinch off blooms from basil so the plant will grow more leaves.

* Cut back lavender after flowering to promote a second bloom.

* One good thing about hot days: Most lawns stop growing when temperatures top 95 degrees. Keep mower blades set on high.

* Once the weather cools down a little, it’s not too late to add a splash of color. Plant petunias, snapdragons, zinnias and marigolds.

* After the heat wave, plant corn, pumpkins, radishes, winter squash and sunflowers. Make sure the seeds stay hydrated.

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