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Sunday, May 19, 2019  Spokane, Washington  Est. May 19, 1883
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Tomato plants will need help beating the upcoming heat

Tomatoes are the hardest hit by hot temperatures.
Tomatoes are the hardest hit by hot temperatures.

Our vegetable gardens are happy to see the cooler weather of the last week or so. Hot weather higher than 85 to 90 degrees delays fruit set and puts a lot of stress on the plants.

Tomatoes seem to be the hardest hit by the hot weather. Once temperatures get over 85 to 90 degrees during the day, pollen being shed by the stamens becomes too dry to stick to the tip of the anther of the tomato flower. As a result the tomato fruit isn’t pollinated and the flower quickly drops off the plant. While the tomato flower is self-fertile, meaning it doesn’t have to have other tomato flowers to be pollinated, the presence of bees helps shake the pollen loose as they visit flowers looking for nectar and pollen. The bees don’t like hot weather any more than we do and don’t venture out when it’s hot.

To help tomatoes get around the return of the heat expected this weekend, here are a few things you can do: Keep plants well watered; plants stressed by a lack of water will go into survival mode and not flower. Keep the soil around tomato plants moist to prevent the formation of blossom end rot, the condition that turns the base of the tomato a leathery brown color. Mulch your plants with 2 to 3 inches of untreated grass clippings, shredded leaves or pine needles to keep the soil cool. Hold off any fertilizer while it’s hot; the plants are in survival mode and can’t use it efficiently. Lastly, provide some shade for your plants in the afternoon by erecting a light-fabric screen alongside the plants. Don’t cover them as it will be too hot under the cover.

Other plants like squash, cucumber, eggplant and peppers will also be affected by the heat if it keeps the bees and other pollinating insects from doing their jobs. They, like us, will have to just wait it out. Regardless, as soon as the temperatures start cooling down in mid-August, we should see a good fruit set. Let’s then start hoping for late first frost.

An update on my vegetable experiments:

Earlier this summer I told you I was experimenting with sweet potatoes, a grafted watermelon and a tomato-potato combo plant called Ketchup and Fries. The sweet potatoes are going great guns. I lost half of the 12 plants I started with mostly due to gardener error. The leaves have completely filled a 4-by-8 garden box and vines are headed off in all directions at last check. The Ketchup and Fries likes being in a large pot on my deck and so far we’ve enjoyed a handful of cherry tomato-sized fruits. We will have to wait until the tangle of tomato and ornamental sweet potato vine die back to see the potato results. Lastly, the watermelon was a nonstarter. The graft was not strong enough to support the plant through the hot weather and the plant died.

Pat Munts is co-author, with Susan Mulvihill, of “Northwest Gardener’s Handbook.” She can be reached at pat@inlandnw gardening.com.

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