It's hot! Here are 5 ways you can protect your garden during a heat wave

If you can’t stand the heat, stay out of the garden.

After the last couple of weeks of our stubborn heat dome, my inbox has been flooded with dozens of “what’s wrong with my ...” and “what will this heat do to my ...” questions.

While I attempt to provide a few answers below, I have to admit that over the last two few weeks, other than some early morning watering sessions, I’ve spent much more time in my basement woodworking shop than in the garden. It’s been hot!

Warmth is a good thing in the garden. We spend all winter waiting for the time when the spring sun’s consistent warmth allows us to plant our tomatoes, cut roses for an arrangement on the dining room table, and generally enjoy the many fruits of the garden. But like anything in life, too much of a good thing can be a challenge. And our recent heat wave has driven home this point with many examples.

What does extreme heat do to my tomato plants?

Tomatoes are warm-season plants. We all know that when we jump the gun with early planting to try to hasten that very first harvest, a cool night or two can turn a vigorous young transplant into a weak, puny, and struggling seedling. They need warm soil and warm sunshine to establish and set themselves on the road to bountiful yield. But once the heat of summer hits we can have the opposite problem.

Heat stress can impact your tomato plants in several ways, literally from the ground up.

Starting with the roots, research is a bit variable but the general trend is clear. Once soil temperature approaches the low to mid-80 degrees Fahrenheit, root activity is drastically decreased. Fortunately for many of us, we’ve stayed below that mark. Western Kentucky University’s Mesonet lists maximum soil temperature at the 4-inch depth this year at about 78 degrees and the 8-inch depth a mere 75 degrees. But tomatoes growing in containers are a different story.

I have a couple of heirloom tomato varieties growing in a 36-inch diameter container on my hot, paved driveway. So last week I used a probe thermometer to check the root zone temperature in two spots – 3 inches in from the edge of the pot and right smack in the middle. At 4:30 p.m. on a 93-degree day and in full, blazing sun, the edge temperature was 94 and the center came in at 88.

Just for fun I watered the container and checked again. Now it was 74 at the center and 76 at the edge.

Containers offer tremendously variable temperature profiles and this reminds us all of the importance of irrigation. For tomato plants in containers, it’s important to keep in mind that irrigation both supplies essential water and also is an effective temperature control tool. If you watered your tomatoes before work and they still look ok when you get home in the evening, it might not be a bad idea to give them a shot of water to cool off those precious roots.

Is there an upper-temperature limit for tomato root growth?

Tomato tops are also sensitive to high temperatures. Checking into published research the story is all over the map. You can find reports that indicate 85 degrees as a maximum daytime temperature above which blossoms abort and drop off the plant. Other studies list 92-94 degrees as the critical maximum. One report listed a whopping 104 degrees as the critical point. For maximum nighttime temperatures, you can find a range from 75 degrees to as high as 82 for the point above which tomato flowers don’t yield fruit.

This bit of online research into tomatoes and high temperatures confirmed two things. First, the vast majority of information out there is poorly documented, copied, and misinterpreted junk information that you should always read with a healthy bit of suspicion. Best to look for land grant university research reports rather than somebody’s brother-in-law’s cousin who’s got a big following on his bottle cap-collecting podcast!

The second thing that is clear from the research out there, there is absolutely an upper temperature limit for optimal root growth, shoot growth, and flower/fruit production. The variation seen in the results is primarily due to variety differences with modern, so-called High Heat tomato varieties performing best in heat studies.

How to protect your garden during a heat wave

Beyond the vegetable garden, we see similar patterns of plant responses to high temperatures. Roses, those constant stalwarts of the hot summer garden, also show reduced flowering and growth when stressful heat waves descend on our gardens. Even the normally iron-clad Knock Out rose showed significantly reduced activity at temperatures as low as 89 degrees. The ultimate result was a significant decrease in root growth, shoot growth, and flower size.

So, what can we do as gardeners to help our plants through what will likely become more frequent heat waves, heat domes, and flash droughts? The answers are a bit complicated but here is a list of a few suggestions:

• Give plants a bit more shade than you might normally think best. The few roses I have grown in my garden have done best in part shade rather than full sun.
• Use larger containers. The smaller the container the more the root zone temperature will follow the air temperature.
• Use irrigation as a cooling technique. Just be careful not to turn your garden bed into a swamp.
• Look for varieties specifically developed for heat tolerance.
• Plant more trees.

Beyond that, I always need an extra set of hands in my basement shop.

Paul Cappiello is the executive director at Yew Dell Botanical Gardens, 6220 Old Lagrange Road, yewdellgardens.org.

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