Tomato Patch: How Tomatoes React to Extreme Heat

When the forecast predicts the temperature will approach 100° F, I water the Tomato Patch just after dawn and, usually, soaked to the skin with sweat, retreat to the shower and then breakfast with Ellen. Throughout the day I check our digital thermometer and wonder how my tomatoes are coping in the heat.
Extreme heat—such as the 106°F officially recorded at nearby BWI Thurgood Marshall Airport on Friday afternoon-- affects tomatoes in several ways.

One is that high temperatures can cause “tomato blossom drop.” In a posting of that title, about.com describes the problem as where tomato blossoms “dry up and fall off the plant before a fruit is formed.” The posting explains: “Tomatoes grow best if daytime temperatures range between 70 F / 21 C and 85 F / 29 C. While tomato plants can tolerate more extreme temperatures for short periods, several days or nights with temps outside the ideal range will cause the plant to abort fruit set and focus on survival. … High nighttime temps are even worse than high daytime temperatures because the tomato plant never gets to rest.”

Perhaps there’s not a specific temperature at which blossom drop begins. Online postings state general figures, like the about.com statement above, ranging from 85 to 95 degrees. The point is that high temperatures can interfere with tomato fruit production. Since our local temperature reached 106°F on Friday, I’ll mention that a University of Nevada website said a 104°F temperature for as little as four hours will cause blossom drop.

Yellow Plum tomato leaves respond to high temperatures

Another tomato response to extreme heat involves its leaves. Perhaps you’ve noticed that some tomato varieties respond to heat by curling their leaves. That’s a defensive mechanism that attempts to slow transpiration of water from plant to atmosphere.

But extreme heat can cause more than leaf curling. In a posting titled “Excessive heat on tomato plants,” ehow.com describes what can happen: “The damage done to a tomato plant in excessive heat can include wilting stems and leaves that become dried and brittle. Also, the tomatoes themselves can be damaged. Their growth can be halted with excessive heat. Even if they look ripe, tomatoes that have been exposed to intense heat can be red outside and green inside.” Other sites mention that tomatoes often stop making red pigments at high temperatures, so red varieties under extreme conditions may turn pink or orange-red, rather than red, when they ripen.

Despite the extreme heat and its adverse effects on the Tomato Patch, there are still reasons for hope. A horticulturist with the Kansas State University Research and Extension commented about blossom drop: “You can’t do anything to prevent it, although some varieties are more prone to blossom drop than others. If you can keep the plants alive and healthy, however, they’ll put out new flowers that produce fruit when cooler weather returns.”

So if your plants suffer blossom drop, don’t despair! Make sure your plants are well mulched and deep-watered, and when cooler weather returns, your plants will start flowering and setting fruit again, hopefully with enough time to produce ripe fruit before fall frost.

Ah, cooler weather—when is that coming?

I thought you’d never ask.

Cooler weather, on the average, begins today!

Let me explain. Several years ago, I found a tab at weather.com that allowed me to research and print out daily average temperatures for Dayton, Maryland, the nearest town for which statistics were available. I printed out the averages (since 1967 apparently) for both high and low temperatures for the entire year.

The highest average high temperature for this neighborhood is 88° from July 18 to 23. The average drops one degree, to 87°, on July 24, which is today. Our highest average low temperature is 64° from July 17 to 26, and our nighttime temperatures will drop one degree on Tuesday.

Now doesn’t the fact that, on average, cooler weather begins today make the recent extreme heat wave seem just a bit more tolerable?

Let’s hope—for the sake of our tomatoes and our electricity bills—that average temperatures soon return.