Fire and Ice: How Extreme Temperatures Impact Your Tomato Plants

The ripening tomato is a sight to behold in the garden. It’s certainly the pomme d’amour of my eye. A warm-season crop, tomatoes are one of the most commonly grown vegetables in the home garden. But while they are relatively easy to grow, I consider tomatoes the “Prima Donnas” of the garden.

For a successful tomato harvest, the growing conditions for the tomato plant must be just right; this begins with seed germination and continues to fruit ripening.

You would assume that being a warm-season crop, the tomato would relish our recent hot temperatures. Unfortunately, this is not the case and you may begin to see the effect on your plants now and even later in the season.

Weather Woes

Weather conditions, such as temperature fluctuations, extreme heat, wind, and rain, greatly impact the tomato plant’s performance. Of these conditions, temperature plays the most important role; affecting proper pollination, fruit set, and the ripening process.

Let’s explore how temperature affects each of these process in the tomato plant.

Pollination Problems

Tomatoes are self-pollinators with “perfect” flowers, meaning the blossoms have both male and female reproductive parts. They don’t rely on insects for pollination (although that can occur).  Often pollination occurs before the blossom even opens.

When pollination doesn’t occur, the blossom dries and falls off the plant. This is known as blossom drop. But, wait… if the tomato is self-pollinating, why wouldn’t pollination occur?

It’s commonly due to weather conditions, especially in our region. The optimal temperature range for successful pollination is between 70⁰F and 85⁰F. When temperatures are below or above this range, pollination is affected and can cause issues like blossom drop or even misshapen fruit.    

I know it’s difficult to watch tomato blossoms drop to the ground, but don’t panic, your healthy tomato plants will continue to blossom and set fruit once it’s a little cooler.

You may also see some odd-looking tomatoes developing on the vine. These are usually the first fruits set on the plant and may exhibit several fruits fused into one, be severely misshaped, or have zig-zagging scars on the skin (or all of the above). Talk about Franken-fruit! 

These abnormalities are also due to pollination issues, but in this case, it’s due to cooler-than-normal temperatures early in the season. Conjoined fruits are the result of “mega-blossoms” or blossom fusing. A mega-blossom should have formed into multiple flowers; instead, it fuses into one large blossom. Blossoms often look like dandelion flowers on the plant. When examined closely, two or more ovaries will be present within the blossom. The result is one very large lumpy, tomato.

Cat-facing, often found in large heirloom tomatoes, is an abnormality characterized by its deeply creased fruit with a brown, cork-like deformity on the blossom end. It’s common on fruit set early in the season when temperatures are more likely to be unstable. (Note: Cat-facing is not the same condition as blossom end rot, although the brown, leathery patch on the fruit may look similar).

Do your tomatoes have scars running down their skin? This phenomenon is known as zippering, and again, it is a condition caused by low temperatures during pollination. When the anther of the tomato blossom becomes stuck to the newly forming fruit it creates a zipper-like scar usually from the blossom end to the stem. Some fruits may develop a closed hole as well.

Given our cooler-than-usual spring, don’t be surprised if you see these abnormalities on your tomatoes. The good news is these are all minor physiological issues. None cause harm to the plant or render the resulting fruit inedible.

Too Hot to Ripen

Not only do tomatoes have an optimal temperature range for pollination, but they also have an optimum temperature range for ripening – 68⁰F and 77⁰F. The further the temperature moves in either direction from that optimum, the slower the process will be.

In addition, when temperatures are consistently over 85⁰F the production of lycopene and carotene, the pigments that give tomatoes their color, stops. This causes fruit to remain in the mature-green stage or a shade of yellow to orange. Ripening may or may not continue once conditions improve.

Our extended periods of extreme heat can significantly slow down or even stop the ripening process in our tomatoes.

Keeping Our Cool

Unfortunately, there isn’t anything we can do to control the weather, but you can keep your tomato plants cool by installing a shade cloth and mulching around your plants to keep the soil cool and moist. Pay special attention to plants growing near walls or structures that radiate excessive heat.

Oh tomatoes, we still love you, despite your especially finicky, fair-weather growing needs. We are thankful to harvest your sweet, delicious fruits, and at summer’s end, pat ourselves on the back for a job well done.