Hot Weather Threatens Tomato Plants 7/15/04

MANHATTAN, Kan. - Extreme temperatures or an abrupt change to hot weather can do a number on tomato plants.

“You have to monitor the plants closely. You won’t be able to counter everything, but some problems will be preventable,” said Chuck Marr, horticulturist with Kansas State University Research and Extension.

Flower drop tends to be the first problem to emerge - especially following an abrupt temperature rise. This blossom drop becomes worse as temperatures go above 95 degrees, particularly if accompanied by hot, dry winds. Plants that are excessively lush or overfertilized are most at risk. Each flower that falls from the vine equals the loss of one tomato.

Harvested Tomatoes
Can ‘Vine-Ripen’

MANHATTAN, Kan. - Every tomato lover knows fresh, “vine-ripe” flavor is by far the best.

At the same time, many don’t know what “vine ripe” means. So, whether growing or buying tomatoes, they may be limiting their access to the top flavor of the season.

“Tomatoes develop their optimum nutrition, color and flavor when they’re in the full red-ripe stage. But getting to that point doesn’t have to occur on the plant,” explained Chuck Marr, horticulturist with Kansas State University Research and Extension.

Tomatoes go through specific steps, Marr said, in developing “vine ripeness”:

* A gas called ethylene regulates the ripening process. Tomatoes start producing this gas internally when they reach full size and become pale green.

* When tomatoes turn about one-half green and one-half pink (called the breaker stage), a layer of cells forms across their stem, sealing them off from the main vine. At this stage, tomatoes can ripen on or off the vine with no loss of quality or flavor.

* Tomatoes don’t ripen at refrigerator temperatures. If harvested in the breaker stage, however, keeping them in a cool place (to a minimum 50 F) will slow down their ripening process. Keeping them warm (to a maximum 85 F) will rapidly bring full ripeness. And, once ripe, they can keep for several weeks in the refrigerator.

“This kind of control allows you to spread out the good eating over a longer period,” Marr said. “A breaker-stage harvest also allows you to protect tomatoes from the heat extremes of summer. Tomatoes can’t form their red pigments when temperatures are above 95 degrees. They’ll still ripen, but they’ll end up a yellowish-orange.”

Leaving tomatoes to ripen on the vine also increases the odds for cracking and other types of damage. Plus, it puts an increasing weight load on the viney plant.

“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,” Marr said.

Heat’s immediate threat to the plants themselves is severe stress, he added. Mulch helps keep the soil from warming up and drying out quite so quickly. Still, garden tomatoes need a thorough watering at least once a week to encourage deep root growth down to the cooler soil zones. Container-grown tomatoes may need watering once or more per day, so long as the weather remains hot.

“Overwatering tomatoes is never a good idea, but you have to stay alert to the fact that hot weather does change the plants’ moisture requirements,” Marr said. “As a bonus, your supplying moisture when the plants need it will also affect the developing fruits. Again, some varieties are more prone to the problem, but a major cause of cracked tomatoes is fluctuations in soil moisture level.”

Temperature alone can affect tomato fruits, however. When daytime temperatures reach 95 F, tomatoes stop producing red pigment, so may ripen to orange. When daytime temperatures exceed 100 F and nighttime temperatures are in the 80s, the ripening process itself slows down, and tomatoes seem to stop maturing.

“Fortunately, neither of these things affects tomatoes’ ultimate flavor and nutrition,” the horticulturist said. “Besides, you always can harvest the full-size fruits that are just beginning to turn red and then finish their ripening process indoors, where it’s not so hot.”

What can ruin the fruits is exposure to intense sunlight. The resulting injury is called sunscald. The tomatoes develop a light-yellow, leathery surface that slowly turns brown. Then they rot.

“Some tomato plants shade their own fruits by maintaining dense, thick foliage. But some varieties lack enough foliage to cover fruit well. And, if a plant disease becomes a leaf killer, that can expose fruit to sunscald, too,” Marr said.

Where leaves aren’t providing enough protection, about the only thing gardeners can do to prevent sunscald is to rig up some kind of artificial afternoon shade.

“It can just be a wood frame with cheesecloth stapled to it,” he said. “Of course, you’ve also got to ensure that your artificial shade doesn’t block the air flow to - and increase the heat around - the plants.

No matter how bad they look, however, neither plants nor fruit will benefit from fertilizer. For heat-stressed tomatoes, a soil nutrition surge can translate into “producing themselves to death,” Marr said.

K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.