MANHATTAN, Kan. – Tomatoes are a warm-season plant that’s averse to summer heat.
When daytime temperatures rise above 95 degrees, the plant’s ripening fruits turn orange, not red.  If those hot days combine with nighttime temperatures above 75 farenheit, the plant’s fruit set is poor.
“Neither reaction is permanent. If you can just keep that plant alive, it’ll produce normally again when the weather’s cooler,” said Ward Upham, K-State Research and Extension horticulturist.
To avoid heat’s “orange” effect, gardeners can harvest when existing fruits first start to ripen from green.
“They’ll finish ripening off the vine,” Upham said. “And, if you keep them in temperatures of 75 to 85 degrees, their red pigment will form just as it should. You’ll get normal-looking, deep-red tomatoes.
“If you don’t want to go to the trouble, though, don’t worry. Orange tomatoes are just as edible as red ones.”
Gardeners can’t force tomato plants to set fruit in hot weather. Heat not only leads to abnormal growth in the flowers’ reproductive organs but also interferes with pollen’s viability, he said. Wind and/or dry weather simply make the responses worse.
“It takes about three weeks for flowers to try to develop fruit before you can see something’s wrong.  At that point, fruit should be within a week of reaching full size and starting to change color,” Upham said.
“Heat-set” tomatoes are now on the market under such variety names as Florida 91, Sun Leaper, and Sun Master, he said. Often, however, they can handle just 2 to 3 degrees more heat before they also develop fruit-set problems.
“Fortunately, going from flower to ripened fruit generally takes no more than 45 days. So, if gardeners can keep the plants alive now, they should have plenty of time to harvest more tomatoes before first frost,” Upham said.