ST. LOUIS COUNTY, Mo. — It's technically fall, but autumn plants will not thrive if the mercury continues to rise to 90° and above. 

Spotty showers have also missed some gardens leaving them thirsty in the sweltering heat. Chip Tynan with the Missouri Botanical Garden said it's the first time in two years that he's had to set a sprinkler out to keep his plants alive. 

"It's interesting," he said, "only in the past two weeks I've had to use the overhead sprinkler."

The late heat, coupled with drought could certainly pose problems for plants if left untended. Tynan said a short round of heat will kill your autumn varieties. 

"If they're just wilting because of the heat, that's OK as long as there is adequate moisture in the soil," he said. "But if you go out there in the morning and find that they are still wilted then you need to go ahead and water that plant."

Providing temperatures do follow the forecast and plummet into the fall-like low 70s, Tynan said both flowers and trees will do great. 

"This would be timely arrival of cooler temperatures and will allow the plants to go ahead and bloom very well before the days get so short that they simply can't continue," he said.

While hot temperatures could cause problems for plants like broccoli and cabbage, summer plants are actually benefiting from the late heat. The start of the growing season was slow because of spring's wet and cool conditions. The current heat is prolonging the growing season, thus giving plants like corn and tomatoes a chance to catch up.

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