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Could a 'false fall' be changing Colorado leaves early?

The weather is likely to blame for what appears to be early fall color on the Front Range, but arborists its likely disease and not true fall color.

DENVER — Sometimes in the late summer on the Front Range, we get something known in local lore as "false fall."

That's not exactly a real meteorological term, but it basically when a cool snap leads people into believing that fall weather will come early. 

And Monday mornings weather would best be described as fall weather. The official Denver temperature dropped to 49 degrees which broke a cold record for Aug. 14. 

The high temperature didn't even make it to 80 degrees on Monday or the day before. And that's the third unusual cool snap of the summer. The high on Aug. 6 was just 70 degrees. In early July, Denver had three consecutive days with highs in the 70's. 

So you can certainly understand why some minds might be drifting to thoughts of an early fall. But then of course, we entered into an extended heat wave which is forecast to last for at least 14 days. Proving that the cool weather earlier in the week was simply false fall. 

Adding to the intrigue perhaps is the scenes of gold, orange and red leaves in isolated patches around the area. 

“We have such weather extremes," said certified arborist Tony Hahn with Denver Commercial Property Services. "That’s what tricks these trees, it’s just brutal.” 

Hahn said Mother Nature's joke is to make Front Range trees think nice and average weather is headed they're way, but the truth is our weather averages are only reached by finding ground in between extremes. 

2023 weather has brought extreme cold and long-lasting snow cover. Extreme wind. Extreme rain and cloudy conditions. Add in an unusually high amount of hail and a tornado that killed an estimated 16,000 trees in Highlands Ranch

Hahn said any discolored leaves should be blamed on the weather, but he said its likely not true fall colors. 

“Maybe what people are seeing are fungal diseases on the foliage because of the weather that we had back in May and June,” he said. 

He said this summer he’s seen more diseases like Hawthorne Rust, Bacterial Fire Blight, and Maple Wilt on the Front Range. Likely due to the extremely wet and cloudy conditions during the spring.  

From a distance those colors can look like beautiful fall color, but a closer inspection would reveal a less appealing appearance. Dull, brown, and unhealthy looking.

He also said that some patches of discolored leaves could be a result of some kind of branch injury.

And he said some of the early golds and yellows might be something called Micronutrient Chlorosis. When the clay soils on the Front Range get too wet, some trees, especially oaks and maples, have a hard time getting the food they need to produce that green chlorophyl.

Credit: KUSA

But when it comes to actual fall color, which is when photosynthesis gets shut down in the leaves – he said it’s the people not the trees that usually get tricked. 

While the weather does get cooler in the fall, it’s the shorter length of daylight that shuts down photosynthesis and causes the leaves to turn. So, despite a few cool mornings, the trees are well aware that it's still summer.

Hahn said it’s best to pre-treat trees before they get a leaf disease but check with your arborist because sometimes treatment is possible. In the case of a nutrition issue causing chlorosis, often the treatment is a macro injection done in the fall by a professional. 

One thing you should do if you have trees with leaf disease is dispose of the leaves after they fall. He said if they sit on the ground over winter, the disease could return next summer.  

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