The signs of fall are well apparent: temperatures are dropping, seasonal items are rolling into stores and pumpkin spice lattes are available once again.
One hallmark of fall – peak foliage – isn’t here just yet. However, you might already be noticing that the trees in your yard are exhibiting signs of changing colors. Trees that are popular in landscaped areas are starting to show the first bits of color, explained Christy Rollinson, forest ecologist at The Morton Arboretum.
The same can’t be said for trees in forest preserves and natural areas; they might not begin changing colors for several weeks yet. Likewise, if you’re looking forward to seeing phenomenal colors — hues of red, green and orange — you’ll need to do a bit of waiting.
Typically, peak autumn color happens in mid- to late-October in the Chicago area. Despite the uncertainty, farmersalmanac.com expects peak color for northern Illinois to fall somewhere between Oct. 5 and 21.
Trees usually start changing as the days continue to get colder and nighttime temperatures get consistently quite cold – into the low 40s, 30s or even lower, Rollinson explained.
While an early peak fall color season isn’t expected, Rollinson noted the possibilities.
“Things can change very quickly as we progress into September and October,” she said. “When what happens day to day really has a strong influence on the process of trees changing color and preparing for winter.”
The past month and much of the growing season have been pretty moderate for trees although the extreme heat from the end of August may have caused increased stress, she said. However, that might not have much of an impact.
Morton Arboretum and its research partners found trees really respond to extreme weather conditions over longer time periods — several weeks or more — compared to short-lived heat waves or dry spells.
“Trees are really well equipped to hang in there and play the long-game,” Rollinson said.
While it appears peak colors won’t emerge for several weeks, you might be wondering what causes trees to change color in the first place — or what causes the vibrant hues.
Trees with leaves that change colors are green most of the year because of the chlorophyll they use to absorb energy from sunlight during photosynthesis, according to the an article by the Smithsonian Institute.
But when temperatures drop and days get shorter, trees get less direct sunlight, and the chlorophyll in the leaves breaks down.
“How much and how fast leaves transform varies by location on the globe,” the article explained. “The best colors are produced when the weather is dry, sunny and cool. Places that are cloudy, damp or warm won’t see the same degree of changing color.”