Area Leaves Slow to Turn

Michelle Anstett

At a time when temperatures should be dropping and leaves should be changing color, for the most part, they’re not.

While many factors — including moisture, tree health and the amount of daylight — influence the length and brilliance of a foliage season, cool nights are really needed to produce that bright red color which is so dazzling to behold.

"Predicting fall foliage is just like looking into a crystal ball," said John Mullen, a naturalist at Forest Park Nature Center. But, he added, if temperatures remain as high as they have been the last several weeks, it will not bode well for brilliantly-colored leaves.

Over the meteorological summer of June 1 through Aug. 31, 2007, Mike Hardiman, meteorologist with the National Weather Service in Lincoln, said Peoria was slightly below normal in precipitation, with 10.59 inches total. Mean temperatures were about two degrees above normal at 75 degrees.

The hottest temperature for that three-month period was 96 degrees, recorded on both Aug. 7 and 12. That is two degrees below the 98-degree high from summer 2006. In total, there were 24 days above 90 degrees, five more than normal.

Leaves are guaranteed to at least change yellow in the fall, as that color is always lying just underneath the green of the leaf. As the trees photosynthesize less, the green disappears and the yellow remains.

The red is created by sugars produced by the trees, and cool nights in the 35-to 50-degree range are needed to seal those sugars into the leaves, giving them color. "The higher the sugar, the brighter the red," Mullen said.

No matter what the weather, the leaves will fall off the trees, as the shorter days and longer nights will cause the trees to decrease photosynthesis, eventually stopping that chemical process altogether. The leaves then fall from the trees, to be raked into big piles for children to jump in.

August temperatures averaged out to 78.3 degrees, "a little over five degrees above normal," Hardiman said. September has continued that trend, running about three degrees above normal thus far.

And the warmth is expected to remain for the next few weeks, Hardiman added, noting there are "no strong chances for cool air" until the middle of October.

That, Mullen said, could be a problem for people who love to see the brilliant hues of autumn. "You’ll lose your leaves before they color up" if the weather remains warm throughout the season.

"If you have warm days and then, bam, it gets cold, you’re going to have a shorter season" for color changing, he said.

One positive aspect of this summer’s weather for trees was the amount of rainfall. Hardiman said Peoria’s total August rainfall of 4.12 inches was about one inch above normal. That moisture will help keep the trees hydrated, keeping leaves on longer in anticipation of cooler temperatures.

While the color outlook may appear grim, Mullen said the best way to view the science of foliage change is to get out and experience it.

"Spend a lot of time on these beautiful days out in the woods," he said. "You’ll see it from the very first maple or hickory changing."

Michelle Anstett can be reached at 309-686-3196 or manstett@pjstar.com.