Autumn Leaves: Peak Of Fall Color Depends On The Weather
By Laurie Casey
The Morton Arboretum
Green grass. Green leaves. We had a whole summer season of green. Now comes autumn, when the landscape changes dramatically. We see plants we didn’t notice before because now they are turning color. Or new plants stand out because other plants have dropped their leaves.
“In autumn, even the light changes,” said Ed Hedborn, The Morton Arboretum botanist and “Color Scout.” “In summer the light was filtered through green leaves. Now, light that filters through yellow leaves brightens the forest floor.”
Did you ever wonder why leaves turn color?
Shorter days and falling temperatures cue plants to get ready for winter, according to Hedborn. With less light, plants make less chlorophyll, the green pigment that has a key role in plants’ food-making process. In the summer, the green chlorophyll is so dominant that it masks other colors in the leaves. But in the fall, when chlorophyll production slows, we are finally able to see the yellows that were there all along.
But where do the brilliant reds, deep purples, and oranges come from? Certain trees have the ability to make red pigments, such as sugar maples, other maples, some oaks, sourwoods, dogwoods, tupelos, black gums and persimmons, Hedborn says. Other trees, such as honey-locusts, elms and hickories can’t make red.
Trees make more or less red depending on the amount of sunlight and rain, day and night temperatures and the plant’s health. In favorable conditions, the reds really pop. When strong red pigments combine with the yellows and oranges, you’ll see fiery reds, deep oranges and bronzes, says Hedborn. When trees make less red pigments you’ll see orange-yellow or mostly yellow leaves.
Many people want to know when the “peak” of fall color will hit. “There’s really no way to predict. It depends on the weather,” he notes.
Rather than calling it a “peak,” Hedborn likens fall color to a sequence, “because, in reality, each plant has a peak that hits at different times.” In September, sumacs, green ashes and hickories, among others, typically turn. This month, it’s usually trees such as redbuds, hackberries, white ash, and white oaks that change. Come November, expect the Japanese maples, callery pears and others to finish up.
Hedborn takes inventory of which trees and plants are changing at the Arboretum, which contains trees and other plants from 40 countries, and publishes free, weekly Fall Color Reports through November. Visit www.mortonarb.org or call (630) 719-7955 for the reports.
One of his favorite fall color trees is the ginkgo, whose fan-shaped leaves turn a pure yellow. “The whole tree turns at once,” said Hedborn. “It’s gorgeous.” But the beauty is fleeting. “You can walk by it one day, and it will be in full glory. The next day, the leaves have fallen into a beautiful ring under the tree. But that’s the fun of autumn…to watch the changes.”
Glad You Asked
Q. Can I add anything to the soil to make my tree display prettier fall color?
A. If soil is too alkaline for pin oak, its dominant colors are brown and yellow. But if you add acid to the soil, and weather conditions are right, pin oak can turn bright red in fall. Other than this one exception, we can’t improve the fall color of our street trees.
Q. Are very colorful trees telling us they are healthy?
A. A colorful autumn is just that—colorful, according to Hedborn. Color does not indicate a plant is healthier or weaker, just that the environmental conditions are good or are poor for color production. “Plants with problems can have leaves that change color, but the color can be brown, yellow, or admittedly in some cases orange or red. But it is not predictable,” said Hedborn.
At the Arboretum
Learn the latest about eco-friendly garden design at Gardeners’ “Night Out” 6:30-9:00 p.m. November 6. $60 (discount for members), includes buffet dinner. Registration open now, call (630) 719-2468.
Laurie Casey is a staff writer for The Morton Arboretum.