When yellow school buses start rolling, look around for gold in the trees. The fall color show is coming.
“Exactly when, we can never say for sure,” said Ed Hedborn, the plant records manager and fall color scout at The Morton Arboretum in Lisle. He reports on the fall color changes at the Arboretum weekly at mortonarb.org/fall-color.
“Generally, trees in the Chicago area turn between the end of September and late October, but every year is different,” he said.
The big variable is the weather. The changing climate is making our weather more volatile and extreme overall, with more periods of drought. That makes fall color less predictable.
Drought in summer stresses trees and can cause them to change color early or have muted colors. “This year there’s been plenty of rain, so most trees should be well set up for fall,” he said. “Of course, that could always change.”
The leaf color we enjoy so much is a side effect of trees’ preparation for winter. To avoid cold and drought, deciduous trees enter a sleep-like state called dormancy until the weather warms up in spring.
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While the trees are dormant, they don’t need leaves to collect sunlight and make food, so they stop investing in them. The leaves quit producing chlorophyll, the chemical that powers photosynthesis and gives them their green color. As the green chlorophyll drains away from each leaf, its underlying colors can be seen.
In most species, that’s yellow or brown. The red pigment that produces dramatic oranges and reds is called anthocyanin and is produced only by some kinds of trees and shrubs. The amount produced in fall varies by species.
“Each kind of tree has its own characteristic color possibilities, which are coded in its genes,” Hedborn said. The more diverse the range of trees in your neighborhood, the more different hues you are likely to see in fall.
Color also depends on the situation in which a tree is growing. It may change color early if it is planted in a stressful situation — for example, in a hot, dry, cramped parking lot island, or in a playground, where the soil over its roots is compacted by foot traffic.
“Generally, the widest range of color occurs when there has been plenty of rain all summer, and when autumn nights are cool and days are sunny and seasonably warm,” Hedborn said.
For homeowners, the best way to help trees and shrubs show their best color every year is to make sure they have enough water all through the growing season, said Sharon Yiesla, plant information specialist in the Arboretum’s Plant Clinic.
Even in a summer like the one just past, when it seemed like there was plenty of rain, make a habit of checking the soil often to make sure there is moisture in the top couple of inches. “Sometimes rainfall doesn’t reach the soil under a tree because it gets blocked by the leaves,” Yiesla said.
New trees, shrubs and perennials are especially vulnerable: “If you water nothing else, be sure you water anything that you planted this year, even back in spring,” she said. It can take years for new plants to develop enough water-storing roots to survive on rainfall alone.
With your watering help, your trees and other plants can thrive to put on their best show in every season, whether it’s spring bloom or the bright colors of autumn leaves.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or [email protected]). Beth Botts is a staff writer at the Arboretum.
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