Gold in a branch on the sunny side of a tree. Scarlet in a shrub. The first flashes of color mean that fall is coming soon.
Leaf color is a sign that trees are preparing for winter by entering the sleeplike state called dormancy, when they temporarily shut down many of their functions and hunker down until the weather warms up
in spring.
“Dormancy is the big change. The fall color display that we enjoy so much is just a side effect,” said Ed Hedborn, plant records manager and fall color scout at The Morton Arboretum in Lisle.
He tracks the progression of autumn colors, from the early red of sumac in September to the last brown oak leaves in November, in the Arboretum’s Fall Color Reports (mortonarb.org/fall-color-report).
The timing and vividness of trees’ color varies from year to year and depends both on the genetics of different kinds of trees and on the weather. In the Chicago area, most trees in the Chicago area change
color between the end of September and late October.
“If some trees turn early, it’s usually because they’ve been under stress from drought or other adverse conditions,” Hedborn said.
A period of drought can affect tree color months later. “Even if it’s rained a lot recently, a tree can still be stressed from drought earlier in the summer,” he said. In urban areas, soils and other conditions vary enormously over small distances, so one tree can be stressed and change color early, while another nearby remains green.
The weather is a major variable. The most intense fall tree color tends to come in times when days are sunny, nights are cool, and there has been plenty of rain. Hot spells often bring a duller color show.
The spectrum also depends on the species you’re looking at. “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 shades you are likely to see in fall.
Shortening days toward the end of summer are the signal for trees to prepare for dormancy. They stockpile food in the form of sugars in their roots to fuel their new start next spring. Then they shut down their food-making process, photosynthesis, which requires water, sunlight, and chlorophyll, the chemical that makes leaves green.
Once a tree stops making chlorophyll, the green drains from its leaves and reveals the underlying color, which is usually some shade of brown or yellow. That’s why the overall color of the autumn woods is
gold.
The reds and oranges that make fall color really pop come from anthocyanin, a pigment produced by only some kinds of trees. “Sugar maples can produce red pigment, but hackberries can’t,” Hedborn said. Years with sunny days and cool nights tend to lead to more red pigment and more varied colors.
Soon after a tree’s leaves turn color, it lets them go and they flutter to the ground. There, they settle into a layer that insulates the roots and feeds soil organisms as it decays.
Evergreen plants that stay green all year, like pines, spruces and boxwoods, never go fully dormant in winter. They retain their leaves and continue to photosynthesize through the cold season, although at a
slower pace.
The rest of the trees, protected by the bark that covers their trunks and branches, wait for spring and the time to grow a whole new set of green leaves.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.