Enjoy the splendor and science behind vibrant fall foliage in northern Illinois

The signs are already here: scarlet flashing on sumac shrubs, a reddish tinge in some kinds of maples, yellow leaves beginning to flutter down. The colors are telling us fall is here.

Typically, the color of trees and shrubs is at its height in early to mid-October in northern Illinois, according to Ed Hedborn, manager of plant records and fall color scout at The Morton Arboretum in Lisle. But there’s no way to be sure when or where trees will be at their brightest, because the color is affected by factors that vary from place to place.

“Trees just a few miles apart may have gotten very different amounts of rainfall over the summer,” he said. “Every location, and every tree, is different.”

Apart from the weather, the largest factor is the genetic makeup of the trees themselves. Every species of tree or shrub is genetically programmed to turn a particular color or range of colors.

Buckeyes, elms and Norway maples turn yellow. Like sumacs, Virginia creeper and poison ivy turn bright red, while oakleaf hydrangeas and white oaks turn burgundy. Serviceberry leaves turn orange. And some species, such as sweetgums, combine yellow, orange and red leaves on a single tree. Sugar maples can be yellow or all the colors of sunset, depending on the weather.

In a neighborhood with only a few kinds of trees, most trees may change color and drop their leaves at the same time. At the Arboretum, which has thousands of kinds of trees from around the world, the color is often more varied and lasts longer. Hedborn reports on the procession of tree color at the Arboretum at mortonarb.org/fall-color-report.

Although genes control what colors a tree can turn, the weather is the big factor in how intense the color is likely to be, Hedborn said. Trees that have been stressed by drought usually have muted colors and change early. Where there has been ample rainfall, colors are likely to be more brilliant.

“We see the best autumn color when there has been plenty of rain all summer, and when nights are cool in autumn and days are sunny,” he said.

The colors of trees’ leaves are a side effect of the big change they undergo every fall: entering a dormant state to survive the rigors of the winter. “You can think of it as going to sleep for a while,” Hedborn said. Dormancy keeps deciduous trees and shrubs safe from winter’s cold and drought and helps them conserve resources for their burst of growth in spring.

As trees sense days growing shorter, they start to temporarily shut down many of their functions and get ready to abandon their leaves. They stop producing chlorophyll, the chemical that powers photosynthesis and gives leaves their green color. As the green chlorophyll drains away from each leaf, its underlying colors can be seen — in most species, yellow or brown.

The red pigment that amps up fall color, anthocyanin, is produced only by some tree and shrub species. The amount varies by species and depends both on that year’s weather and on the situation in which the tree is growing.

“A sugar maple growing among other trees in the woods will produce more anthocyanin at the top of the tree, where leaves are in full sunlight,” Hedborn said. “It will only have red and orange high up in the sun. The same species of tree growing by itself might be red and orange all over, because the sun can reach more of the leaves.”

The timing of autumn tree color depends on so many interrelated factors, and is so localized, that scientists are still trying to figure out how climate change is likely to affect it, said Christy Rollinson, a forest ecologist at the Arboretum.

“It seems to be highly dependent on where you’re looking and what species you’re talking about,” she said.

The best way to enjoy the colors of autumn, Hedborn said, is to pay attention to the changes all season long, rather than waiting for one perfect day.

“Each autumn is different, and it’s different in every place,” he said. “Every autumn day is a new experience.”

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.