As you watch leaves turning yellow on buckeye trees, red on Japanese maples and burgundy on white oaks, you may wonder: Why does this happen? Why don’t all trees stay green all year, like a pine or a spruce?
“The reason is that different kinds of plants have different ways of handling the challenge of winter,” said Ed Hedborn, plant records manager and fall color scout at The Morton Arboretum in Lisle.
Deciduous trees — the ones that lose their leaves — duck the challenge altogether, by temporarily shutting down many functions and entering a sleep-like state called dormancy until the weather warms up in spring.
“These trees don’t need their leaves when they’re dormant, so they let them fall,” Hedborn said. “The color change is just a side effect of the process of entering dormancy.”
As trees sense days growing shorter in late summer, they start to shut down 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 with oranges and reds is called anthocyanin and is produced only by some tree and shrub species. The amount produced in fall varies by species and depends both on that year’s weather and on the situation in which the tree is growing.
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“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.”
Color differences between tree species are coded in their genes. Every deciduous species has its own characteristic range of color possibilities. “Buckeyes and hackberries can only turn yellow,” Hedborn said. “They don’t have the genes to make red, like a sugar maple or a Japanese maple.”
Evergreen plants that stay green all year never go fully dormant in winter. They retain their leaves and continue to photosynthesize, although at a slower pace. To keep going through the winter, they pay a price: Their leaves are usually small and long, like needles, to expose less surface area to the drying winds, and are coated with protective wax. Evergreens’ needles still run a risk of drying out, especially if they don’t store up much water in autumn.
The timing and intensity of deciduous trees’ color is not the same from year to year and from place to place. It’s unpredictable because it depends on many factors, from the tree’s genetic makeup to long-forgotten weather early in the growing season. “The most attractive color will be seen on trees that have been healthy all year,” Hedborn said. “Drought stress in spring can affect tree color months later.”
Generally, the widest range of vivid color occurs when there has been plenty of rain all summer, and when autumn nights are cool and days are sunny. Typically, the color peaks in the Chicago area in about mid-October.
Hedborn 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 Report (mortonarb.org/fall-color-report).
“We’re just getting started at the end of September,” he said. “It’s exciting to see what autumn will bring.”
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.