The trees in your garden and your neighborhood endured a serious dry spell this year, which may affect their fall leaf color.
“The weather is the biggest factor that determines when the color change occurs and how vivid it is,” said Ed Hedborn, plant records manager and fall color scout at The Morton Arboretum in Lisle.
“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.”
Rainfall was not plentiful in late summer and fall this year. Illinois did have a wet July, but the tap shut off in August, according to the Illinois State Climatologist’s office. By mid-September, parts of the state were officially in a moderate drought. September’s daytime temperatures were more like the heat of summer than autumn.
“Dry and hot weather tends to produce less vivid tree color and earlier leaf drop,” Hedborn said. Typically, trees’ leaf color peaks in the Chicago area in mid-October.
Whether recent rainfall can change the fall color prospects remains to be seen. “It’s different every year, so watching the change of seasons is always an adventure,” he said. He reports on the changes at the Arboretum weekly at mortonarb.org/fall-color.
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.
While the trees are dormant, they don’t need leaves to collect sunlight and make food, so they cut them loose. “If the trees are already under stress from drought, they may go dormant and drop their leaves a little earlier,” Hedborn said.
Soon-to-be-abandoned leaves 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 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. It also depends both on each year’s weather and on the situation in which the tree is growing.
“A sugar maple, for example, produces more anthocyanin in the leaves that are most exposed to sunlight,” Hedborn said. “Leaves on lower branches that are shaded in a forest won’t show much red or orange, just yellow.”
In a home landscape, the best way to help trees and shrubs show their best color every year is to water them, said Spencer Campbell, Plant Clinic manager at the Arboretum.
Plants need watering in autumn because their roots are still growing, even after they drop their leaves. Evergreens need to store up water to keep their broad or needle-shaped green leaves from drying out over the winter.
“Water even if a storm has passed through,” Campbell said. “When soil is dry and baked, a short rainfall will just run off the hard surface. Trees and plants need a long, slow soak, so the water can soften the topsoil and seep several inches down into the soil.
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.” It can take years for new plants to develop enough water-storing roots to survive on rainfall alone. “We need to help them by watering even in normal years, much less when there’s drought,” he said.
Don’t neglect mature trees. “Drought stresses all trees, and it can have lasting effects that lead to decline years later,” he said.
With your watering help, your trees and other plants so 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 plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
