A maple here, an ash there. Fall color has been arriving a month or more early for some trees in the Chicago area, says Peter Bristol, curator of woody plants at the Chicago Botanic Garden in Glencoe.
Normally, the leaves start changing in mid-September, according to Ed Hedborn, plant records manager for The Morton Arboretum in Lisle. The early color change seen in some trees this year is usually a sign of stress, he says. “That just means the plant is in trouble.”
Autumn color change for healthy trees is due to a combination of lower temperatures and decreasing day length. As trees begin to shut down for their winter dormancy, they no longer need chlorophyll. When the chlorophyll-making structures die off in the leaves, the green color recedes and reveals the yellow tint of other chemicals that have been masked all season. Some trees create a red pigment.
For early-turning trees, stress came this year from the weather, according to Bristol. “We had a very wet spring and early summer, but then a fairly severe dry period in August,” he says. Because the ground was saturated in early summer, the fine feeder roots of many trees had grown up toward the surface, trying to get more oxygen, he says. There, they were vulnerable when the dry spell hit.
With fewer feeder roots, some trees started to shut down their leaves so they wouldn’t have more foliage than the roots could support.
Another factor was the cool weather during much of August. “The leaves aren’t functioning as they would in warmer weather,” he says, which triggers them to shut down and fall.
The early color usually is seen in a tree here or there that is especially challenged, according to Hedborn. The trees may be newly planted, or may be in a hot, sunny, dry site or a confined area with little room for roots; may have its soil compacted by foot traffic; or may have been planted incorrectly. Gardeners should make note of the trees that are turning early and plan to water them better next year, Bristol says.
Even now, if you dig with a trowel down 4 to 6 inches and find the soil dry, it’s good to water, Hedborn says.
What kind of a color season will we have this fall? It depends on the weather, Hedborn says; the brightest, deepest, longest show comes if we have a September and October of bright, cool, sunlit days.
