Chicago-area residents are noticing that leaves are beginning to change color on some trees already, and experts from the Morton Arboretum say that the region’s recent weather is having an impact.
According to Dr. Christy Rollinson, a PhD in forest ecology at the Arboretum, early-to-mid September is a critical time in determining how quickly leaves will change their colors.
“It’s kind of a critical season. There’s always trees that turn colors a little early, but we’re getting a little bit more color than normal,” she said. “If that trend continues, it’ll be driven by the dry weather we’re having.”
Rollinson said that warm weather by itself doesn’t impact how quickly leaves will change color, but that it can help dry out soil, which limits moisture that flows to leaves and can encourage them to change color and die off more quickly.
“Warmer weather will impact fall foliage more toward the later end of fall,” she said. “Warm weather will impact it by the moisture in the soil available. Drought is a combination of how much water is available for plants, and how much they need, and how much they need is driven by the temperature.”
According to the U.S. Drought Monitor, most of the Chicago area is currently considered “abnormally dry” due to low amounts of rainfall in recent weeks.
That hot and dry weather could continue for at least the next week or two, with the National Weather Service’s Climate Prediction Center calling for above-average temperatures and near-normal precipitation over the next 8-to-14 days.
Leaves are green because of the chlorophyll they use to absorb energy from sunlight during photosynthesis, but as temperatures drop and days get shorter, trees get less direct sunlight, and the chlorophyll in leaves starts to break down.
As that occurs, trees begin to close off the veins that carry water and nutrients to and from their leaves. Once that process is complete, leaves begin to die and weaken at the stem before falling to the ground, according to experts.
According to American Forests, patchy leaf coloring will start to occur late this month, with colors nearing their peak in mid-October. Most of the Chicago area will be past their peak of colors by the end of October.