How might this summer's extreme weather and the now-historic El Niño impact Chicago region fall color?

Experts at The Morton Arboretum monitoring conditions closely; Walking Plays and Cider and Ale Festival kick off fall color events

LISLE, Ill., Sept. 22, 2026 /PRNewswire/ -- As the seasons change, the tree experts at The Morton Arboretum are monitoring weather factors—including the record-breaking El Niño—most likely to impact fall leaf color in the Chicago region.

"We saw a lot of weather extremes this summer, from drought to heavy rainfall, which will likely impact trees and their foliage very differently and have isolated effects," said Christy Rollinson, Ph.D., senior scientist in forest ecology at the Arboretum. "The warming impact of El Niño in our region, particularly as we progress toward winter, may result in a compressed fall color season with leaves changing all at once in late fall."

Drought, which was seen across the region earlier this summer, would normally be linked to earlier fall color, Rollinson said. And while some fall color has been observed in area trees already, those are species that typically turn early, like the autumn blaze maple. The periods of drought at the start of summer have since been offset by significantly above-average precipitation over the last 90 days, as reported by the Illinois State Climatology Office.

The earliest colors have already appeared in individual trees in "high-light and high-stress environments," such as parking lots, in late August and early September, Rollinson said. On average, maples will develop their brightest and most consistent color in mid-October, and oaks in late October, she said. Overall, leaves tend to stay on trees in the Chicago region through early- to mid-November.

Rollinson and her team are also monitoring the strengthening El Niño, expected to bring warmer-than-average temperatures to October, November and December, as indicated by the most recent seasonal outlooks from the National Weather Service. El Niño refers to a warming of the Pacific Ocean's surface that impacts weather patterns worldwide.

"If this holds, we expect El Niño to delay autumn color in general," Rollinson said. "But once we see several nights of freezing temperatures that leaves can't survive for long, their colors will begin to change quickly."

There is consistent evidence that warmer autumns are generally associated with later fall color in the Chicago region, she said. But the length of daylight is another important factor at play and tree species respond to cues differently. As scientists assess how the season might continue to shift in comparison to its official calendar start, they consider that balance between light and temperature.

"Although our autumn temperatures may become more like southern Illinois' in future years, the timing of the temperature relative to day length won't be equivalent, and we don't know yet exactly what that will mean for the start of the season," Rollinson said.

The Arboretum, which has tree collections containing nearly 4,100 different types of plants, has a long fall-color season to both enjoy and study, as tree species change color at various times. Arboretum visitors can track fall colors as they emerge throughout its 1,700 acres in a weekly Fall Color Report beginning this week on its website.

Fall Color Festival

The Arboretum celebrates its changing fall landscape each year with a Fall Color Festival featuring festivities for all ages throughout October. Special events include Walking Plays: Wuthering Heights, Saturdays and Sundays, Sept. 19 through Oct. 25; Cider and Ale Festival, Oct. 3; the 16th annual Glass Pumpkin Patch, Oct. 9 through Oct. 11; a new Autumn Reserve Wine Experience, Oct. 15; new Plant Bingo, Oct. 17; and the Fall Color 5K Run and Walk, Oct. 24. Seasonal concessions will be available on weekends in October and Monday, Oct. 12 (Indigenous Peoples' Day) in the Arbor Court area outside the Visitor Center, including apple cider doughnuts, taffy apples, cotton candy, warm kettle corn, warm pretzels, chili, and a selection of craft beers and warm beverages.

The Arboretum also recognizes OAKtober: Oak Awareness Month, with programs highlighting the beauty and importance of oak trees. Learn about and connect with trees through classes, guided walks, Forest Therapy and more throughout the month, including 250 Years of American Woodlore, Oct. 3; IAA Careers in Arboriculture Demonstration Event, Oct. 7; and In Trees: A Conversation with Robert Moor, Oct. 19.

For more information about Arboretum programs, special events and admission, visit mortonarb.org.

About The Morton Arboretum
The Morton Arboretum is a globally recognized leader in tree research and education. Its 1,700-acre site cares for 106,714 specimens representing 4,067 different kinds of plants. The Arboretum's Center for Tree Science, Global Tree Conservation Program, Chicago Region Trees Initiative, and Center for Species Survival: Trees are contributing scientific knowledge and technical experience to secure the future of trees locally, nationally, and worldwide. Information about the Arboretum's scientific work and how it contributes to a greener, healthier world where people and trees thrive together can be found at mortonarb.org. 

SOURCE Morton Arboretum