Lisle, Ill. – September 22, 2026 -- Scientists at The Morton Arboretum are tracking how a strengthening El Niño could push back peak fall foliage across the Chicago region this year, potentially compressing the color season into a shorter, more intense stretch in late autumn.
Summer weather extremes are already scrambling normal leaf-color timing
Christy Rollinson, Ph.D., senior scientist in forest ecology at the Arboretum, said this summer's swing from drought to heavy rainfall will affect trees unevenly across the region. Early-season drought typically triggers earlier fall color, and some early turners like the autumn blaze maple have already begun changing in late August and early September, particularly in high-stress spots like parking lots. But the Illinois State Climatology Office recorded significantly above-average precipitation over the last 90 days, offsetting that early drought stress.
Warmer temperatures from El Niño may push color changes later into the season
The National Weather Service's latest seasonal outlooks point to warmer-than-average temperatures through October, November and December due to El Niño, a warming of the Pacific Ocean's surface that shifts weather patterns globally. Rollinson said that if the pattern holds, it should delay the arrival of fall color overall. Color will then shift rapidly once several consecutive nights of freezing temperatures occur, since leaves cannot tolerate sustained freezing for long.
On average, maples reach their brightest color in mid-October and oaks in late October, with most leaves clinging to trees through early-to-mid November. Day length remains a separate driver of leaf timing, layered on top of temperature, and Rollinson noted that even as regional autumns trend warmer, the relationship between temperature and daylight hours will keep shifting the season in ways scientists don't yet fully understand.
The Arboretum is tracking colors weekly across its 1,700 acres
The Arboretum, home to nearly 4,100 kinds of plants, began publishing a weekly Fall Color Report on its website this week so visitors can follow changes as they emerge.
A month of festivals runs alongside the color season
The Arboretum's Fall Color Festival runs through October, including the Walking Plays production of Wuthering Heights (weekends, Sept. 19–Oct. 25), the Cider and Ale Festival (Oct. 3), the 16th annual Glass Pumpkin Patch (Oct. 9–11), a new Autumn Reserve Wine Experience (Oct. 15), Plant Bingo (Oct. 17), and the Fall Color 5K Run and Walk (Oct. 24). October is also OAKtober, featuring events like 250 Years of American Woodlore (Oct. 3) and a talk with author Robert Moor (Oct. 19).