Why unseasonably warm September weather may delay peak fall color in some trees

The first week of fall is bringing milder temperatures to the Twin Cities, but the unseasonably warm September has delayed the peak of fall foliage in some trees, weather and forest experts say.

Prior to Sunday, the first official day of fall, the Twin Cities had an abnormally warm two-week stretch, with highs in the 80s every day except for Sept. 14, which had a high of 76.

Mike Griesinger, a lead forecaster for the National Weather Service in Chanhassen, said that while the shrinking amount of daylight is the primary factor driving fall color, the warm weather has delayed the transition.

“By now we should’ve seen some color,” Griesinger said last week. “A warm September like this can help push that peak fall color out a week or two, but beyond that it gets hard to do.”

He added that because of climate change, the month of September has been feeling more like summer than fall.

Brian Schwingle, a forest health program coordinator for the Minnesota Department of Natural Resources, said it’s difficult to generalize how warm weather can impact fall foliage timing. But he said it can affect some trees, and cited a study out of Massachusetts that suggests some trees take longer to turn in warmer weather.

The study looked at 12 species of deciduous trees, including two species of oak found in Minnesota — red and black — that were found to have delayed fall colors in warmer temperatures. Affected species had noticeably delayed fall foliage, while some others saw no significant impact.

Schwingle noted that the most obvious widespread change in foliage happens when there are severe weather events, like an unexpected overnight freeze that can force trees to drop their leaves prematurely.

The timing of peak fall colors — yellow, orange or red — is hard to predict unless someone monitors one specific tree, Schwingle said.

Another factor is the large amount of rain the Twin Cities saw this spring and early summer. Increased rain can help extend trees’ green color in fall, but it can also create a favorable environment for fungal leaf disease. This year, an estimated 10% to 30% of all aspen trees suffered from leaf disease, Schwingle said. The disease has caused many aspens’ leaves to turn brown or fall off, which is reducing overall fall color of some forests, he added.

“If you were just to snap a photograph of Minnesota, and somehow were able to analyze fall color brilliance, it would be less this year,” Schwingle said.

A general guide to peak fall colors, based on the past 14 autumns in Minnesota, is available online at www.dnr.state.mn.us/fall_colors/index.html. In some forests, the warm early fall will lend itself to a diverse mix of colors, Schwingle said, since some trees are staying green while others are changing.

“Beauty is in the eye of the beholder,” he said. “You don’t always get the whole landscape in sync in terms of fall leaf color change.”