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Warm Days, Cool Nights Spur Bright Fall Colors


It’s turning into a beautiful fall in the Northwoods.  In much of the state, leaves on deciduous trees started to change color a little later than usual.  Bill McNee, DNR Forest Health Specialist in Sheboygan says that’s thanks to recent wet weather.

“We had a lot of soil moisture and the trees are less stressed. Which means the trees aren’t in a hurry, as they would be in a year when there’s not enough moisture.”

The physical process behind the change in color may surprise you. Trees change color because they’re moving chlorophyll and sugars out of the leaves and into the root system.  With less green pigment, McNee says leaves reveal colors of orange and yellow that have been there all along. 

“As the nights start getting longer, and the temperatures start getting cooler at night, the trees reduce their production of the green pigments. And what this does is it exposes the oranges and the yellow colors that were always there – they were just masked by the green pigments.”

Red and purple pigments are produced in the fall…as a sort of sunscreen to protect the leaves.  McNee says these shades are the most variable from one season to the next.

“What we typically find is the orange and the yellow colors remain relatively constant from year to year. What does change in response to the temperature are the red and purple pigments…because these are the ones that are actively manufactured in the fall.”

Ideal weather for maximizing color includes warm sunny days and cool nights that aren’t quite freezing…which is what’s in the forecast for the next couple of days.    

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