MINNEAPOLIS (WCCO) — The first frost of the season goes a long way towards closing the book on summer. So, with a new season coming, we’re already thinking about fall foliage. But, why do leaves change color? Good Question.

“The shorter days and the cooler temperatures mean the chlorophyll in the leaves breaks down,” said Pete Moe, director of operations for the University of Minnesota’s Arboretum. “That’s the green pigment that actually masks the yellow and orange pigments that are in the leaves all summer.”

Those golden colors we’re used to seeing are technically always there, but red, is a different story. Once the chlorophyll, the green, is gone, what basically amount to a wall goes up between the leaf and the tree — the flow of nutrients back and forth, stops.

Good news for red lovers.

“Sugars build up in the leaves and then if it’s sunny those sugars in the leaves will turn the bright red colors,” Moe said.

And this should be a great year for the reds. In fact, Thursday morning’s near-freeze was a perfect example of that.

“Chilly nights do help, too. It allows the sugars to build up in the leaves instead of being used up — that increases the red fall colors later on,” Moe said.

After a rainy spring, a relatively dry and sunny summer and a few more chilly nights, you should see fall’s finest in just about three weeks.

  1. Richard in Minneapolis says:

    I like the explanation my mother gave me all those years ago. Leaves change colors because they are God’s traffic signal telling birds it’s time to go. (Even if the colors are reversed – green, then yellow, then red).