MINNEAPOLIS (WCCO) — Experts at the Minnesota Department of Natural Resources predict a beautiful display of fall colors across most of Minnesota.

That’s because of the all the rain we got earlier this year.

But how do leaves know when it’s time to change? Good Question. An expert helped WCCO’s Heather Brown shed some light on the answer.

Somehow, the trees just know. Early October is a time for change.

“It’s really kind of simple,” Val Cervenka with the DNR said. “There’s a signal and the trees signal is the days are getting shorter, that is key.”

You see, leaves get their green color from chlorophyll. Its green pigment actually masks the leaves’ true yellow and brown color all summer long.

And when there’s less sunlight…

“The photosynthesis kind of shuts off — that’s what causes the decrease in the pigment and the chlorophyll breaks down,” Cervenka said. “That shuts down and allows the yellow colors to shine through.”

So if it’s rainy and cloudy, would there be a different color?

“Temperature has an effect on the color, actually not much when they change,” Cervenka said. “If we have warm nights and warm days, the color is not going to be as brilliant.”

That’s because the sugars in the red and purple leaves are what make them colorful. Colder weather at night stops the flow of the sap, letting the sugars build up and shine through.

“Everything is really green right, but every so often, you see a tree that’s turned. Why is that? That’s because the tree is stressed,” Cervenka said.

It could be too much or too little water, or something happened to the roots.

So we should see brilliant colors this fall, unless it stays hot or gets very rainy this month.

Heather Brown

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