Chicago blanketed by white cottonwood tree fluff in a banner year for the stuff

Chicago has been inundated with white fluffy stuff over the past few weeks.

It isn’t snow. The fluff is from female eastern cottonwood trees. The tree’s seeds are blanketed by thin white floss, sort of like a dandelion, allowing them to hitch a ride on the breeze away from their parent tree, perhaps to somewhere they might take root.

The trees — one of the most common deciduous varieties around Chicago — are going through a masting event, producing far more seeds than usual.

“It won’t be like this every year or next year because that’s the nature of the masting events ... they are relatively random,” said Christy Rollinson, a forest ecologist at Morton Arboretum in Lisle.

Other trees and plants — like oaks, spruces and pines — go through similar cycles every few years, putting out a lot of extra seeds in nature’s effort to help ensure that at least a few survive.

The trigger is a combination of biological and environmental factors, according to Rollinson, though exactly how and why it happens isn’t known.

This year’s cottonwood masting might have something to do with last year’s weather, Rollinson said.

“Leaves really have not been on the trees for that long — it may seem like it at this point,” she said. “But particularly things that are putting fruit in in the spring, a lot of times they are pulling from resources from last year.”

Last year was a great year for trees: a little dry but not too hot, contributing to brilliant fall foliage.

The cottonwood tree buds were more or less formed before winter. In the spring, those buds blossom into male and female flowers. The male flowers, dangling clusters known as catkins, release pollen that fertilizes the female flowers — and trigger seasonal allergies. Those female flowers become capsules a few weeks later that release the fluffy seeds into your yard, your street, your air conditioner, everywhere. At least the sees aren’t to blame for allergies.

“These are new potential trees that are blowing around,” Rollinson said. “Since they’re big and you can see them, they’re not irritating your allergies.”

Pat Nabong / Sun-Times

The forecast for next season isn’t looking as favorable. Eastern cottonwoods like wetter conditions, and May was an exceptionally dry month.

Morton Arboretum and several other institutions recently got a grant from the National Oceanic and Atmospheric Administration to study the effects of drought on urban trees. For now, this summer is on track to be as dry or worse than the 2012 drought, according to Rollinson.

Some trees that put out leaves late are “already looking a little scraggly,” Rollinson said. If Chicago doesn’t get more rain soon, some drought-sensitive trees, like certain birches, could see their leaves start to yellow.

“Now is a good time to give your trees a little extra love and a little extra water,” Rollinson said. “People love to water their grass, but the tree does more for you — so give it some love, too.”