It’s not snow in May — Cottonwood fluff’s back in Bellingham. Will it affect your allergies?

If you walked outside in Bellingham last weekend, chances are you were greeted by a few flurries of white fluff. The fluff, which comes from cottonwood trees, largely went away with Tuesday’s rain, but it was back Wednesday morning. Here’s why cottonwood fluff clouds pop up sporadically in Bellingham, why it was so heavy this week, and how it could affect your allergies.

What is cottonwood fluff?

Cottonwood fluff occurs when cottonwood trees release their seeds each spring, according to Lynn Pillitteri, a professor of biology at Western Washington University whose research focuses on plants.

“Each one of those little fluffs are a seed, maybe a couple stuck together,” Pillitteri said. “They’re almost like dandelion seeds, so they’re very tiny seeds and they’re attached to that piece of fluff that the wind can take them away. They can go for miles and miles.”

Why does cottonwood fluff come in waves?

Most years, cottonwood trees produce relatively few seeds, but occasionally they’ll see a masting year, which brings a drastic increase in seed production, and with it, a drastic increase in fluff.

“Cottonwoods — things like sycamores, oaks, maples, those sorts of things — they kind of have this up-and-down schedule of producing a ton of seeds and then lower amounts of seeds,” Pillitteri said. “So there’ll be some years that are actually just a real boom, they can produce tens of millions of seeds.”

The strange pattern is an evolutionary quirk that keeps predators away, according to Pillitteri.

“The years preceding that, they’re kind of keeping the predator population down — so things like ducks and birds mostly, probably insects [too]. And then when they have this big boom of seed production, so many seeds get produced that in the periods of time when they have this large boom, there aren’t enough predators or animals to eat all of the seeds.

But just because there’s cottonwood fluff in Whatcom County right now doesn’t mean this is a masting year.

“It isn’t regular, but for cottonwood it’s every three to four years, or something in that range. But certainly even in the low years, they still produce a whole lot of seeds,” Pillitteri said.

Another factor is the weather, according to Pillitteri. Bellingham’s relatively dry weather the past few weeks allowed the seeds to float in the air and spread. But the rain midweek stopped them in their tracks.

“If it’s dry-ish or warm while the seeds are coming out, more of them get spread around as opposed to wet [weather], rain that kind of knocks the seeds off the tree,” Pillitteri said.

Will it affect your allergies?

Since it’s made of seeds and not pollen, cottonwood fluff likely won’t affect your allergies, according to Pillitteri.

“The fluff is actually the seed, so it’s not highly allergenic,” Pillitteri said.

However, if you’ve noticed your allergies acting up lately, it’s not a complete coincidence. There are two types of cottonwood trees — one’s that produce seeds and ones that produce pollen. In the weeks leading up to the release of cottonwood seeds, pollen-producing cottonwoods shed in order to pollinate the seeds when they’re released.

“In the weeks prior to that, when the pollen was being shed from the pollen-producing trees. That’s probably when people felt their allergies,” Pillitteri said.

This story was originally published May 23, 2024 at 12:00 PM.