In backyards across Des Moines, a quiet battle is unfolding between trees and the creatures determined to eat them out of house and branch.
Sometimes, even people get caught in the crossfire. Oak trees loaded with acorns can snap under the weight, sending heavy limbs crashing onto homes, garages and anything unlucky enough to be beneath them. The trees themselves are often perfectly healthy, just overburdened.
Aaron Steil, consumer horticulture extension specialist at Iowa State University Extension and Outreach — an acorn expert — said oaks are exhibiting a phenomenon called masting, when trees produce a surplus, or “bumper crops,” of acorns all at once.
The inside of Rachel Stassen-Berger’s garage after an oak limb, heavy with acorns, crashed through the roof.
“This is actually a relatively regular phenomenon in a lot of different trees, but oaks are probably the one that we notice it the most on,” Steil said. “This year seems to be a year for a lot of trees to have this really heavy seed set.”
Most oak species go through a mast event every two to five years, he said. The pattern is irregular and difficult to predict, tied to weather conditions and pollination. That includes the bur oak, Iowa’s state tree.
“When we have really favorable conditions, especially early in the spring for pollination and initial fruit set, then we might have a larger than normal crop,” Steil said. “Some years, we’ll have a late freeze that kills some of the flowers or prevents some of the fruit set to occur, and then we’ll have a really low amount of acorns that year.”
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It is not just oaks. Silver maples did it this spring, dropping so many helicopter seeds that their leaf production could be temporarily stunted. Hickories, walnuts and even apple trees follow the same boom-and-bust cycle.
Why trees do this comes down to survival, said Steil. A mast year is meant to overwhelm the predators who relentlessly gorge on the tree’s future generations — we’re looking at you, squirrels — along with deer, turkeys and even insects.
“If the tree produced the same amount of seed every year, then those predators would rise to a population level that would basically consume all the seed every year,” said Steil. “If we do have this irregular cycle of really large seed crops, it overwhelms the seed eaters out there and allows for some of the acorns to actually not be eaten and hopefully more are successfully able to sprout or to germinate and grow.”
Acorns blanket the ground in Rachel Stassen-Berger’s yard.
But all those extra acorns put strain on branches, causing them to easily fall.
And mast years also come with folklore. One tale holds that a bumper crop of acorns signals a harsh winter ahead. Steil, though, said there is no evidence to back that claim up.
Another misconception is that more acorns in the fall mean heavier pollen in the spring. In reality, Steil said, the opposite is more likely.
For homeowners worried about the abundance of nuts in their yard, Steil said this is rarely a problem.
“It is part of the tree’s natural kind of long-term life cycle. And so it’s not necessarily unusual for the tree, although it is a bit unusual for us, because we don’t experience it as often,” he said.
He recommends giving struggling trees a lift if they don’t straighten up on their own or manually thin acorns and branches to take off some weight.
What it means for most Iowans, though, is more acorn sweeping, raking and dodging. For squirrels, it means a feast that only comes around once in a while.
“I can tell you, probably, from a squirrel’s point of view, this is probably one of the best years ever,” Steil said.
Nick El Hajj is a reporter at the Register. He can be reached at nelhajj@gannett.com. Follow him on X at @nick_el_hajj.
This article originally appeared on Des Moines Register: Acorns are weighing down Iowa oak trees during mast year