Trees across metro Detroit suddenly dropping tons of acorns: Here's why

Plink. Roll. Plink. Roll.

That's the sound of acorns hitting your roof. 

Throughout Michigan, especially metro Detroit, residents are asking why there seems to be an abundance of oaknuts this year. 

One posted to a Facebook group for suburban gardeners that she had lived in her home, which has two "epic oaks" that are easily each a century old, for nearly 30 years and until this year, they had never dropped an acorn.

Suddenly they were falling like crazy, but looked small.

She asked the group: "Should I be concerned?"

The short answer is no unless you are parking under an oak tree. In that case, move your car, especially if it is leased, because when you turn it in, you're likely going to have to pay to repair all the tiny dents that those nuts leave on your vehicle's roof.

There you have it.

If that's all you want to know, you can quit reading. The complete answer is more interesting and complex. 

Biologists call the overproduction of acorns — and other nuts, like walnuts and hickory nuts, — masting. 

"It's a mast year," said Jeremiah Sandler, 28, a certified arborist in Royal Oak. He owns a company Tree First. Just before he was about to shimmy up and prune a hickory tree, he added: "It's not just oaks. Hickory and walnut trees are experiencing the same thing."

The abundance of acorns is mostly just a nuisance.

Walnuts, which are also called "stone fruits," are coated in husks. Depending on the species, walnuts can be bigger than golf balls. When they fall, especially from a tall tree, they can come down with considerable velocity, Sandler said, and really do damage.

Sandler said he gets a lot of calls this time of year about the extra tree nuts. Homeowners tell him they've never seen so many acorns and are worried. He calms them down by telling them that they have nothing to fear. Nature is doing just what it's supposed to be doing.

"I like mast years," Sandler said. "It gets more people looking at trees."

There also was an abundance of acorns in 2010. Back then, several people also called the Michigan State extension service about the bumper crop of acorns. The MSU experts explained that there's a natural masts cycle between very heavy years, like this one and 2010, and those with very little mast.

Several factors, they said, go into determining the mast.

Sometimes it's cyclical. Heavy years, preceded by light years.

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And there's also the weather.

Oak catkins — the cylindrical flower cluster that develop acorns — are produced from buds formed the previous year. Drought can affect catkin production the following year. It takes between six months and two years for an acorn to mature.

Rainy weather during flowering also can reduce pollination and seed set.

Along with the number of acorns, residents, like the Facebook poster, also notice an acorn's smaller size.

Last December, the Farmers' Almanac published an article — "Attack Of The Acorns! What Does It Mean?" — that said folklore dictated that an abundance of acorns in the fall was an indication of what kind of winter to expect.

The author reasoned that "five years ago, my lawn was filled with buckets and buckets of acorns" and "it was a snowy winter."

That doesn't sound like a very scientific confirmation, but, it's the Farmers' Almanac.

Bert Cregg, a horticulture professor at Michigan State University, recently told Michigan Radio that this year's mast was triggered by stress during last year's summer. Stress is one of the things that trigger trees to flower and thus produce more acorns.

"If a tree is under stress and thinking — in quotes — that it's 'not going to make it,' its first thing is to try to reproduce," he said. "And that's why we get this stressed flower, if you will, that occurs this year even though the stress was last year."

It doesn't hurt the tree.

But making acorns means the oak probably isn't growing as much.

For the most part, Cregg and other tree experts said, this year’s large mast of tree nuts shouldn’t be problem, aside the annoyance of running over them with the lawnmower, stepping on them and listening to them fall on your roof.

Plink. Roll. Plink. Roll.

But the nuts also are food for wildlife, so it should be a good year for the squirrels.

Contact Frank Witsil: 313-222-5022 or fwitsil@freepress.com.