Wisconsin's Cold Snap Might Make Trees Explode

Wisconsin and much of the upper Midwest are bracing for rapidly dropping subzero temperatures, and some viral social media posts are warning people to watch out for "exploding trees."

Sounds ridiculous, right? Except it's actually real. Kind of.

Bill McNee, a forest health specialist with the Wisconsin Department of Natural Resources, says it's not uncommon for a sudden temperature drop to cause "frost cracks" in trees. These vertical cracks can run the full length of a tree and come with a loud bang that'll make you think someone just fired a gun in your backyard.

Why Trees Crack in the Cold

Frost cracks happen because the thick sap inside a tree freezes at a colder temperature than water. Usually, that sap stays liquid even when temperatures are well below freezing—a state called "supercooling."

"It's going to get cold enough that this sap may actually finally freeze. And when it does that, like ice cubes in your freezer, they expand very quickly," McNee said. "That just creates a lot of physical pressure that can lead to the frost cracking appearing suddenly, branches can fall off, and people hear this really loud crack from their tree, almost like it's a gunshot."

There's another factor at play too. When temperatures drop suddenly, the exterior of a tree gets much colder than the inner wood. That causes the bark cells to shrink, leading to "unequal contraction" between the tree's outer and inner tissues. The tree basically can't handle the stress and splits.

Do Trees Actually Explode?

McNee says it would be extremely rare for a tree to fully explode from frost cracking. Usually, the crack just becomes part of the tree and life goes on.

But it can happen.

"I've never seen the damage of it, but from what I have seen and what I read online is that it is rare for there just to be so much pressure that is suddenly released inside this tree that it almost does explode," McNee said.

Frost cracks are most likely to show up in thin-barked trees like maples, birch, lindens, and sycamores, but they can happen in other species too. Any tree with sap could crack under the right conditions.

What Happens After a Tree Cracks

Frost cracking is common for many trees every few winters. Some trees have adapted by dehydrating themselves before winter hits, which makes physical damage less likely. For most trees that do crack, the living part typically heals over the "wound" over time.

The problem is that a crack can expose a tree to fungal infection or decay, which could eventually kill it. McNee advises anyone who spots a severe crack in a tree to call an arborist and get it evaluated. Severe frost cracking could cause trees to fall, which is a safety hazard nobody needs.

So, if you're in Wisconsin or anywhere else dealing with this cold snap and you hear what sounds like a gunshot coming from your yard, don't panic. It's probably just your tree having a really bad day.

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