What are ‘exploding trees’? The winter phenomenon may not be what you think

A woman snaps photos of snow- and frost-covered trees near Kottgeisering, Germany. (Sean Gallup/Getty Images via CNN Newsource)

John Seiler was strolling across Virginia Tech’s campus with his students Thursday morning when something stopped them in their tracks: a sweet cherry tree with an unusual jagged scar running along its trunk.

Seiler, a professor and tree physiology specialist at the university, knew immediately that the scar was the aftermath of what social media has dubbed a “tree explosion.”

The tree “had broken open in the cold,” he said.

As more than half of the United States braces for a powerful winter storm, some meteorologists on social media are warning that these “exploding trees” are possible.

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But while heavy snow, ice and bitter cold can absolutely wreak havoc on trees, Seiler said there’s one important thing to clear up: They’re not actually exploding, at least not in the way the phrase suggests.

What some people call “tree explosions,” scientists call “frost cracks,” Seiler said.

They happen when temperatures drop suddenly or trees don’t have time to adjust to the cold, and the sap or water inside starts to freeze.

“That water expands as it freezes, and it can happen usually under very, very drastic drops in temperature,” said Doug Aubrey, a professor at the University of Georgia’s Warnell School of Forestry and Natural Resources.

That expansion puts intense pressure on the bark and wood, which sometimes causes it to crack or split apart, producing a loud bang that may sound like an explosion.

“It’s more of a sound like a gunshot, a really loud crack,” Seiler said. “You know when sometimes you’re in a hurry to get a cold Coke, and you put it in the freezer, and you forget about it and the can splits open? That’s what’s happening with the tree.”

There are “numerous historic and current observations” of trees exploding due to extreme cold, according to the National Forest Foundation.

Seiler said these types of events aren’t necessarily dangerous for passersby or the tree itself.

“It’s going to be loud, but it’s not dangerous. Wood doesn’t go flying through the area,” he said. “And for the tree, when it splits open like that, that’s not going to kill it. … But because the bark split open, it could be killed by insects getting inside, or infections from like a fungus or bacteria.”

Additionally, Aubrey said, “if the bottom of a stem freezes, then the entire tree might die, but that also depends on the species since many trees can grow new stems from buds underground. If freezing occurs in a branch, then perhaps everything on that branch might die, but the tree will survive.”

During the storm, there is higher risk of heavy branches falling from trees due to the weight of ice or snow, Aubrey warned. A heavy tree branch could harm a home, a vehicle or even a person.

“Tree size factors into the types of impacts that you can have,” Aubrey said.

“Longleaf pine trees, which have longer needles, have the potential to accumulate more ice on them than a loblolly pine tree or something that has shorter needles,” Aubrey said. “In terms of branches breaking, large trees that are more open-grown tend to have more large branches and leaf area, compared to a dense forest where the trees are closer together.”

Seiler agreed that people should be more worried about heavy ice accumulation on trees, which could cause branches to fall or the tops of trees to snap.

“If there’s a heavy ice accumulation and wet snow accumulation, you don’t want to have a branch fall on your head,” Seiler said. “That’s extremely, extremely dangerous.”

By Jacqueline Howard, CNN