What’s causing 'exploding trees' in the Chicago area?

GLENCOE, Ill. - As the region braces for Friday's brutal cold, it won't just be people feeling the chill — trees could be in trouble, too.

Social media is buzzing over the potential for 'exploding trees' caused by extreme and rapid temperature drops, but experts are breaking it down. 

What we know:

The natural winter phenomenon is technically called a 'frost crack.' While it is a real occurrence, arborists say it's not as dramatic as it sounds. 

It happens when moisture and sap inside a tree freeze during plummeting temperatures. As pressure builds, it can result in a long, vertical split in the trunk.

"When we have this really fast fluctuation of temperatures, the water can actually expand, and as water freezes the molecules can expand up to 10%, and if it happens quickly the tree doesn't have time to adjust for it and it can cause this kind of splitting to happen," explained Tom Tiddens, supervisor of the Plant Health Care Department at the Chicago Botanic Garden. 

Certain trees are more vulnerable than others, especially those with smooth bark like sycamores. 

At the Chicago Botanic Garden in Glencoe, several sycamore trees have visible frost cracks. 

Tiddens, who is also a certified arborist, has been keeping track of them for years. He said the fractures even expand and contract with the seasons. 

"I expect by morning this frost crack will be opened up two-plus inches," Tiddens said. "If a tree has never experienced a frost crack before, the first time it happens, when it hits that certain point, the crack all of the sudden is like a split in your car windshield. It will kind of just kind of happen. And then that kind of fracture will be in the tree for the rest of its life," Tiddens shared. 

The noise it makes can apparently be startling. 

"I've heard it can sound like a gun going off," Tiddens said. "I'd love to experience it, but I don't think I'm going to be here in the middle of the night."

Tiddens said he finds it to be a fascinating quirk of Mother Nature. 

"I saw this first one happen about 15 years ago and it's like an annual thing. As soon as we get the first prediction of a night going down to zero, I'm out the next day taking a look at it, because once this happens, it is causing a flaw in the tree. It is a little bit concerning. Does that mean I need to take down the tree? No, no. But it does mean I need to keep a little bit of a close eye on it," he said. 

Tiddens said frost cracks can create avenues for insects and decay, which is why they should be monitored over time. 

Tiddens said tree wrap can sometimes protect younger trees during extreme cold, but it’s most effective when applied in the fall — not mid-winter.

What's next:

For community members with questions or concerns about tree and plant care, the Lenhardt Library and Plant Information Service at the Chicago Botanic Garden is available to help.