When temperatures dip well below zero Fahrenheit, especially if they fall precipitously, things pop. Wood siding creaks. Frozen lakes and ponds emit ominous groans, snaps, and booms that reverberate through the ice. If soil moisture is high and frost is deep, even the earth can shift in a harmless localized cryoseism, or “frost quake” that produces a nerve-rattling bang.
If you live in a wooded area, you’ve probably heard trees popping and cracking during a deep freeze. It’s an eerie sound on an otherwise still night. Native peoples from northern regions were very familiar with this sound, and some even named one of the winter months in honor of it. The Lakota call February cannapopa wi, ‘moon when trees crack from the cold.’ The Arapaho consider December the tree-cracking time; for the Abenaki, it’s January.
I once found a reference in a novel to exploding trees. In the book, a lost teen boy survives a northern winter that’s so cold trees explode into smithereens. I’d lived through winters with minus-40oF temperatures but had neither seen nor heard of exploded trees. What did this author know that I didn’t?
After poring through scientific journals, I realized this: fiction is sometimes, well, fictional. As I thought, trees don’t blast themselves to bits. But since that first reference I’ve encountered the exploding-tree myth a number of times. So what does happen when trees go ‘pop’ in the night (or day)?
As we all know, when water freezes, it expands. Luckily, sap is not pure water. It’s endowed with antifreeze in the form of sugars, and, to a small extent, dissolved minerals.
The more sugar (or any solute) that’s mixed with water, the lower the freezing point becomes. This is due to something known as the “Colligative Property of Solutions,” as you no doubt recall from General Chemistry. (Actually it’s just as well to forget such terms; they clutter one’s head and make it harder to keep track of appointments, car keys and coffee cups. It’s the basic idea that’s important.)
There comes a point, however, when even sugar-fortified sap will freeze and expand. This may rupture the bark of a tree, resulting in an audible crack as well as a visible one. In many cases frost cracks close with no long-term ill effects, but sometimes they do become perennial.
Since it’s a weak point, a frost crack will pop open in cold spells. Then each spring and summer the tree makes callus tissue in an attempt to grow over the injury, resulting in a raised lip along the seam. Such trees have reduced timber value and an increased potential for decay to set in.
There’s nothing one can do for frost-cracked forest trees in terms of prevention or treatment. You can protect young landscape and fruit trees, though, with light-colored trunk wraps, or even a coat of interior-grade white paint, on the lower trunks. Wraps should be removed promptly in spring, and cracks or wounds should never be coated.
In truth, trees do explode occasionally—if someone has placed explosives in them. A friend of mine contracted with the US Forest Service in Oregon in the 1980s to create habitat for cavity-nesting birds. He climbed large spruces and firs, drilled a hole in the trunk halfway up and inserted dynamite, which was later detonated. I’m pretty sure he preferred to do this work when it was not below zero.