Trees do ‘explode,’ but don’t expect a mushroom cloud

The most dramatic example of the awesome power of nature that I’ve witnessed came last summer after a loud bang reverberated through the neighborhood. The cause became clear after the rain subsided and a walk down the street revealed shards of wood scattered far and wide — 2-foot-long daggers stuck in the ground 200 feet away from a large tree that had been shattered by a lightning strike.

The tree had exploded.

That is why I was expecting something more sinister when recent subzero temperatures led to an “exploding tree warning.” The danger zone, as illustrated by a map of dubious derivation, was centered over Wisconsin.

Social media users wasted no time mocking the latest potential weather catastrophe with photo illustrations of trees going up in mushroom cloud blasts.

So it may be disappointing to learn that while trees can “explode” due to extreme cold, it’s not nearly the dramatic event it was made out to be.

Frost cracking, as it is more accurately described, happens when sap, which is mostly water, freezes and expands at the same time that the tree’s outer bark contracts. Such cracking can happen at around 20 degrees below zero and it’s more likely when temperatures drop rapidly, according to the Wisconsin Department of Natural Resources.

As you might imagine, cracking wood can be rather loud, leading to a sound that might resemble an explosion.

Any kind of tree can sustain frost cracks, but maples, apple, crabapple and beech trees may be more prone to the damage, which can leave large cracks through the bark or weaken limbs.

With the most severe temperatures of the recent cold snap apparently past us, Southeastern Wisconsin may be safe from exploding trees for the time being, but a similar danger can extend into late winter and even early spring when the sun has more heat and can warm the bark, causing it to expand. Then when temperatures drop suddenly, the outer bark contracts and the pressure difference in the tree caused by contraction can cause a failure in the bark. These failures, which occur on the south side of trees, are known as frost cankers, and do not extend into the stem’s wood like true frost cracks do.

There isn’t a lot that can be done to prevent frost cracks and cankers other than avoid having a tree going into winter stressed from over or under watering. Tree wraps may help prevent frost cankers, but a study in the Journal of Arborculture suggested paper wraps may be more harmful than helpful.

Many frost cracks can heal on their own as it is a clean crack with smooth edges, and if bark is shredded you can trim it off. Arborists can perform lip bolting on large, serious cracks in some cases.

Many southeastern Wisconsin homeowners probably won’t know if their trees sustained frost cracks until spring when they are outside actually looking at trees. Much like trees falling in a forest, even though the cracks can come with a bang, if there’s no one outside to hear them, did they really happen?