The answer to the title question is that they often do freeze and break. Have you ever seen cracks running up and down the trunk of a tree? This is caused by the freezing and thawing of water in the outer portions of the trunk. In the tree care industry, these are referred to as frost cracks.
Some have compared trees freezing and breaking with plumbing pipes bursting. However, it isn’t the same phenomenon at all. Pipes burst when they are filled with water, which then freezes, expands and causes a weak spot in the pipe to burst. In the case of trees, there are thousands of liquid-carrying “pipes” – the phloem and xylem – and they are more elastic than plastic or metal pipe. Besides, there is less liquid flowing through these vessels when the tree is dormant.
Trees are most apt to freeze and break when the temperature plummets before the tree goes completely dormant. Even then, in our area, it is usually only the vessels near the perimeter of the tree, closest to the bark, that are apt to freeze enough to break. Thick bark trees can often prevent the trunk from breaking but thin bark trees don’t have that protection. Consequently, they are the trees that most often split and form frost cracks.
Although frost cracked trunks on thin bark trees are most common, weak xylem and phloem vessels anywhere inside a tree can break. But these interior broken vessels don’t affect the bark, and there are so many of these vessels running up and down the trunk that these breaks won’t affect the tree’s health. Also, the sugar in the phloem reduces water’s freezing point.
There are a few ways to protect thin bark trees from frost cracks. One is to wrap the trunk with burlap or tree wrap. If you do this, be sure to remove the wrap in the spring. You can also build a wood structure around the trunk. When planting a thin bark tree, it’s a good idea to keep it out of the path of the prevailing wind. Also, a good layer of mulch will help keep the roots warm and, as a result, the liquid that flows in the vessels will be warmer.
On the plus side, a frost cracked tree is not in imminent danger of dying. As I said above, there are so many fluid transport vessels that they can take over for those that broke and caused the frost crack.
If you have any questions about protecting your trees this winter, we will be happy to answer them.