Warming, cooling may be cause of tree splitting

We have a prairie fire crabapple tree that has very severe splitting in almost all of the major branches. There is some evidence of old bark splits that have healed over that we did not notice until recently. The tree seemed to be slow leafing out this spring until we gave it a spraying with permethrin. Is there anything we should do this winter?

— Jim

I’m not totally sure what’s going on with your crabapple. One question I have: When did these cracks first appear? You mentioned some evidence of old cracks that have healed over, but when did these new ones show up? Another thing that would be helpful is how has the tree looked overall in the past? How did it look overall this summer and fall? Have you noticed any oozing or sap running related to the cracks? Bark cracking can be caused by a number of different things and knowing this might help me narrow things down a bit.

The causes can be generally divided into two groups. The first are cracks caused by pathogenic factors like insects and diseases. Certain insects like borers and diseases like cankers and vascular diseases harm the plant by killing the vital tissue directly underneath the bark called the cambium layer. When cambium is killed, that patch of stem dies. As the bark tissue above this dead area dries out it shrinks, causing the cracking and peeling that we’ll see occasionally when a tree is dealing with problems like this.

The second cause of cracking bark is non-pathogenic factors like temperature and sunlight. I don’t think the problems you’re seeing are from a pathogenic agent. The longitudinal cracks your tree is showing are usually from a problem called frost cracking.

Frost cracking happens during the winter. During the day with our bright sunshine beating down on the trunk and branches, the tissue will warm up. Think of a tree trunk as a solid cylinder inside a snugly fitting pipe. The “pipe” on the outside represents the bark tissues of the tree. The solid cylinder is what we call the sapwood and heartwood of the tree.

During the day, when things warm up, both of those tissues expand, and during the night, as it gets cold, they contract. However, when temperatures drop quickly, the bark tissues and the wood tissues constrict at different rates. Being on the outside, exposed to the cold, the bark constricts quicker than the wood beneath it. Sometimes, that is enough that the bark will split along the length of the trunk or stem like your crabapple.

Frost cracks tend to develop where there’s a weak spot in the branch tissue — whether that’s a spot where the tree was injured in the past or where a side branch is or used to be. These also tend to be more common in thin-barked trees like maple, sycamore, crabapples, horse chestnut and ash.

Frost cracks are hard to prevent. Wrapping the trunk and exposed branches with a commercial tree wrap should prevent the problem by shading the bark tissue, reducing the warming during the day. Apply it in the fall and remove it in the spring. Commercial apple growers often paint the trunks of their trees with white paint to prevent frost cracks and sun scald, but this is unattractive in landscape situations.

Frost cracking problems also tend to diminish with time as the thinner, juvenile bark is replaced with thicker adult bark.

Keep in mind that frost cracks are not really all that uncommon and are usually not a significant threat to the life of the tree. In most cases, there’s not much for you to do about it once it happens. I am a bit concerned about what you said about the tree leafing out late this spring — that can indicate a more severe problem going on than simple bark splitting.

Dennis Hill is the nursery manager at Bookcliff Gardens, bookcliffgardens.com. Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or email .(JavaScript must be enabled to view this email address).


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