Last week a large limb broke away from a very mature live oak in Riverside Park during some high winds from the passage of a cold front. Unfortunately, it damaged the tree significantly. However, it has also provided a great opportunity to investigate why some branches fail and perhaps prevent it from happening in our own landscapes.
An inspection of where the branch split apart revealed a condition called included bark. Included bark is where the bark is included or embedded in the union of a branch with the trunk or another branch. The included bark acts as a barrier that gets trapped in between the two branches and does not allow a strong branch union to form.
In the case of this very old tree, the included bark started forming perhaps more than a century ago and was not revealed until the branch union could no longer support the weight of the branch.
Fortunately, scientists and arborists have identified what causes included bark and if identified early, can be removed or reduced. Included bark is formed when two branches that are close in size to each other originating from the same point in the trunk form a “V.” The two branches are called codominant stems. As the two codominant stems grow in diameter, the bark becomes trapped or embedded between the stems forming included bark.
Conversely, branch unions that are created between small branches and larger trunks form what is called a branch collar. The branch collar helps to form a strong branch union and is much less likely to break. Also, these stronger branch unions are more “U-shaped“ shaped than “V-shaped.”
There are a few things that can be done to identify and reduce codominant stems in the landscape. First, purchase quality trees. Trees that do not have good structure require more pruning to create better structure. The better-quality trees have a single dominant trunk. Don’t purchase trees from the nursery with codominant stems. Trees with two or more leaders (codominant stems) are weaker and may split apart as they grow.
Next, have your established trees inspected regularly by an ISA Certified Arborist. Certified Arborists are trained to identify co-dominant stems and how to correct them. Correcting defects early, or when the trees are young, and the branches are smaller and lighter is much better than having to fix the problem as the trees grow older. To find an ISA Certified Arborist go to https://www.treesaregood.org/findanarborist/findanarborist.
In younger trees, codominant stems can be resolved by structural pruning to develop a single trunk. Developing a single trunk with one dominant leader starts by identifying the stem that will make the best leader. Then either remove or shorten the competing stem(s) by using a pruning technique called a reduction cut. A reduction cut is performed by cutting back the codominant stem to a lateral branch that is at least ⅓ of the diameter of the cut stem.
This does two things: It reduces the weight of the codominant stem, as well as causes the codominant stem to grow at a slower rate than the main leader. For more information on structural pruning of young trees go to: https://hort.ifas.ufl.edu/woody/documents/ch_12_mw04.pdf.
In large mature trees, codominant stems can also be shortened by using reduction cuts. This should be done carefully and judiciously. Pruning mature trees requires more specialized equipment and skill and is more expensive than pruning younger trees.
Furthermore, larger, mature trees are less tolerant of heavy pruning, and need more time to close over large wounds. In addition to pruning, Certified Arborists can also install cables or braces to reinforce a weaker branch union. For information on pruning mature trees look up: https://hort.ifas.ufl.edu/woody/documents/ch_13_mw06.pdf.
There is no way to completely eliminate the risk of a tree or branch falling unless you are willing to remove all trees and the benefits that come with them. With regular inspection of mature trees by Certified Arborists, homeowners can significantly reduce the risk by identifying defects in their trees and taking care of them before they become a problem. By planting quality trees, residents can reduce the risk of future branch failures. More information on tree care and maintenance can be found on the University of Florida “Ask IFAS” website at https://edis.ifas.ufl.edu/.
Larry Figart is an urban forestry extension agent from the University of Florida/IFAS.
This article originally appeared on Florida Times-Union: Garden Help: Anatomy of a tree branch failure