Hiring a qualified arborist to prune young trees is one of the best landscape investments a homeowner can make. It is also one of the most overlooked or neglected practices, and it inevitably leads to premature tree loss. I will use this and my next column to expand on this and better explain why pruning young trees is so important.
Most homeowners will look at the outer canopy of a tree with the perception that a full and uniform shape is a sign of a well-formed tree. A trained arborist is less concerned with the shape of the canopy. Arborists look at the trunk and branch structure, which needs to support the tree. We attempt to balance canopy shape, mostly for aesthetic reasons to appease a homeowner, but it is secondary to good structure.
Trees are large and long lived. They need to develop a strong structure to hold their weight and to withstand wind, snow and other loads that are placed on them. When growing in a wooded area, this occurs naturally as a group by forcing a strong leader to capture sunlight and self-pruning lower branches through shading. In urban areas where trees are planted further apart, they have sunlight available to them from all directions. Because of this, they develop much fuller canopies and retain lower branches, which often compete to become "the leader" or get excessively large compared to the trunk.
Pruning to promote a single leader is my first consideration when I look at tree structure. If more than one shoot grows upright, creating multiple leaders, it is basically like multiple trees growing on a single trunk. As the multiple leaders increase in size, there simply isn’t enough space for them on the trunk below. As multiple leaders grow in diameter, they force each other apart and split the trunk, causing the tree to fail. Since failure commonly happens when storms cause wind-load on a tree, most people don’t understand that it was poor tree structure that caused the tree to fail, and not the wind. This condition is very preventable if young trees are pruned properly as they develop.
The second thing I look for are branches with poor attachment, or "included bark." These are branches that grow at narrow angles and have bark sandwiched between them and the trunk. The top of the branch will appear to have a seam where it attaches to the trunk. A well-attached branch will have a ridge of bark visible on the upper surface, where it attaches to the trunk.
Included bark prevents proper wood development between the branch and trunk and creates a weak branch attachment. These branches are very likely to break off of the tree under wind or snow load, or even from their own weight as they develop in size.
Dead, diseased or declining branches are also considered for removal. Sometimes these are obvious, and sometimes signs of branch decline are quite subtle. These are less important to remove unless a disease has the potential to progress through the rest of the tree or if they pose a risk.
In my next column, I will discuss the other considerations when pruning a young tree. These include selecting permanent scaffold branches, removal of temporary branches and managing growth rate in branches.
Actually performing the work properly involves a thorough understanding of tree structure and physiology. One must be able to understand how the tree responds to pruning practices and be able to visualize future growth to achieve desired goals.
Ed Gilman from the University of Florida has an excellent guide to pruning young trees. This publicationgoes into more detail than I can cover in my column.
Do you prune your trees in dormant season?
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