Almstead Tree & Shrub Care Blog

Wednesday, November 13, 2013

Why Do We Prune?

Everyone knows that it’s past time to prune when a large branch drops onto their lawn or car. But many people wonder exactly when a tree first needs to be pruned. When it’s 50 years old? 20? 10?

Think younger.

Pruning serves many purposes – and eliminating dead branches is just one of them. For a young tree, pruning offers the opportunity to correct many of the issues that will cause problems later on – and even lead to an early demise.  One example: co-dominant stems. A co-dominant stem simply means the tree splits into 2 or more vertical stems (trunks). While for some trees, such as birch, multiple stems growing from the base are common and natural, for most trees this is undesirable.

When stems are competing in a tree, there is inherent weakness. In a mature tree, the double trunk may not be strong enough to support the heavy canopy. Tree failure is common in this situation. When we see this developing in a young tree, we have the opportunity to choose one trunk and prune away the other wood, leading to a safer, healthier tree. Even in a mature tree, when it’s too late to eliminate one of the stems, the crown can be reduced to lessen the weight load supported by the weaker stems.

When arborists look at a tree, we look at the “scaffold”: the arrangement of stem and branches. We want to see strong unions between each branch and the stem. Usually, each branch is joined to the trunk through a “branch collar” a ring of strong wood that supports the developing branch. The union between stem and branch should be smooth, like the letter “U.” If branches or stems are too close together, as in the case of a co-dominant stem, this smooth, strong union is unable to form correctly. Instead we see a “V” between the two rivals. This V zone between the stems or branches is vulnerable to splitting (see the picture). With periodic pruning throughout the life of a tree, we can create a strong, balanced scaffold as the tree develops, which leads to a healthier and safer tree.

As trees mature, we want them to grow with a certain symmetry. This isn’t just because it is pleasing to the eye: symmetry means the weight of branches and leaves will be balanced, making the tree more stable. An arborist will also remove branches that are crossing or competing for the same space. This eliminates damage from branches rubbing into each other, and also opens up more space among the branches. This space increases airflow beneath the canopy which creates a less hospitable climate for insects and disease to spread.

Pruning a tree regularly throughout its life will actually reduce problems later on. It’s similar to seeing a dentist: periodic checkups and minor dental work throughout your life are preferable to procrastinating until you need a root canal or lose a tooth.

There are, of course, many other reasons to prune: to keep a tree from impinging on a house or wires, or to open up a view, to name a couple. Although pruning can be done at any time of year, I feel winter is the best time. Winter creates a more sanitary environment (no bugs!), and of course, it’s easier to get a clear view of a tree without its leaves.

Here’s an example of a tree with co-dominant stems. The V-shaped attachment makes the tree vulnerable to splitting: the process has already begun, and this tree is at high risk of failure.