Pruning Young Trees

The thought of pruning a large, mature tree is daunting. It requires skill, caution, and can be expensive if hiring someone to do the job. Young trees, on the other hand, are a little more manageable for many homeowners. Beginning a training pruning regimen on trees that have been in the ground for a few years can actually help to improve their structure and avoid some of those expensive problems later on. It can also be less stressful on a tree, due to the reduced size of pruning cuts. Trees do not actually ever “heal” their wounds. Instead, they produce growth around a wound to seal it off so that decay will not enter and spread. A large pruning cut exposes a large area for a tree to seal off and greatly increases the opportunity for decay. It is best to remove a limb with foreseeable problems when it is a small branch, instead of after it has grown in size and become an issue. Always be sure to prune a limb just outside of the branch collar, the raised area around a limb’s attachment to another branch. Cutting into the branch collar (a flush cut) or leaving a stub can create greater potential for decay to spread in a tree. This applies to removal of an entire branch from the trunk or removal of just a portion of a branch as well.
young tree pruning 1.jpg
Pruning cuts should be made only to achieve a specific objective, such as to improve branch structure or to remove dead wood. Thoughtful, purposeful pruning will improve the structure of your tree, while random or unplanned pruning can cause greater stress and damage. When it comes to pruning trees, less is often more. Prune a small amount, and step back to observe and see if more needs to be done. You can always prune a little more off of a branch, but once it’s gone, it’s gone. Finally, always make sure not to remove more than 30% of the canopy at once. Removing too much could stress the tree and result in unwanted sprouting or decline in overall health. Some pruning objectives, such as removal of a codominant stem, may need to be addressed over time to avoid excessive pruning.

Hand pruners and hand saws are excellent tools for pruning young trees. Avoid using hedge trimmers, as they are not meant for cutting woody tissue and do not make very clean, straight cuts. Approach tree maintenance carefully and make sure you are working in a safe manner to protect yourself and others working with you.

There are a number of things to look for when pruning, such as weak structure and issues that could impact a tree’s surroundings or harm the tree as it grows. See below for some potential pruning considerations.

Codominant Stems and Narrow Branch Angles

Codominant stems are two (or more) stems in the same tree that are very close in size and are competing to be the “leader”, or main stem of the tree (or branch). Some small growing species such as crape myrtle are supposed to have a multi-trunked growth habit, but most, larger trees should be trained to have one main stem to minimize the risk of splitting. To retrain a tree into a single-stemmed structure, first choose which of the codominant leaders to retain as the main trunk. Then reduce the size of the other stem(s) by pruning back to another branch that is at least 1/3 the diameter of the branch being pruned. You can do this gradually each year until the stem has been removed.

One of the reasons that codominant stems are considered a defect is because of the narrow branch attachment. When looking at codominant stems, the space between them often looks like a “V” shape, whereas a strong branch attachment looks more like a “U” shape. As codominant stems (or branches with narrow attachment angles) grow larger, bark gets trapped in between them. The trapped bark creates a weak point and acts as a wedge as the branches grow larger. Branches with this trapped, or included, bark are much more likely to fail in high winds and storm events. For example, this is a common problem with the structure of Bradford pear trees.

In addition, the size ratio between a branch and parent stem impacts how securely a branch is attached. Branches that are more than 1/3 the diameter of the parent stem have a weaker attachment than those that are smaller than 1/3 of the parent stem or trunk. Branches that are too large in relation to the stem they are attached to should be pruned early on, before they increase in size and failure potential.

Branch Spacing

Multiple branches growing from the same area of the trunk are more weakly attached than branches that are spaced along the trunk. This is due to less wood growth around the bases of the branches from the lack of spacing between them. When pruning to improve structure, look for limbs that are clustered. Observe the branches and identify those causing the greatest problem, or that may be the most suppressed. Crowded branches may be corrected over time to avoid removing too much tissue all at once.

Sometimes limbs will grow across one another, and as they grow in size the wood begins to rub with the natural movement of the tree. This can cause wounds which lead to decay, making branches more susceptible to breakage. Removing a branch to make room for another to grow larger without later incurring damage by nearby branches can help to improve the structure of a tree as it matures. This should not be confused with the thinning of branches just for the sake of thinning, however, which can cause stress to a tree when performed in excess.

Dead/Decayed/Broken Limbs

“Crown cleaning” involves the removal of dead, decayed, and broken branches. These limbs pose greater risk than other branches to the safety of people and property around the tree and should be included in the most basic of pruning sessions.

Branches Conflicting with their Surroundings

Low branches over a sidewalk or touching a building, fence, or other structure are a very common maintenance issue that may be minimized by some training pruning when a tree is young. This may be accomplished by pruning limbs and small branches that are growing downward into the street or toward a building. Growth of a branch may often be redirected by pruning back to a branch that is growing in a more favorable direction, such as up over the street or away from the side of the building.

When removing limbs from the lower trunk, always be sure to retain branches on the upper 2/3 of the entire height of the trunk at a minimum. Removing branches to a greater height on a tree can place more wind force on the upper canopy and potentially lead to breakage, as well as other issues such as sprouting and weak structure in the upper canopy.

A little basic structural pruning can go a long way in preparing strong form in a young tree as it matures. Remember: use proper pruning cuts, wear personal protective equipment, prune with a purpose, and “less is more”!

Check out other topics from the Winter 2015 issue of Edmond Tree Mail