What's Blooming: Structural Pruning

 

What’s Blooming:  Structural Pruning
by Jacob Mares, Arborist and Horticulturist, Gardens on Spring Creek

 

Winter is a great time to take a moment to assess the plants in our landscapes. Deciduous trees remain dormant during these stretches of low temperatures, and the absence of leaves provides us with an opportunity to critique and correct structural issues.  Often, the care of trees in our landscape is relegated to what happens at ground level. While watering and mulching are integral parts of establishing healthy trees, the shape of your trees’ crown can develop problems over time that can lead to limb failure and an untimely demise. We can mitigate this damage by correcting structural defects early and often. Keeping an eye out for these problems starts the minute we purchase the tree and should continue well into the tree’s lifetime.

Most trees can be assessed and / or pruned every other year during establishment, and every three to five years once mature.  This type of pruning, referred to as structural pruning, should be done when the tree is completely dormant. A pruning cut made during the active growing season diverts energy, needed for growth, to compartmentalize the wound. Making cuts during dormancy allows the tree time to close the wounds during the growing season. Good structure can mean many things; a strong central leader, solid attachment points for branches, and scaffolding help create healthy canopies. How we assess and apply this practice can be subjective to which species you are working with. Research your tree, and its growth habits, to help you make informed decisions when pruning.

Having the right equipment and knowing how to make a proper pruning cut is imperative to the pruning process. Here is a list of what a basic set of pruning tools should entail:

• Bypass pruners (hand pruners)
• Hand saw
• Ladder
• Personal protection equipment including gloves, eye protection and head protection

First thing, first - if the planned work is higher than you can reach safely on a ladder, it is time to call a certified arborist who is trained in structural pruning.  But if you plan to tackle the project yourself, make sure that pruners and saws are sharp, as dull blades tend to rip and crush tissues leaving wounds that are harder for the tree to close. Make sure the area you intend to work in is free of debris and level enough for a ladder. Try to keep this area clear as you prune to avoid tripping accidents. Always use gloves when making cuts and be mindful of where your hands are when cutting, as a sharp pruning saw has no problem cutting through most clothing.

There are two types of pruning cuts associated with “training” young trees for structure: (1) removal cuts and (2) reduction cuts. The removal cut takes the entire branch back to the trunk of the tree or the parent branch. Avoid making a “flush cut” by starting the cut just past the branch ridge collar and mimicking the angle of attachment.  Take care to reduce the size of the branch before the final cut to avoid a branch tearing the bark on the main leader. Removing any portion of the branch ridge collar will greatly reduce the trees’ natural ability to compartmentalize the wound. The reduction cut focuses on removing a portion of a branch back to a branch union to “reduce” a branches growth. The size of the branch to be pruned should be one third or less the diameter of the remaining branch. Anything larger can create undesired suckering growth and is considered a bad pruning practice. Choosing which type of pruning cut to use will depend on your assessment.

The first step in the assessment process is to decide on your central leader. Most deciduous trees will have a pronounced, central stem that is larger than any that are competing against it. Choosing the main leader will allow you to make decisions to avoid co-dominant stems (two or more main leaders). If you have a situation where two stems are competing to be the central leader, choose the healthiest and remove the others.  Once you have decided on the central leader you can start to plan what pruning cuts should follow. Branches with good attachment points will have a pronounced branch ridge collar (an area of overlapping growth rings) and have an angle no tighter that 45 degrees to the main leader. 

Some genera, like Linden (Tilia spp.), naturally have tight attachment points, so use your research to correctly apply these steps. Removing branches that are clustered together will improve branch spacing, or scaffolding, and prevent sections developing included bark. Branches that will not remain on the tree as scaffolding can be treated as temporary and planned to be removed at a later date. At this point you can generally start with the lowest branches and begin assessing them based on attachment point, size in relation to the main leader and height. 

Something to keep in mind as the tree grows into maturity is most shade trees will have their lowest permanent branches roughly 10 feet off of the ground. Removing these branches early will keep the wound smaller and easier to compartmentalize. Branches approaching half the size of the main stem should be removed or slowed down with a reduction cut. Once you have assessed the branches in question you can decide on “dosage”.

Dosage refers to the amount of pruning you apply to the tree. A general
rule is to never take more than one third of the tree’s limbs during one pruning cycle / growing season. Dead branches that are removed during the process do not count towards that amount. I like to pile the dead and the living branches separately and within sight. This allows for a reminder of how much your current dosage is and how much you can safely remove. Sometimes you will find the tree only needs one or two cuts to correct any problems. 

You will need a combination of your research and structural assessment to accurately gauge what consists as a problem and how much dosage to apply. Some trees like Elm (Ulmus spp.) will need yearly pruning and usually require a hefty dosage to maintain proper structure, while others like Oak (Quercus spp.) might require small amounts every few years.

Keeping a close eye on your trees growth and assessing its structure on a yearly basis will provide you with a plan of action when it comes time for pruning. There are many online resources for in depth looks at structural pruning and why it is important for tree health. My favorite can be found at Colorado State University Extension.