ISA Rocky Mountain Chapter

Pruning

Pruning is the deliberate removal of tree branches and limbs to achieve a specific objective in the alteration of a tree’s health and form. Pruning is the most significant practice due to costs and impact on the tree, but can extend the useful life of a tree in your yard for decades.

Trees may need to be pruned to:

  • Remove dead or hazardous branches
  • Maintain vehicular, pedestrian, and sight clearance
  • Improve the tree structure, e.g. balancing crown weight to avoid future leaning
  • Increase light or air penetration
  • Improve tree aesthetics

Avoid Harm to You and Your Tree

Although we are providing these basic instructions on tree pruning, we recommend contacting an ISA Certified Arborist for anything more than basic tree care. Pruning trees incorrectly can not only damage your trees but also result in injuries or death for untrained individuals. An arborist is a specialist in the care of individual trees. ISA Certified Arborists are knowledgeable about the needs of trees and are trained and equipped to provide proper care.

When to Contact an Arborist:

  • The tree cannot be pruned from the ground.
  • The tree has been identified as hazardous.
  • The tree is near electrical or other utility lines.
  • The branch(es) that need to be pruned are large.

Safety Tips:

  • Keep pruning equipment sharp, clean, and in good operating condition.
  • Make clean cuts.
  • Be careful with all tools.
  • When pruning trees that show evidence of disease, disinfect pruning equipment between trees. During extreme infestations, disinfect equipment between cuts.
  • Always wear personal protective safety equipment, including safety glasses, while pruning.

When to Prune

The best time to prune living branches is late in the dormant season or very early in spring before leaves form. Growth is maximized and wounds close faster. Flowering trees should be pruned after blooming. Routine maintenance pruning of dead or dying branches can be done at any time. However, your tree species may be an exception to these general rules.

For new trees, inspect for pruning needs annually. Prune trees regularly throughout their life to keep them healthy, safe, and aesthetically pleasing.

Do not defer pruning until limbs get large. Large limbs equal large wounds, which are more difficult for a tree to seal and leave the tree open to disease, insects, and rot. Do not prune trees on a crisis-only basis. Do not attempt to reduce tree size as a substitute for proper tree selection and placement. Known as topping, this is incredibly damaging to trees.

What to Prune

Young and mature trees have different pruning needs. On new trees, prune only dead, broken, crossed, or rubbing branches.  A young tree can survive the removal of up to one-third of its foliage in a growing season, but do not remove more than one-quarter of the foliage of a mature tree in any one growing season. Do not make indiscriminate cuts on large branches in an attempt to lower the height of the tree. This is called topping and is one of the worst things you can do to your trees.

You may wish to prevent future hazards in mature trees by removing branches that may become problematic in the future. Branches with splits and cracks at a joint can be weak. Multiple branches attached to one spot on the trunk can also be trouble spots. U-shaped joints are stronger than narrow V-shaped unions, which can harbor disease-causing debris. Broken branches, whether partially attached or completely separated from the tree, are called hangers or widow makers. They are extremely hazardous and likely to fall; they should be removed promptly. The same is true for deadwood.

Training

To reach their full potential in maturity, young trees should be trained. Training is careful, thoughtful pruning that creates strong trunk and branch structure and a visually pleasing form. This influences future performance, landscape potential, and safety.

Correct pruning of young trees will improve structural stability, increase tree longevity, and decrease maintenance costs. Trained trees will have fewer branches but better spacing. With fewer structural defects when mature, trained tress reduce the need for costly corrective measures later.

The process of training young trees directs growth to fulfill the landscape function, reduces structural defects that may lead to tree failure, and ultimately decreases hazard potential and liability risks. Well-maintained trees are an asset to any landscape.

How to Prune

Proper pruning takes skill and practice. To minimize the amount of exposed wood, make small cuts and conserve as many living branches as possible. Excess end weight should be removed with preliminary cuts to avoid tearing bark. Always prune trees back to the parent branch or a lateral branch that is at least one-third the diameter of the branch being pruned. Avoid cutting the trunk or branches that you are not actively pruning. Do not remove more than one-quarter of the foliage from a branch unless you are removing the entire branch.

Every branch has a swell at the base, where it meets the trunk of the tree. This is known as the branch collar. All pruning cuts should be made further away from the trunk than the collar.

Three-cut pruning method diagramThe three-cut pruning method. Adapted with permission by Alex Shigo, Modern Arboriculture.
  1. Make a shallow cut on the underside of the branch, away from the collar. This will prevent bark tears if the branch drops suddenly.
  2. Just beyond the partial cut, cut through the branch to remove the bulk of the weight.
  3. Finish the prune by cutting through the branch just outside the branch collar.

The two most common pruning errors are known as "flush cuts" and "stub cuts." Both of these errors happen during Cut 3. A flush cut is a cut that injures or removes the branch collar. A stub cut leaves too much branch past the collar. Stub and flush cuts can open your tree to pests, disease, and decay.

Remember, tree wounds should be left uncovered so the tree's immune system can take care of them.

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