Pruning

Trees and large shrubs are usually pruned for one of three reasons:

• Safety: Removing branches that have grown into structures or other objects, removing branches that are unsafe and may case injury or damage when they fall, and removing branches that endanger pedestrians or motorists.
  
  
• Health: Removing diseased or damaged wood, thinning to improve airflow and reduce pests, and pruning to remove rubbing or crossing branches.
  
  
• Aesthetics: To improve the shape of a tree, to increase flower or fruit production, or to open a vista.

Young Trees

When a tree is young the pruning emphasis should be on creating a strong structure. Always have a specific purpose or goal in mind before you make a cut. Small cuts do less damage that larger cuts and are easier for the tree to heal.

What you are trying to achieve with a young tree is a strong framework that will serve the tree well as it matures. In general, you want the primary scaffold branches to be well-spaced, both vertically and radially around the trunk. You want to avoid branches with tight crotches or narrow angles of attachment to the trunk, for these are weak and may lead to branch failure. And you want to maintain a single dominant leader, the highest branch tip on the tree, which controls overall growth.

Mature Trees

With a mature tree, you generally prune to maintain it in a healthy, safe and attractive condition. The best method of pruning a tree varies with the species, so learn about your tree and its growth characteristics prior to pruning. The types of mature tree pruning include:

• Cleaning: Removing dead, dying, diseased, and weak branches from the crown of the tree.
  
  
• Thinning: Selective branch removal to increase air and light penetration into the crown.
  
  
• Raising: Removing lower branches to provide clearance around the base of the tree.
  
  
• Reduction: Reducing the size of a tree while maintaining the form and structural integrity of the tree.

Pruning Tips

• Trees and shrubs have a wide variety of growth characteristics: single or multiple trunks, broadleaf or conifer, alternate branching or whorled, differences in how flowers and fruits are set, etc. Before you start pruning, learn about your plant and how it grows naturally.
  
  
• Whenever possible, cuts should avoid damaging or removing the branch collar or ridge, the raised bark where the branch originates. These areas have specialized tissues which aid in preventing disease and in healing the wound left after pruning.
  
  
• Avoid "stub cuts" (leaving too much of the branch in place after it has been pruned at the trunk) and "flush cuts" (cuts immediately next to the trunk, removing the branch collar and ridge). Cuts such as these delay wound closure and can provide access points for diseases.
  
  
• Use "three-step" cuts on large branches to avoid ripping of the bark when the limb falls. This method uses an undercut to prevent bark tearing from extending to the trunk. Whenever possible, support branches that are being cut.
  
  
• Make your pruning cuts on branches at stem nodes. This allows re-growth without leaving dead wood at the end of the branch.
  
  
• Avoid topping (cutting large, upright branches between nodes) and tipping (cutting lateral branches between nodes). To reduce the size or height of a tree, crown reduction is preferred.
  
  
• To prevent the growth of excessive, weak sprouts below the cut, make your cuts at a lateral branch that is at least one-third the diameter of the branch being cut.
  
  
• Avoid painting or applying wound dressings to cut surfaces. These do little good, and may actually delay healing and increase the chance of disease entry. Where wound dressings may be necessary is when a tree suffers from severe mechanical or storm damage.
  
  
• Avoid removing more than one-quarter of the living crown of a tree at one time. If it is necessary to remove more, do it over a span of successive years.
  
  
• The tools you use for pruning must be the appropriate type and size, sharp and well-maintained, and sanitized between cuts with alcohol or diluted bleach when dealing with diseased plants. Make sure you have safe and stable footing.
  
  
• Do not prune in situations involving electrical wires, overhead obstructions, or unsafe conditions. Call a professional or the utility company to deal with these.

Resources

WSU Extension Library.

• Good Cuts vs. Bad Cuts. Tips on making pruning cuts.
• Plant Sanitation. Discusses identifying, removing, and disposing of diseased wood, along with proper sanitation practices.

Pruning articles from the Monthly Regional Garden Column, WSU Extension Library.

• Pruning.
• Rogueing - Removing Unwanted Plants.
• Winter Pruning - It's all in the tools.
• Renovating the Rhododendron Jungle.
• Pruning out Disease to Keep Trees Healthy.
• Living with Big Trees.
• Checking Trees for Winter Problems.
• Is Your Big Tree a Hazard?
• Checking Tree Safety Before Winter Storms.

US Forest Service, USDA.

• How To Prune Trees.
• How To Recognize Hazardous Defects in Trees.

International Society Of Arboriculture. A nonprofit organization supporting tree care research around the world and dedicated to the care and preservation of shade and ornamental trees.

• Pruning Young Trees.
• Pruning Mature Trees.
• Why Topping Hurts Trees.