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Archive date:  April 5, 2001

Pruning Trees and Shrubs

Why and When to Prune

Proper pruning enhances the health and beauty of almost any tree or shrub. However, improper pruning will quicky (and often irrevesibly) damage a plant, reducing its health and beauty. Therefore, it is important to be aware of why, when, and how to prune.

There are four major reasons to prune a plant.

  1. To remove dead, diseased, or damaged (weak or broken) branches.
  2. To maintain the plant's natural beauty.
  3. To control flowering, fruiting or colored twig effect in certain plants.
  4. To control the plant's size and shape.

If a plant has become so large that more that 1/3 of it needs to be removed in order to reach the desired size, it should be replaced, not pruned. Replacing the plant with a dwarf variety would be best, especially when planting in limited spaces. This reduces pruning needs and improves the long-term health of the plant.

It is important to note that trees should never be topped. It damages the health and beauty of the tree and simulates the growth of weak branches. These branches become a hazard as they are easily damaged during storms and may fall on roofs, cars, and anything else in close proximity.

Pruning Specific Plant Types

Broad-leaved evergreens

Rhododendrons may occasionally need some light pruning. Both azaleas and rhododendrons should be pruned after they have flowered. Never severely prune rhododendrons or azaleas. This will cut into areas of bare stems or sparse foliage that will not flush back out. When a fairly severe pruning is attempted, no more than one-third of the branches should be cut back in a single season.
Cane type plants, such as Mahonia, may develop old shoots with bare stems. These shoots may be removed at ground level. This will thin out the plant and make room for new growth.

Deciduous shrubs

Spring-flowering shrubs such as forsythia, flowering almond, and spirea produce flowers from buds formed the previous summer or fall. If these shrubs are pruned before they bloom, many of the flower buds will be removed, reducing the flowering display for that season. To ensure maximum flowering, these shrubs should be pruned as soon as possible after blooming is completed.

Plants such as pyracantha and Japanese quince produce flowers and berries on branches more than 1 year old. For this reason, these plants should never be severely pruned during one season.

Summer- and fall-blooming shrubs include such plants as abelia, beautyberry, butterfly bush, rose of Sharon, and crepe myrtle. Most of these plants flower on wood that is produced during the current season's growth. Therefore, these plants should be pruned now, before this new growth begins. Butterfly bush and beautyberry may be cut back completely to the ground level to control size and ensure vigor.

Pruning hedges

A dense hedge must be developed slowly. Never try to make a hedge reach the desired height in a single season or it will be thin and open at the base. To develop a hedge that is well filled at the base, always trim so that the base is wider than the top (fig. 2). If the top is allowed to become wider than the base, the base will become thin and open.

Plants such as privet or barberry need severe pruning immediately after planting and at the beginning of the second year to make them bushy.

Pruning Trees

The most important aspect of pruning a tree is making a proper cut. Cuts should be made just outside the branch collar (fig. 3) to allow the tree to seal the wound. The branch collar ats as a bandaid. If it is cut into, it will not cover the wound and the tree may even suffer decay in that area. If too mush of the branch is left outside the branch collar tiny shoots will form on the end of the cut. These shoots are weak and should not be allowed to develop into branches.

For more information and detailed diagrams of different pruning techniques visit the links below.

Pruning mature trees

Pruning young trees.

Proper Pruning Techniques