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Why Prune?

Most pruning is done for “people reasons.” Trees growing in the urban forest must endure stresses not found in the wild, and human safety and aesthetic preferences dictate certain pruning requirements. Arborists must understand the biology of trees and their basic requirements in order to optimize the health and structure of trees through pruning.

Poor Pruning Cut 

Poor pruning cuts leave stubs that impede
healing and increase the risk of disease

Pruning cuts must be made with an understanding of how the tree will respond to the cut. Improper pruning can cause damage that remains for the life of the tree.

Removing foliage from a tree by pruning branches affects its future growth. Removing leaves reduces the tree’s overall photosynthetic capacity and may reduce overall growth, causing dwarfing. At the same time, growth after pruning takes place on fewer shoots, so unpruned parts tend to grow more than they would have without pruning. This is call shoot invigoration. These principles should be considered when pruning trees and other woody plants.

Because each cut has the potential to change the growth of a tree, no branch should be removed without a reason.  In most cases pruning is of a corrective or preventive nature, with goals such as:

  • Improving tree structure or health
  • Accommodating human needs such as clearance pruning or hazard mitigation
  • Increasing light penetration
  • Providing a view

Pruning Techniques

Pruning Young Trees (PDF, 897KB)
Pruning Mature Trees (PDF, 1.9MB)

When should I prune?

The best time to prune trees depends on the result you desire. As a rule, growth is maximized if pruning is done in early spring just before the buds swell. Pruning when trees are dormant can minimize the risk of pest problems associated with wound entry and allow trees to take advantage of the full growing season to begin closing wounds.

Plant growth can be reduced if pruning takes place during or soon after the initial growth flush, so pruning at this time is usually not recommended. This is when trees have just expended a great deal of stored energy to produce foliage and early shoot growth. Removal of many live branches at this time can stress the tree. A few tree diseases can be spread when pruning wounds allow spores access into the tree. Susceptible trees should not be pruned during active transmission periods.

Flowering can be prevented or enhanced by pruning at the appropriate time of the year. Landscape trees that bloom on current season’s growth are best pruned in winter, prior to leaf emergence, or in the summer after the bloom has occurred. Plants that bloom on last season’s wood - such as many fruit trees - should be pruned just after bloom. Often, fruit trees are pruned during the dormant season to enhance structure and distribute fruiting wood, and are pruned after bloom to thin fruit.

Maples and birches should not be pruned in the early spring when sap flow is heavy. These trees tend to “bleed” or drain sap heavily from pruning cuts. Although research has shown that sap drainage has little negative effect on tree growth, it is unattractive and best avoided. Most routine pruning and removal of weak, diseased, undesirable, or dead limbs can be accomplished at any time with little negative effect on the tree.

Under Alberta legislation, elm trees may only be pruned between October 1 and March 31 to help prevent the development and spread of Dutch Elm Disease.  This fatal disease is spread by elm bark beetles, who are attracted to the smell of wounded and dying elm trees.  Help keep Alberta DED free - do not prune your elm tree between April 1 and September 30!

Pruning Rules of Thumb:

  • Deadwood can be removed at any time of the year.
  • Prune live wood in the dormant season, or if it must be done sooner, in late June.
  • Conifers usually require no pruning. However, if shearing/pinching is desired, do it in June after the new shoots are fully developed.
  • Do not remove the bottom branches on spruce or fir trees.  Grass will not grow under the heavy branches anyway, and their shading moderates root temperature.
  • Prune off no more than 10% of total leaf area at one time.  If it is necessary to remove more, you can remove up to 25% of the leaves with dormant season pruning, but expect excessive shoot production near these cut ends.
  • Remove suckers in July or August on plants that sucker.
Last updated: 12/10/2014 9:24:01 AM