Pruning is the act of removing branches from a tree to accomplish a specific purpose.
Reasons to Prune:
- Hazard Pruning - for safety reasons is top priority such as the tree/limb having the potential to fail (break or fall), it existing in a location that may contribute to its failure, and if a person/property would be injured or damaged should the failure occur
- Crown Raising - lower branches are removed to provide clearance for traffic, sight lines, and pedestrians.
- Crown Cleaning - removal of dead, dying, diseased or broken branches.
- Crown Thinning - removal of obstructing, weak or crossing branches to improve light penetration, air circulation, and the structural integrity of the tree
When to 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. Some 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.