Trees growing within an urban landscape require pruning to maintain them in a safe and aesthetically pleasing manner. Pruning should be undertaken with an understanding of how trees respond to each cut, as improper pruning can cause damage that could last the life of the tree or reduce a tree’s life expectancy.
Pruning can either help or harm a plant according to if, where, when, how, and why it is applied. When undertaken correctly, benefits include reducing risk of branch and stem breakage, improved health and appearance, and increased flowering. Functional benefits of pruning include providing better clearance for vehicles and pedestrians as well as maintaining clearances of structures and reduced litter.
The act of pruning creates a wound upon the tree. In response to a wound, the tree reacts by walling off or compartmentalizing the wound. The affected plant part is naturally sealed by wound wood tissue. This response limits the development of decay as a result of wounding or natural branch death.
It is preferable to remove smaller branches thereby causing smaller wounds. Removal of large branches creates larger wounds that may not form wound wood allowing more extensive decay to form.
If a large limb is to be removed a three cut method should be used to reduce the weight of the branch and prevent tearing of tissue back into the branch collar. First, make an undercut 300 mm to 450 mm from the branch’s point of attachment. Make the second cut from on top of the branch further out from the first cut. This will leave a stub that can be cut at the branch collar to finish the branch removal: (see figure 1).
Figure 1. Relieving cut prevents bark stripping back to the branch collar or beyond the final cut within a branch scaffold, causing irreparable damage to the tree.
Cutting tree branches flush with the trunk is to be avoided as this removes the branch collar and opens the tree to extensive decay formation. In order to minimise the extent of wounding, the final pruning cut should be made just outside the branch collar: (see figure 2).
Pruning to branch collars
Figure 2. Pruning to a branch collar (can also be referred to as target pruning).
To reduce the height or spread of a tree, pruning back the leaders and branch terminals to internal lateral branches or stems is preferable to the unacceptable practice of lopping: (see figure 3).
Figure 3. Reducing the height of a tree using a drop-crotch cut.
As trees grow older, less living healthy wood should be removed from the tree. There is no absolute rule for the amount of living wood that can be removed from an older tree. The Australian Standard – AS 4373 – 1996 Pruning of Amenity Trees states that when pruning a tree, as little foliage as possible should be removed and that the trees natural habit should be maintained (other than specific pruning requirements, e.g. remedial pruning, pollarding). The International Society of Arboriculture and the American Standard ANSI A300 caution that not more than 25% of the foliage should be removed from a mature tree within a growing season.
Wound dressings were once thought to accelerate wound closure and reduce incidence of decay however, research has found that this is not the case. Wound dressings are to be avoided and the target pruning method described above, utilised to reduce the extent of wounding trees.
Pruning large trees can be dangerous. If pruning requires working above ground with power equipment it is best to hire a trained arborist. Pruning of large, mature trees also requires a thorough knowledge of tree physiology and pruning methods. All pruning should be carried out in accordance with the Australian Standard – AS 4373 – 1996 Pruning of Amenity Trees.
Some information within this Fact Sheet has been obtained from a number of published sources. For further reading, refer to the Trees – Further Reading Fact Sheet.