Summary by Stuart Rennie
The term AS4373 is often quoted by arborists and council officers; `pruning must be in accordance with Australian Standards AS4373-2007 Pruning of amenity trees.’
The average homeowner is not expected to understand these guidelines but unfortunately it seems there are a lot of tree loppers who don’t either (judging by some of the poor pruning that goes around).
The Australian Standard AS4373-2007 `Pruning of amenity trees’ is a document produced by Standards Australia to provide a guide for the pruning of trees.
This guide has been developed for Arborists and Council Tree Officers to bring uniformity to the industry using up to date and sound pruning techniques based on the widely accepted theories of compartmentalisation of decay in trees (CODIT).
So basically, it is a guide that outlines pruning techniques that won’t harm your tree.
Council Tree Management Policies require that pruning is undertaken in accordance with AS4373-2007. Failure to do so is considered a breach of Council’s Tree Management Policy and fines can be imposed.
A common request from tree/ home owners is for topping or lopping of their trees as a means to control the growth potential of their trees.
Most tree owners do not understand why this outdated pruning technique is not permissible and not compliant to AS4373.
History has proven that topping or lopping was detrimental to the long-term viability of trees as this technique failed to recognise the process of compartmentalisation of decay in trees.
To put it simply, topping or lopping is an unacceptable practice for the following reasons:
- The natural habit of the tree is destroyed
- The regrowth or epicormics are weakly attached and become prone to failure.
- Branch stubs can decay
- Pruning wounds predisposes the tree to fungal infections and insect attack
- The lifespan of the tree is usually reduced.
More often, trees that have been lopped are generally removed a few years later.
The Standard specifies methods for pruning of trees and gives guidance on the correct pruning techniques.
1. Prior to pruning being undertaken, an arborist with a minimum qualification of AQF Level 3 in arboriculture needs to assess the tree to determine the need for pruning.
The inspection should consider the trees species, age, vigour, condition, hazards, wounds, wildlife hollows, wind loading and stability. The arborist needs to consider whether the tree will be adversely affected by pruning.
Note that some councils allow 10% pruning each year, even if the tree doesn’t require pruning. This pruning is not compliant to AS4373.
2. Equipment that can damage the bark like spurs or spikes shall not be used. This includes lowering systems such as friction drums that may bruise the bark and disrupt the vascular cambium.
3. Sharp tools are required so that clean cuts will be made.
4. When removing a branch, a pre-cut or undercut should be made to avoid splitting or tearing of the bark. The position of the final cut should be at the branch collar. In the absence of a visible collar, the branch bark ridge shall be used to determine the appropriate angle. The Standard also describes the final cut for the removal of a co-dominant stem.
5. When dropping or lowering branches, care must be taken to avoid damage to parts of the tree that are not being pruned.
6. When pruning a tree, the minimum necessary to achieve the aim of the pruning should be pruning.
It is important to remember that excessive pruning may have an adverse effect on a tree’s vigour or condition.
The Standard promotes the following types of pruning:
- Crown maintenance retains the structure and size of the tree and includes deadwooding, crown thinning, selective pruning and formative pruning. These pruning types are suitable for all trees.
- The Standard states the minimum diameter for deadwood and the maximum diameter for crown thinning needs to be specified when prescribing pruning recommendations. Also, the percentage of canopy being removed should not have a detrimental effect on tree vigour.
- Lions tailing should also be avoided. This seems to be a common practice where all the interior branches are removed leaving only a small amount of foliage on the branch ends. This is not crown thinning and may lead to structural hazards.
- Crown modification is pruning that changes the structural appearance and habit of the tree. This includes crown lifting, reduction pruning, pollarding, remedial pruning and line clearance. Whilst crown lifting and line clearance are suitable for all trees, reduction pruning, pollarding and remedial pruning are not suitable for all trees and are specific to certain species, age and damaged trees.
- The standard states that clearances and the maximum diameter and location of the branches to be removed should be specified.
- Important notes to remember; Pollarding is suited mostly to deciduous trees that have been pruned that way from a young age. Trees pollarded initially and not regularly maintained can become hazardous. Pollarding should not be carried out on mature trees that have not previously been pollarded.
- Remedial pruning shall only be carried out on trees that have been damaged. This method should only be used when replacing the tree is difficult. It is only a way to prolong its life and reduce the hazard potential.
- Line clearance is pruning to maintain clearances.
Pruning of palms
- Palms are often pruned to remove old or dead fronds and fruit.
- The terminal shoot of a palm should never be pruned to avoid killing the palm.
- The unnecessary removal of healthy fronds should be avoided as it may place the palm under stress.
- Spikes should be avoided to prevent wounding to the stems. Wounds may provide entry points of decay pathogens.
Root Pruning is also discussed under the standards. As roots are responsible for the uptake of nutrients and water and give the tree stability, prior to pruning any roots, specialist advice must be sought by a minimum AQF Level 4 Arborist to assess whether root pruning will adversely affect tree vigour or even affect stability.
Minimally destructive techniques such as hand-digging or compressed air must be used to expose roots. Any roots cut must be done so with sharp tools such as secateurs or handsaws.