AS 4373 Pruning Standard Explained | Pruning Classes & Rules

AS 4373-2007 Pruning of Amenity Trees is the Australian Standard for pruning urban and amenity trees. It sets out how cuts should be made, how much of a tree may be removed, the recognised types (“classes”) of pruning, and the practices that are never acceptable, such as lopping, topping and flush cutting.

It covers street trees, park trees, garden trees and palms. It does not cover fruit or timber production, hedging, topiary or espalier, and it does not deal with wildlife habitat or the safety of the person doing the work.

The current edition was published in 2007 and reconfirmed by Standards Australia in 2020. Standards Australia lists it as pending revision, but until a new edition is published the 2007 version is the one that applies.

Why the standard matters to you

A standard is not law on its own. It becomes one when a law points to it, and in South East Queensland several do:

Brisbane. The Natural Assets Local Law 2003 defines pruning as maintenance or remedial pruning done in accordance with AS 4373. Its list of damage to vegetation includes lopping or topping as the standard defines them, and cutting branches other than in the course of pruning. So on a protected tree, a cut that doesn’t meet the standard isn’t pruning in the eyes of the law: it is damage, and it needs a permit like any other damage. Using climbing spikes on a healthy tree other than as the standard allows is listed as damage too.
Gold Coast. The City Plan’s vegetation management code points to AS 4373 for general guidance on pruning.
Redland. Council asks that routine pruning on private property follow AS 4373, recommends an arborist qualified to at least AQF Level 3, and accepts a dead tree on rural land being made safe by pruning it to a habitat structure under the standard.
Ipswich. The City Plan asks for vegetation work described in a vegetation management plan to follow the standard.

Even where no law names it, AS 4373 is the benchmark a council officer, an insurer or a tribunal will measure pruning against. If a contractor’s work is ever questioned, “was it done to AS 4373?” is the first question.

Who the standard says should do the work

The standard sets minimum qualifications for each step:

Assessing the tree and specifying the pruning: at least AQF Level 3 in arboriculture.
Carrying out the pruning: at least AQF Level 2, or equivalent experience and demonstrated competence.
Root pruning: specialist advice from someone with at least AQF Level 4 before any roots are cut.
Trees with hollows or likely habitat: may need further assessment by an ecologist or wildlife specialist.

Before any pruning: the inspection

The standard expects the tree to be inspected before pruning is prescribed. The inspection looks at the tree’s health, structure, stability and surroundings, and decides whether pruning is needed at all. If it is, the pruning required now, and any follow-up, should be specified.

The inspection should weigh up hazards, habitat, species, age, condition, wind loading, location and the time of year, along with how much foliage will be removed and how large the wounds will be. It should also check any legislation that applies: planning rules, heritage listings and protected species. The guiding principle is that the pruning should not harm the tree.

How the cuts must be made

The procedures rest on how branches attach to trunks and how trees seal off wounds and decay (compartmentalisation). The key requirements:

Tools. Sharp tools only, so every cut is clean. Equipment that wounds the bark, such as climbing spikes, hooks or chained platforms, must not be used on a tree, or the part of a tree, being kept.
Disinfection. Where disease could spread between trees, tools must be disinfected between trees, and the assessment should say when that applies. Palms of the genus Phoenix are a standing example, because of a fatal Fusarium wilt.
Pre-cutting. Large branches are undercut or pre-cut first, so their weight can’t tear the bark down the trunk. The stub left is then removed with the final cut.
The final cut. This goes as close as possible to the branch collar (the swelling at the base of the branch) without cutting into it or leaving a stub. Where there is no visible collar, the branch bark ridge sets the angle. For a codominant stem, the stem bark ridge does. The bark at the edge of every cut must stay firmly attached.
Reduction cuts. When a branch is shortened, it is cut back to a lateral branch at least one third of its diameter at the cut. Cutting to anything smaller is lopping.
Lowering branches. No other part of the tree, or any neighbouring tree, may be damaged as branches come down.
Hanging branches. Every broken or cut branch caught in the canopy must be removed before the job is finished.

How much can be removed?

The standard does not set a single percentage for every tree. Instead:

Remove the minimum necessary to achieve the aim of the pruning, and avoid excessive pruning.
The amount to be removed must be specified before work starts, by the pruning class, for each tree or group of similar trees.
How much a tree can tolerate depends on the species, health, age, condition and location, so the assessing arborist sets it.
Pruning should keep the tree’s natural shape, and leave foliage and weight spread naturally along the branches.
On young trees, at least half the foliage should sit on branches arising from the lower two-thirds of the trunk, to build good trunk taper.

In practice, a quote that says “prune tree” doesn’t meet the standard. A compliant specification names the class and the amount, for example “crown thinning (T), 15%, branches up to 50 mm”.

The pruning classes

The standard groups pruning into two families, each class with a one-letter code for specifications.

Crown maintenance works with the tree’s growth habit. It keeps the tree’s size and structure and doesn’t reduce the volume of the crown.

Crown modification changes the tree’s form and habit.

Class	Code	Family	Applies to
Deadwooding	D	Maintenance	All species
Crown thinning	T	Maintenance	All species
Selective pruning	S	Maintenance	All species
Formative pruning	F	Maintenance	All species
Reduction pruning	R	Modification	Trees with suitable secondary branches
Crown lifting	C	Modification	All species
Pollarding	P	Modification	Deciduous trees formatively pruned for it
Remedial (restorative) pruning	H	Modification	Damaged, declining or diseased trees only
Line clearance	L	Modification	All species

Deadwooding (D)

Removing dead branches. The specification must give the minimum branch diameter and where in the tree. Dead branches can be habitat, so that is weighed first. Where a dead branch has been in the tree a long time, the collar may have grown out along it; that collar is left intact.

Crown thinning (T)

Reducing the density of the canopy by removing smaller, lower-order branches while keeping the main structure. It lets in light and air or opens a view, without changing the tree’s overall size. The percentage of crown to be removed must be specified. Thinning must not strip out the inside of the canopy and leave foliage only at the branch ends (“lion’s tailing”), which the standard warns can create structural hazards.

Selective pruning (S)

Removing particular branches that are causing a specific problem, such as one rubbing a roof or blocking a driveway. Those branches are identified at the assessment.

Formative pruning (F)

Pruning young and establishing trees to build a sound structure: improving form, heading off structural weaknesses before they develop, preparing a tree for more specialised pruning later, and steering growth away from buildings and services. Done early, it prevents many of the structural problems that are harder and more expensive to fix in a mature tree.

Reduction pruning (R)

Reducing a tree’s height or spread by cutting branch ends back to internal lateral branches or stems, each at least a third of the diameter of the branch being cut. The extent of the reduction is set at the assessment. The standard is explicit that reduction pruning is not lopping or topping. The difference is where the cut lands.

Crown lifting (C)

Removing lower branches to give clearance over a path, road, driveway or roof. The clearance required, and the largest branch to be removed, should be specified. On young trees, the foliage rule above (half on the lower two-thirds of the trunk) still applies.

Pollarding (P)

A specialised system that cuts branches back to the same point every one to three years, forming knobbly “pollard heads” that sprout vigorous shoots. It suits mainly deciduous trees trained this way from a young age. It must not be started on a mature tree that has never been pollarded, and a pollarded tree that is then neglected can become hazardous. Pollarding is not lopping.

Remedial (restorative) pruning (H)

For trees that have lost their natural form to storms, vandalism, earlier lopping, dieback or disease, usually when other options have failed and the tree is hard to replace. Damaged or lopped branches are cut back to healthy tissue, and the new shoots are then managed over time, through reduction and thinning, into a new crown. It is done in stages, the tree needs monitoring, and the standard notes that removal of a dangerous tree should be considered.

Line clearance (L)

Keeping trees clear of overhead power lines and other services. It is reduction pruning applied to a clearance, and state electrical safety law sets the clearances themselves. Pruning near power lines is specialist work for trained line clearance crews.

Palms

Palms are pruned mainly to remove old fronds and fruit that could fall. The growing tip must never be removed, because that kills the palm. Healthy green fronds should generally be left alone; removing them stresses the palm and is rarely needed. Climbing spikes must not be used on palms, and tools must be disinfected between palms because of Fusarium wilt.

What the standard rules out

Lopping and topping

Lopping is cutting branches or stems between branch unions; topping is reducing a tree’s height by lopping. The standard calls both unacceptable, because they:

trigger a flush of fast, weakly attached regrowth that is prone to failure
leave stubs that decay
destroy the tree’s natural shape
can shorten the tree’s life
open the tree to fungal infection and insect attack.

A lopped tree is often more dangerous within a few years than it was before it was cut, which is the opposite of what most owners are paying for.

Flush cutting

A flush cut removes or damages the branch collar. The collar holds the chemical defences a tree uses to seal off a branch wound, so cutting into it leaves a bigger wound that is prone to decay, costs the tree stored energy to deal with, and can lead to cavities later.

Wound paint

The standard advises against wound dressings and paints. Research shows none of them prevents decay, most do nothing for wound closure, and some damage tissue or help decay fungi. A correctly placed cut lets the tree’s own defences do the job.

Root pruning

Root pruning is largely outside the standard, but it gives the essentials. Cutting roots can stress a tree, let in root-rotting fungi and destabilise it, and the effects are not always predictable. Specialist advice (AQF Level 4 or above) should come first. Roots should be exposed with hand digging, air or water excavation, or located with ground-penetrating radar, and cut cleanly with sharp tools rather than torn.

For trees near building work, AS 4970-2025 sets the root protection zones, and a root investigation finds where the roots actually are before anything is cut.

How to check a contractor’s quote

Before you book pruning, check the quote or specification against the standard:

It names the class (or classes) and the code: D, T, S, F, R, C, P, H or L.
It states how much will be removed, as a percentage, a clearance or the specific branches.
It says “to AS 4373-2007”, and the work is assessed by someone with at least AQF Level 3.
No spikes will be used on a tree being kept.
No lopping or topping. “Cut it back by half” or “take the top out” is lopping unless every cut lands on a suitable lateral branch.
Protected trees have a permit, or the work clearly falls under an exemption. In Brisbane, pruning that isn’t to AS 4373 isn’t covered by the pruning rules at all.

If you’re not sure whether pruning will help, or how much your tree can take, a tree health or risk assessment answers that before anything is cut.

AS 4373-2007 Pruning of Amenity Trees: The Australian Pruning Standard Explained