Assessing Potential Tree Hazards

Trees provide great benefits for the urban environment. These include providing shade, defining and softening of the landscape, reducing noise levels, improving the quality of air and providing habitat for different forms of fauna and flora.

Trees however, can at times become hazardous to both people and property. This can be associated with storm damage, fungal attack, climatic conditions, growth patterns and damage by animals and humans. Periodic assessment can help identify problems where remedial works can be implemented to prevent tree removal and to maintain tree vigor.

Generally tree managers can program remedial work to reduce the risk of any future hazard and increase the viable lifespan of a tree. The current program for streets involves an annual inspection and subsequent pruning to provide clearance distances from over footpath, roadway and from electric lines. Council will also respond to requests from the public with concerns on all tree issues.

For select reserves and other Council's areas where the public use is regular, periodic tree assessments are carried out to determine tree health.

The following information is a brief overview of the potential hazards, which can often be easily recognised. Residents may find this useful to assist council in managing trees.

The diagram identifies conditions that could be potentially dangerous if left untreated.

Tree hazards diagram (pdf, 61KB)

In some reserves, Council will monitor and manage non functioning trees for their habitat value.

Giant Pine Scale

Giant pine scale is an insect that lives by sucking the sap of pine, fir and spruce trees. The insect produces white, waxy cotton-like discharge that is visible on infected trees.

More information: Giant Pine Scale FAQs (pdf, 89KB)

Identifiable Tree Hazards

1. Poor branch structure

Trees with acute angled branches or “V” shaped crotches are more likely to split than trees with less acutely angled branches. There may be included bark in the branch union, which weakens the branches’ attachment.

Trees with multiple trunks have a higher chance of splitting than single trunk trees.

Poor branch structure

2. Poor condition of the tree

Symptoms such as a high proportion of dead branches and a reduced amount of foliage tend to indicate the tree could be under high levels of stress.

 

3. Detached or hanging branches

This can indicate a weakness in the tree or create a wound causing future problem if left unchecked. Hanging branches may become detached, falling and causing damage.

 

4. Decayed timber or cavities

Cavities or visible signs of decay could indicate that the timber with in the trunk may be structurally unsound.

 

5. Fungi

Fungi in the tree can deteriorate the timber within the branches or trunk, compromising the trees structural integrity. Other fungi can appear as mushrooms at the base of the tree.

Fungi

6. Root damage

Roots that have been damaged or severed can affect the stability of the tree.  Fungal fruiting bodies such as mushrooms at the base of a tree could indicate that a root rotting fungi may be present.

Root damage

7. Deep bark splitting

May indicate high stress loads are evident on that area of the tree.

 

8. Borer attack

Drill like holes generally caused by moth larvae which can occur on the trunk or where branches are attached. Damaged branches may be weakened. Sawdust at the base of trees can also indicate borer attack.

Borer attack

9. Leaning tree

Has it noticeably shifted? Is there movement in the soil at the base of the tree or near tree roots? Tree stability may be unstable normally associated with root damage.

 

10. Large overgrown scare

Indicates previous damage and the trees natural mechanism to seal off the wound. Can be prone to fungal and insect attack.

Large tree scar

11. Swelling

Indicates the tree may be in a stressed condition.

 

 

Remedial Tree Works, To Reduce Tree Hazards

The type of treatment possible are dependent on several factors including the tree’s age, size, species, severity and type of damage and location. The following are works which could be expected:

  • Pruning the tree. Pruning to remove dead or diseased branches can reduce tree hazards.Weight reduction pruning can also be used to reduce stress loads on branches and can help reduce the chance of failure.It is important that pruning is done by qualified tree workers and is in accordance with Australian Standards for pruning of amenity trees
  • Cabling the tree. Cables and bracing rods are at time used to reduce the stress on branches and reduce the chance of branch failure, or uncontrolled failure.
  • Constant monitoring of hazardous trees. Trees identified as potential hazards can be monitored at regular intervals to assess whether they are becoming more of a risk or not
  • Remove the target. Often trees are perceived to be dangerous when in fact they are safe. While it is not realistic to move a home, sometimes it may give piece of mind if outdoor furniture, cars and landscape features are moved clear of large trees
  • Removing the tree. Sometimes the removal of a hazardous tree is the most appropriate option and replanting with a suitable species should be a consideration.The safe useful life expectancy of a tree is often the deciding factor when determining whether living trees require removal.

Other action such as soil aeration, fertiliser treatment and chemical treatment can also be considered although their success can be variable and difficult to measure over a short period.

If you notice any such hazards

If you have any queries on street trees or notice any of the previously mentioned potential hazards, please call Councils Customer Service Centre on 9518 3555.  The tree can then be inspected by a qualified arborist to determine any action needed.

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Last updated: 09 February 2018