Hazardous Defects in Trees

Tree that have defects can cause harm to people and damage property. Recognising the features of hazardous trees and taking proper corrective actions can prevent loss of life and property. A tree with hazardous defect can cause failure (toppling) of all or part of the tree, which can hit a "target".

Evaluating trees for hazardous defect can be complex because of the variability of trees, the severity of their defects, and site differences. This article highlights guidelines, not absolute rules for recognising and correcting hazardous defect. Corrective actions should be undertaken by certified arborists.

What to Look For

1. Dead wood

Dead trees and branches must be removed as they can fall at any moment. Dead branches that have already broken off and hanging are all the more dangerous.

2. Cracks

A crack is split deep into the wood, sometimes completely through the stem/trunk. Cracks are dangerous as they are an indication of potential tree failure.

3. Weak branch unions

Areas where branches are not strongly attached to a tree are called weak branch unions.

When two or more branches grow very closely together, the area where they join swells with ingrown bark, which does not have the strength of wood, and thus the union is weaker than one that does not have included bark.

Trees with a tendency to form upright branches, such as Alstonia, often produce weak branch unions. They are also formed after a tree is topped (a cut at a right angle to the direction of growth).

4. Decay

Decay leads to wood that crumbles or is soft, or a cavity where wood is missing, creating a hazard.

Signs of fungal growth such as mushrooms, conks, and brackets growing on the base of the tree, stems or branches can be indicators of advanced decay.

5. Cankers

Cankers are caused by mechanical injury or disease, occuring at a localised area where the bark is sunken or missing. A canker that covers half or more of the circumference of the tree may be hazardous even if exposed wood appears sound.

6. Root problems

Root problems may be due to the raising or lowering of soil grade near the tree, severing of roots, installing of pavement over roots, driving vehicles over roots, or advanced decay in root and root flares.

As defective roots are underground and not readily seen, aboveground symptoms such as dieback and off-colour leaves serves as an indication of root problems.

7. Poor tree form

Poor tree form could be caused by damage from storms, improper pruning or unfavourable environment. These trees are structurally unsafe. Trees that lean excessively are also a hazard.