Occasionally, holes develop in the trunk of a tree. Sometimes these are visible, but other times they are not easily detected. What causes this and will it kill the tree?

The short answer is that a hollow tree may continue to thrive for many years, and the decay of the inside deadwood is a natural part of the life cycle of a tree. The centre of the tree trunk was once the sapling timber, and as the tree grows, stronger layers are built around this sapling wood, until the strength of the outer trunk is what supports the canopy.

The trunk of a tree has two purposes – to support the canopy and to carry water and sap from the roots to the tallest parts of the tree. The wood in the middle of the trunk, previously the sapling, is no longer used for the transportation of food to the canopy, as each new layer of timber and bark take over that job. Bacteria invade the dry hardwood at the centre of the trunk, and as decay begins, insects and small animals make their way into the hollows created. Year by year the hollow parts extend within the trunk, and become visible from the outside. Larger animals may now make their home in these hollows.

Unless the foliage of the canopy of the tree is showing signs of distress, there is no cause for alarm. Birds, possums, bats, maybe even tree kangaroos could make their home in your hollow tree.

If the foliage on the canopy becomes sparse and discolours, then it would be time to confer with an experienced arborist regarding the future of the tree. If the tree trunk is unstable, not able to support the tree, the tree may collapse during a strong wind or storm. It is always better to prevent such an event in a built up area.

In days gone by, it was thought that filling visible holes in a tree trunk was a good idea, however it is generally considered to be detrimental to the tree these days. Filling the hole may cause agitation for the tree trunk as it expands and contracts with the weather. This could cause unnecessary damage to the area around the hole and damage the sections of the tree transporting sap and water and supporting the canopy. If you really want to block a hole in a tree trunk, use a thin metal flap or screening covered with plaster. This will prevent animals and water from entering the hole and create a surface that the bark and outer living layers can eventually grow back over.