Understanding hidden decay and how non-invasive tools can detect it before failure occurs.

Why Tree Decay Deserves Attention

Tree decay is one of the leading causes of tree failure and often, it goes unnoticed until it’s too late. Trees can look perfectly healthy on the outside while breaking down from within.

Following on from our recent blogs, “Why Tree Safety Matters” and “Reactive or Proactive? Why Tree Risk Management Needs to Go Below the Surface,” this new blog focuses on understanding what tree decay is, why it matters, and how you can detect it early without damaging the tree.

If you manage an estate, hotel grounds, parkland or public space, this is something worth knowing.

Tree decay is the process of internal wood breakdown (usually caused by fungi) that weakens the structure of a tree over time. It often starts when fungi enter through wounds, cracks, pruning cuts, or natural ageing.

There are different types of decay, like white rot or brown rot, but they all lead to one thing: reduced strength in critical areas such as the trunk or major limbs.

The challenge? Trees with advanced decay can still produce leaves, flower, and appear full of life until they suddenly fail.

If you’d like to explore this topic further,  the Arboricultural Association offers valuable insights into the science behind decay and the questions still worth asking.

We’ve seen many trees that show no visible warning signs, only to reveal significant decay once they fall or are inspected. That’s why relying solely on visual assessments can leave risks undetected.

Waiting for a tree to fail can result in much more than just a lost tree. The consequences can include:

• Damage to buildings, footpaths, fences, or parked cars
• Safety risks for staff, guests, or the public
• Emergency works and unexpected costs
• Legal implications where duty of care is in place

A recent example of this came during our walkthrough at Dunsany Castle in collaboration with IrishTrees.ie, after Storm Éowyn. Several mature trees came down in the storm—some of which had internal decay not visible from the outside. Early inspection might not have stopped the tree to fall during the storm, but it could have changed the outcome.

At Holly Arboriculture, we use a combination of expert visual assessments and non-invasive testing tools to detect decay safely and effectively.

One of these tools is the Arborsonic 3D Acoustic Tomograph.

This advanced equipment allows us to:

• Send sound waves through the tree
• Measure how the waves travel through the wood
• Create a colour-coded cross-section scan showing areas of healthy wood, decay, and cavities

This method is completely non-invasive, meaning the tree remains undamaged and results are immediate.

Paul Holly using the Arborsonic 3D Acoustic Tomograph to assess internal tree decay during a site inspection in Dunsany Estate in collaboration with Irish Trees.ie.

What This Means in Practice:

• No need to drill or cut the tree
• Visual data supports confident decision-making
• We can prioritise which trees need action and which can be retained with care
• Helps reduce unnecessary removals

This technology is already in use on several sites in Ireland, including during our work at Dunsany Castle, where it supported our post-storm risk assessments. This technology helps us identify which trees are genuinely at risk and which can be retained safely with the right care

3D tomograph map displaying the internal health of a tree, with colour-coded sections indicating areas of sound wood, decay, or structural weakness.

If you’re responsible for managing sites with mature trees and regular foot traffic, detecting decay before failure happens is part of your duty of care. This applies to:

• Private estates and heritage properties
• Hotels and accommodation grounds
• Parks, golf courses, and recreational areas
• Schools, universities, and public campuses
• Councils and infrastructure projects

Identifying decay early not only helps protect people and property it also supports tree preservation by allowing for targeted, proportionate responses.

A fallen tree can cost far more than the time it takes to inspect it. That’s why we encourage clients to take a proactive approach to tree risk management including decay detection where appropriate.

We offer:

• On-site walkthroughs and visual assessments
• Decay detection using the Arborsonic 3D Acoustic Tomograph
• Clear, practical reports tailored to your site
• Tree risk management strategies with long-term value

We’re based in Cork and work with clients across Ireland. If you’d like to discuss your site or explore options for assessing mature trees, get in touch here.

You can also visit our Case Studies page to see how this approach has been applied across estates, parks, and public spaces.