Arborist Consulting & Permits
Tree Resistograph Mapping for Decay Detection in Greater Victoria
Tree resistograph mapping uses resistance drilling, a fine needle driven slowly into the wood, to record how wood density changes across a trunk, root flare (where the trunk widens into the roots) or limb and map hidden decay, cavities and cracks. Oakstead Tree & Property Care's Certified Arborists provide resistograph mapping in Victoria, Saanich and the Cowichan Valley as part of tree risk assessments, so decisions to retain, prune or remove a tree rest on what is inside it.
In plain words
A fine needle is drilled slowly into the wood. It records how dense the wood is along the way. This shows hidden rot, holes and cracks inside. Oakstead uses it as part of tree risk assessments.
What is resistograph mapping?
A resistograph is a resistance-drilling instrument. A thin needle is driven into the wood at a steady rate while the instrument records how much resistance it meets. Sound wood resists the needle, while decayed wood, cavities and cracks offer little resistance. The result is a profile of wood density along each drilling path.
Mapping means taking readings at several planned points and heights around the trunk, root flare or a major limb, then reading them together. That shows where decay sits, how far it extends and how much sound wood remains, rather than relying on a single reading.
How it informs a tree risk assessment
A visual assessment can reveal signs of internal decay, such as fungal fruiting bodies, cavities, seams or swelling, without showing how extensive the problem is. Resistograph mapping fills that gap. Oakstead uses the results, alongside increment bore samples where helpful, to judge the likelihood of failure and to recommend whether a tree can be retained, should be pruned or monitored, or needs to be removed. The findings go into the written report.
When resistograph mapping is useful
- Large or heritage trees showing signs of internal decay
- Trees near homes, driveways, play areas or other high-use spaces
- Trees that need a documented assessment for a tree cutting permit or tree management plan
- Trees affected by root damage, construction or past storm damage
- Checking the same tree over time to see whether decay is advancing
Resistance drilling leaves a narrow hole in the wood, so the arborist plans drilling points to gather the information needed with as few readings as practical.
If you’d like to just get the conversation started, or want to schedule a viewing of your trees in the Saanich and Victoria area, please give us a call at (250) 888-9222 or request a consultation online through the request form on our Get a Quote page, and we’ll get right back to you.
Tree Resistograph Mapping: frequently asked questions
What does a resistograph show?
A resistograph records the wood's resistance to a fine drilling needle and produces a density profile along the drilling path. Low readings point to decay, cavities or cracks, and consistently higher readings indicate sound wood.
Does resistograph testing harm the tree?
It leaves a narrow drilling hole, which is a minor wound compared with the information gained. An arborist limits the number of drilling points and places them where the readings will be most useful.
Why map several points instead of taking one reading?
Decay rarely spreads evenly through a tree. Readings at several points and heights show how far decay extends and how much sound wood remains, which is what a tree risk assessment needs to judge the likelihood of failure.
Is resistograph mapping part of a tree risk assessment?
It can be. Oakstead uses resistograph mapping as an advanced diagnostic step within tree risk assessments when visual signs suggest internal decay, and the findings are included in the written report and recommendations.
Ready to talk to a Certified Arborist?
We respond to every request. Serving the Saanich Peninsula, Greater Victoria and the Cowichan Valley.