Tree risk assessment is a complex multi-level process applied to an individual tree.

Risk Evaluation

Tree risk assessment is a complex multi-level process applied to an individual tree. In assessing and managing trees, we try to strike a balance between the risk that the tree poses and the benefits that individuals and communities derive from the trees.


Risk assessment begins with a visual tree assessment (VTA) of a tree from the ground up. While there may be obvious signs of problems including cavities, cracks, fungal conks, broken branches, and other signs, internal or underground defects may also exist. Once the VTA is completed, it forms the basis for further investigation or if a more advanced type of evaluation is indicated.


What happens next depends on the type of defect, the tree owner’s tolerance for risk, and the presence of targets. Targets include people, buildings, cars, and anything of value.


Below you will find a description of the more common types of test / analysis available for risk assessment.

The ease of drilling and the condition of the wood extracted indicate the presence or extent of decay.

Twist Drills

This is a simple, low tech method for assessing decay in trunks, branches, and roots. A standard cordless drill is used, typically with a 1/8 inch twist drill bit installed.


The ease of drilling and the condition of the wood extracted indicate the presence or extent of decay. Using a twist drill requires experience to assess the results and leaves large drilling wounds. This method is really only adequate for finding voids within the tree. It will not detect incipient decay and does not accurately determine bark thickness or sound wood thickness. Drilling is, however, well suited for detecting decay in roots.


Some diagnosticians are making use of the Shigometer to detect internal discoloration and decay in the tree and also to provide a relative measure of its vitality.

Resistance drilling is a more sophisticated type of decay detection than a simple twist drill.

Resistance Drilling

Resistance drilling is a more sophisticated type of decay detection than a simple twist drill. The RESI 400F utilizes a 3mm wide drilling needle and measures wood resistance as it drills. Healthy wood provides more resistance to the drill bit than decayed wood.


As the drill penetrates the wood, resistance is recorded on a strip of graph paper or electronically. Resistance drills have the ability to show bark thickness, annual rings, incipient decay, more advanced decay, and voids, as well as, their depth within the wood.


While drilling into the tree is required, the holes are small in diameter. Resistance drilling is particularly useful for measuring sound wall thickness in decayed trunks and branches.

Soil is removed from around the base of the trunk and the roots to inspect this area for decay.

Root Collar Excavation

Soil is removed from around the base of the trunk and the roots to inspect this area for decay. Typically the soil is blown away using a Supersonic Air Knife® or similar tool. . This allows the removal of soil without damaging roots. Once excavated, roots and the root collar, the base of the tree where the roots emerge from the trunk, can be examined and tested for decay.