An arborist assessing a hazardous tree is answering three questions together, and a company that only looks at the first is answering a third of the problem. How likely is a part to fail — from cracks, decay, a moved root plate, a weak union, dead limbs? How likely is it to strike something if it does, given fall distance, direction and what stands in it? And how bad would that be, depending on whether the target is a dwelling, a parked car, a walkway or an empty field corner? That is why two trees in identical condition get opposite recommendations. A long-standing lean with undisturbed soil is often stable reaction growth; a new lean with cracked or raised soil means the anchoring roots have torn. Some defects have a pruning answer that genuinely lowers the load. A compromised root plate, a decayed base or a crack through the stem does not.
A defect on its own decides nothing
- 2. Look for root movement, fresh soil cracks and trunk defects from a distance
- 3. Identify occupied and frequently used targets
- 4. Consider whether pruning or target management can reduce concern
- 5. Plan how a crew and equipment can work without creating new damage
The three questions behind every risk decision
Risk is not condition. It is condition combined with what sits underneath, which is why the same tree gets opposite recommendations in two different yards.
| Question | What is assessed | What raises it |
|---|---|---|
| How likely is failure? | Defect type, size, position and load | Cracks, decay at the base, moved root plate, dead limbs |
| How likely is it to hit something? | Fall distance, direction, and what is in it | Targets directly beneath; a consistent wind direction |
| How bad would that be? | What the target is, and whether it is occupied | A dwelling, a parked car, a walkway, a play area |
An assessment that only looks at the tree is answering one of the three.
What to observe before the assessment
Half of a risk judgement is what sits underneath the tree, and that half is entirely yours to describe.
- What is beneath the tree, and how often anyone is there
- What specifically changed, and when
- Soil at the base: cracked, raised, or undisturbed
- Any fungal brackets, cavities or seams you can see
- Digging, paving or grading near the roots in the last decade
Why no assessment declares a tree safe
No assessment declares a tree safe. It records what was observable on a date, the likely failure modes, the targets, and a review interval.
12 questions this raises
What does an assessor look at first?+
What is underneath. The condition of the tree is only half the question, and it is the half that cannot be answered on its own — a hollow trunk over an unused field corner and the same trunk over a driveway where a car parks every night are the same tree and completely different risks. An assessment that never looks down is not an assessment.
How is 'likelihood of failure' actually judged?+
From the defect and the load it has to carry. A dead limb will fail; the question is when. A crack through a union means the section is already separating. Decay is judged against how much sound wood remains and whether the shell is continuous. Lean is judged by whether it is new, which the soil tells you, not the angle.
Is a leaning tree hazardous?+
Not by itself, and this is the most common false alarm. A tree that has leaned the same way for twenty years has grown reaction wood to hold that position and is often perfectly stable. What matters is whether the lean is new or increasing — look for cracked or raised soil on the high side and a mound on the low side. Undisturbed turf and moss to the trunk suggests a long-standing lean.
How much decay can a trunk take?+
The rule most assessors start from is that a trunk retaining roughly a third of its radius as sound wood around the full circumference has meaningful strength, because a hollow tube resists bending far better than intuition suggests. That assumes a continuous shell. An open cavity, a seam, or a crack through the shell changes it completely, and so does a lean loading one side.
What do fungal brackets at the base mean?+
That decay has been working for years — the bracket is the fruiting body, not the beginning. Which fungus matters a great deal: some decay only dead heartwood and leave a stable shell, others attack the roots and the base, where failure is sudden and total. A bracket at the root collar deserves prompt attention. Photograph it before it deteriorates, because shape, colour and position are what allow identification.
Why does access affect a hazard decision?+
Because it changes what mitigation is realistically available. A tree that can be climbed offers options a tree that cannot does not; a tree a bucket or crane can reach can be dismantled in a controlled sequence that a hand-rigged removal cannot match. Two trees in identical condition can get different recommendations because one can be worked safely and the other cannot.
Can the whole tree be saved by removing just the bad part?+
Sometimes, and it is worth asking. Reducing end weight on an over-extended limb, removing a co-dominant stem, or crown-reducing a top-heavy tree can lower the load enough to change the assessment. What cannot be pruned away is a compromised root plate, a decayed base, or a crack through the main stem — those are structural, and taking weight off does not restore what has already failed.
What does 'high consequence' mean in practice?+
Something under the tree that is occupied or valuable and cannot easily be moved: a house, a bedroom in particular, a driveway where cars sit, a walkway people use daily, a play area, a road, a neighbour's building. Consequence is what turns a defect into a decision, and it is also the one variable a property owner can sometimes change.
Will an assessment tell me the tree is safe?+
No honest one will, and be wary of one that offers to. What a written assessment records is what was observable on a given date, the likely failure modes, the targets, the reasonable management options and a review interval. No tree is risk-free, decay is largely internal, and conditions change — it is a dated professional opinion, not a certificate.
How urgent is 'urgent'?+
Treat it as immediate if the tree is over an occupied space, is in contact with electrical equipment, is actively moving, or has a lifted root plate. Treat it as prompt-but-scheduled if the defect is stable and the target is intermittent. The distinction is whether something worse can happen before a crew can reasonably get there.
Should I get the assessment from the company that would do the removal?+
It is worth having at least one opinion from someone who is not bidding on the work, particularly before a large removal. An assessment can legitimately conclude that no work is needed, and that is a conclusion a removal quote has no incentive to reach. The cost of an independent look is small relative to taking down a mature tree unnecessarily.
What should I have ready for the assessor?+
The history. What changed, when you noticed it, what the weather was doing, any previous work on the tree and by whom, and — most importantly — any construction, grading, trenching or paving near the roots in the last decade. Root-zone disturbance is the cause most often missed, because it happened years ago and left no visible trace above ground.
Risk-assessment and safety standards
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