Tree risk assessment: what an arborist checks

Homeowners usually ask "is it going to fall?" The honest answer needs a trained set of eyes on the tree itself. This guide walks through what those eyes look for, so you know what you are paying attention to when an arborist walks your yard.

What does an arborist look at on the tree?

The ISA's tree owner checklist is a good picture of the starting point. It asks about large dead branches, detached or hanging branches, branches that have already fallen, loose bark, cracks or splits in the trunk or at branch unions, cavities or rotten wood, mushrooms at the base, unusual yellowing or small leaves, and whether the tree has been topped or heavily pruned.

An arborist walks around the whole tree rather than glancing at one side. The brochure also names specific defects to look for: poor taper, codominant stems, cracks and splits, external signs of decay, dead branches and human-caused wounds.

• Crown: dead wood, thin or undersized leaves, heavy limbs with weight concentrated at the ends
• Trunk: cracks, cavities, cankers, bark that is coming loose
• Branch unions: two stems of similar size joining at one point
• Base and roots: fungal growth, soil cracks or heaving, lean that looks new

Why does the ground and the site matter?

The same defect matters more in some places than others. The ISA lists recent construction, changes in soil level and new lawn or pavement as questions to ask, because root loss from that kind of work can show up years later as a weak tree. Cracks or separations in the soil can warn of root movement, especially if the tree leans.

For more on how site work hurts roots, see protecting trees during construction and trenching and grade changes near trees.

What is a "target" and why does it change the answer?

A target is whatever could be hurt or damaged if part of a tree fell: a house, a parked car, a play area, a driveway, a power line. A leaning pine in an empty back lot and the same pine over a bedroom roof carry the same defect but very different consequences.

That is why an assessment is about more than the tree. The ISA notes that trees falling into utility lines can injure people and cause outages and fires. If any part of a tree is touching a line, stay back and call NV Energy. Never go near a downed line.

What can come out of an assessment?

The ISA brochure lists the usual options: prune the tree and remove defective branches, cable and brace to support weak branches and stems, provide routine care, or remove a tree whose risk is unacceptable. It adds that supports are not guarantees against failure.

Most trees leave the assessment with a pruning recommendation, not a saw job. When removal is the answer, our arborist consultation and tree removal pages explain how the work is planned, and crane-assisted removal covers trees that cannot be dropped in pieces by hand.

Who should do it, and when?

The ISA recommends that anyone who answers yes to any checklist question have the tree examined by an ISA Certified Arborist. At Battle Born Tree Service, a certified arborist assesses every job before work begins, and we have two ISA Certified Arborists on staff. You can verify any arborist's credential on the ISA's Trees Are Good site.

Do not wait for a storm. Look at your trees in calm weather, after a big wind, and after any digging or paving near them. If something worries you, call 775.232.8243 for a free estimate. If a tree is down on a house or anyone is hurt, call 911 first.