A dangerous tree isn't just an unhealthy tree - it's the combination of a defect, a likelihood of failure, and something worth hitting underneath it. Arborists call this a hazard assessment, and you can do a basic version yourself from the ground. Here's what we look for.
Start with the target, not the tree
The same defect in two different trees can mean completely different risk levels. A hollow old totara in a back paddock threatens nobody. The same hollow in a tree over your kids' trampoline, the driveway or the neighbour's bedroom is a different conversation.
So the first question is always: if this tree (or part of it) failed, what would it hit? Trees over houses, sleepouts, parking areas, play areas and public footpaths deserve the closest attention.
Warning signs in the canopy
Large dead branches are the most common hazard we deal with - deadwood over about 50mm diameter can fall without warning, and it doesn't need wind to do it. A canopy that's thinning, browning on one side, or producing unusually small leaves is a tree telling you something is wrong below.
Also look for hanging broken branches lodged in the canopy after storms - so-called widow makers - and for dense clumps of regrowth from old lopping cuts, which are weakly attached and fail readily in wind.
Warning signs on the trunk
Vertical cracks, seams of oozing sap, large cavities and bark falling away in sheets all warrant a professional look. Pay particular attention to unions where two similar-sized stems meet in a tight V - these 'co-dominant unions' with included bark are one of the most common failure points in mature trees.
Fungal brackets on the trunk almost always indicate internal decay. The visible mushroom is the fruiting body - the fungus itself is well established inside the wood by the time you can see it.
Warning signs at the base
The root zone is where the most serious failures start. Look for: soil heaving or cracking around the base (especially on the side opposite a lean), fungal growth at the root collar, and roots that have been cut or damaged by trenching, driveways or construction within the canopy's dripline.
A tree that has recently changed its lean is an emergency - call an arborist the same day and keep people away from the fall zone.
After storms and during wet winters
Waikato's wet winters and spring storms are when marginal trees fail. After any major wind event it's worth walking your trees and checking for new cracks, lifted soil, and hangers in the canopy. Saturated ground halves the anchorage of a large tree - a lean that was stable in summer can move in a wet July.
What to do if you're concerned
Don't test it yourself, and don't send anyone up the tree. Keep people and vehicles out of the potential fall zone and get a qualified arborist to assess it - most reputable operators, us included, will look at a genuinely concerning tree quickly and give you a straight answer.
Not every defective tree needs removal - many can be made safe with targeted pruning or simply monitored. Call 022 167 1268 for a free onsite hazard assessment anywhere in Waikato or Hauraki. Related reading: our guide to the 7 signs a tree needs to be removed.
