Is a leaning tree dangerous? How to tell

Old lean or new lean: the question that decides it

Trees lean for ordinary reasons. A tree grows toward light, so one that started out beside a taller neighbour, a building or a woodlot edge often leans away from the shade. Over decades the trunk adds wood to compensate, and the tree is balanced in its leaning position.

The tell-tale sign of a lean like that is the shape. The trunk usually curves, sometimes in a gentle J, so the top section grows back toward vertical. The base is thick, the roots on the side the tree leans away from are well developed, and the ground around the trunk is undisturbed.

A lean that has appeared or increased recently is a different thing. It means the tree has moved, and the roots that were holding it have lost their grip. This is what happens before a tree falls. Straight trunks tilted as a whole, rather than curved, are the usual pattern of a recent failure.

What to check at the base

Almost everything that matters is at ground level, on the side of the tree away from the lean. That is where the roots are being pulled up.

Soil heaved, cracked or domed on the side opposite the lean. This is the root plate lifting.

A gap opening between the trunk and the soil, or soil that looks pushed up in a mound.

Roots torn or exposed on the high side, or snapped roots at the surface.

Fungus at the base, such as mushrooms or shelf-shaped brackets on the trunk or roots, which often points to decay.

Cavities, cracks or a trunk that looks hollow near the base.

Other trees nearby that are also tilting. If several lean the same way, the soil itself has probably shifted.

Any of these on a tree that is also leaning means the lean is a structural problem, not just a habit.

Why late fall and winter matter

Saturated soil is the main thing that lets a leaning tree go over. Roots hold a tree in the ground by gripping soil, and soil that is soaked after a wet autumn grips much less. Add a wind storm, or the weight of wet snow or ice on the crown, and a tree that has stood for years can go over.

Freezing and thawing also move soil around roots. If you notice a lean after a storm or after the ground freezes and thaws, take it seriously, since the tree has already carried the load once and may have been weakened.

Our guide to what to do when a tree comes down in a storm covers what to do if one has already fallen.

What is most at risk

The danger depends on what is in the direction of the lean. A leaning tree in the back of a large lot with nothing under it is a monitoring job. A leaning tree pointing at a house, a driveway, a play area, a neighbour's building or a power line is a priority, even if the lean is old.

Some features raise the risk further: a heavy, one-sided crown that adds weight on the leaning side, a trunk with a crack or decay, and a tree that is also dead or dying. If you are not sure whether it is alive, see how to tell if a tree is dead. Dead Ash, which becomes brittle quickly, is a special case covered in dead Ash trees.

What not to do

Do not try to pull it upright with a rope, a truck or a winch. A tree with failing roots can come down suddenly and in an unpredictable direction.

Do not prop it with posts or beams. They rarely hold the weight and give a false sense of safety.

Do not stand or park under the lean, and keep children and pets away until it has been looked at.

Do not cut the top off to lighten it. Topping weakens the tree further. See why not to top a tree.

A fresh lean with heaved ground is a job for a crew with the right equipment, because the tree can move while it is being cut.

What an arborist can do about it

For a tree with a new lean and a failing root plate, removal is usually the safe answer, and it should not wait. The tree cannot be re-anchored, and an emergency call is reasonable if it is pointing at something valuable or occupied.

For an older lean with no signs of root movement, the answer can be gentler: an inspection, a note of the lean's angle so you can tell whether it changes, and sometimes pruning to take weight off the heavy side. If the tree is sound but the structure is weak, support systems can be considered, but only after an arborist has checked the roots and trunk.

If the tree has to come down, see our tree removal service. Check first whether a permit is needed in your municipality: the Durham Region permit guide explains how to find out. A tree whose trunk grows on the property line belongs to both neighbours, so talk to them first.