How to Spot a Dangerous Tree Before It Falls: Five Warning Signs
When a tree comes down in a storm, it rarely happens without warning. In most cases the tree has been showing signs for months or years: mushrooms at the base, a hollow trunk, leaves that suddenly brown in summer. These are the tree telling you it is close to its limit.
We are a small tree care and landscape business in Mie Prefecture, central Japan. Every autumn, after the typhoon season, we are asked the same question: is this tree safe? This page sets out the five places we look, and the guideline that tree specialists in Japan use to decide when a hollow tree should come down.
Sign 1: Look at the base
The base of the tree tells you more about the risk of falling than anything else. We check three things.
First, mushrooms or brackets. When fungal fruiting bodies appear at the base or low on the trunk, wood-decay fungi are already at work inside. By the time you can see them, the decay is usually well advanced. White rot breaks down lignin as well as cellulose and leaves the wood pale and fibrous. Brown rot removes the cellulose first, so the wood turns brown and cracks into small blocks.
Second, the ground. If the soil around the base is heaving up or cracked in lines running out from the trunk, the roots may be losing their hold. In strong wind, such a tree can be pulled out of the ground roots and all.
Third, the roots themselves. Roots at soil level that are blackened, or soft when you press them, are a sign of root rot.
Sign 2: Look at the trunk
In Japan, a common first check is to tap the trunk with a wooden mallet. The Japan Greenery Research and Development Center explains that sound wood holds water and gives a slightly dull, muffled sound. Weakened wood has lost its water and gives a lighter sound that rings.
Many people expect the opposite, so it is worth listening to a tree you know is healthy first. The difference is easier to hear when you have something to compare.
For the next step, the same center describes a guideline used in Japan: if the cavity takes up 50 percent or more of the cross-section, or if an open cavity spreads across 120 degrees or more of the trunk, felling is considered. Large branches are measured too, not only the trunk.
Long vertical cracks are another warning. A crack that looks small from outside can be much larger inside.
Sign 3: Look at the leaves
Leaves show the health of the whole tree. Watch for leaves that suddenly brown or wilt in spring or summer, a crown that is clearly thinner or has smaller leaves than in past years, and heavy leaf fall at the wrong time of year. Each can mean that the roots or trunk are no longer moving water up the tree.
Autumn colour, of course, is normal. If you are unsure, compare with photos from the same month last year.
Sign 4: Check for climbing vines
In Japanese woods and neglected gardens, wisteria, kudzu and wild grape climb into trees. They do not usually strangle a tree outright. But they cover the crown and take its light, and their weight adds to the load in wind. Thick vines can also press into the bark and squeeze the layer just under it that carries water and sugars.
Wisteria is especially strong, and it grows back from the root if you only cut the stem. If you find it climbing a tree, cut it at the base and check again each year.
Sign 5: Look at the lean and the ground around it
A tree that leans is not always dangerous. Many trees grow at an angle toward the light and are perfectly stable. What matters is change. A tree that leans more than it did last year needs attention soon.
Trees standing in soft ground, or where water collects after rain, are more prone to root rot and to being pulled over in strong wind.
Winter: do not mistake a bare tree for a dead one
Every winter in Japan, some trees are cut down because their owners thought they were dead. Deciduous trees such as konara oak, Japanese maple, zelkova, cherry, persimmon, Japanese apricot, ginkgo, dogwood and wisteria are normally bare from about November to April. Evergreens such as pine, chinquapin, evergreen oak, camphor, camellia, sasanqua and sweet osmanthus keep their leaves all year.
On a bare tree, you can still check whether it is alive. Bend a thin twig and see if it springs back, look for buds, and look at the bark. A living twig bends; a dead one snaps.
Why trees fall
Most trees that come down are brought down by strong wind and typhoons. But a sound tree does not fall easily. The trees that fall in a storm are very often the ones that already had decay or a cavity inside. Root damage from road works or building nearby is another cause that is easy to forget, because the damage happens years before the tree falls.
When you cannot tell by looking
Everything above can be checked by eye. Decay inside a trunk often cannot. For that, specialists use a mallet, and instruments that read the inside of the trunk with sound waves or a fine drill. If a tree near a house, a road or a place where people gather worries you, ask a local arborist to look before the storm season, not after.
Common questions
What does it mean if mushrooms grow at the base of my tree? A fungus is decaying the wood inside. The decay is usually well advanced by the time you see them, so have the tree checked.
Does a hollow tree always have to come down? No. A guideline used in Japan considers felling when the cavity is 50 percent or more of the cross-section, or when an open cavity spreads across 120 degrees or more.
How does a healthy trunk sound when tapped? According to the Japan Greenery Research and Development Center, sound wood gives a slightly dull, muffled sound, and weakened wood gives a lighter, ringing sound.
Is a leaning tree dangerous? Not always. A lean that is getting worse year by year is the warning sign.
