A tree that looks solid from the street might be hollow inside. The bark appears healthy. The canopy is full. Nothing seems wrong until you put a saw to it and the trunk crumbles.

Reading decay before cutting is how arborists stay alive. A hollow tree falls differently than a solid one. The hinge wood we rely on to control the fall might not exist. The trunk might snap instead of hinge. The whole plan changes based on what’s happening inside.

Here’s what I look for before I make the first cut.

External Signs

Decay leaves clues on the outside. You just have to know where to look.

Mushrooms and conks growing from the trunk or root flare are the most obvious sign. These are the fruiting bodies of fungi that have colonized the wood inside. A single mushroom means the fungus has been working for years. The visible part is a fraction of the organism. The rest is inside the tree, digesting wood.

Different fungi cause different types of decay. Ganoderma produces shelf-like conks near the base and causes white rot in the roots and butt. Laetiporus, the chicken of the woods, appears higher on the trunk and indicates heart rot. Knowing the fungus tells you where and how severe the decay is likely to be.

Cavities and hollows are obvious entry points. Where a branch fell or broke years ago, decay organisms moved in. The tree tried to seal the wound but lost the race. Now there’s a hole, and the hole connects to rot inside. The size of the cavity hints at the size of the decay column behind it.

Cracks in the bark can indicate internal problems. Vertical cracks sometimes form when decay weakens the wood and the trunk can’t handle normal stress. Horizontal cracks are worse. They suggest the trunk is already failing structurally.

Bulges and swelling often mark areas where the tree added extra wood to compensate for internal weakness. The tree knows something is wrong even if we don’t. It’s trying to shore up a failing structure. A bulge near the base is a red flag.

The Sounding Test

I carry a mallet or the back of my hatchet for sounding. Tap the trunk and listen. Solid wood produces a sharp, resonant tone. Decayed or hollow wood sounds dull and flat, like knocking on a cardboard box.

Work your way around the trunk, tapping every few inches. Mark spots that sound different. A tree might be solid on three sides and hollow on one. That tells you where the decay is concentrated and how it might affect the fall.

Sounding isn’t perfect. A thin shell of solid wood over a hollow core can fool you. But combined with visual inspection, it gives you a rough map of what’s inside.

Probing

When I suspect decay, I probe. A long screwdriver or a specialized increment borer lets you physically test the wood beneath the bark.

Pick a spot where you suspect problems. Push the probe into the wood. Solid wood resists. Decayed wood gives way. You can feel the transition from sound wood to soft, punky material. How far in before it gets soft? That tells you the thickness of the remaining shell.

Probe at multiple heights. Decay columns aren’t uniform. Rot might be severe at the base and minimal at four feet, or the opposite. You need a three-dimensional picture of what’s inside.

An increment borer pulls out a core sample. You can see the wood directly, measure the sound shell thickness, and identify the type of decay by its color and texture. This is the most accurate field method, but it takes time and creates a wound that needs to be considered.

Root Inspection

Trunk decay gets the attention, but root decay is more dangerous. A tree can stand with a hollow trunk if the roots are solid. A tree with rotted roots can fall without warning, even if the trunk looks perfect.

Look at the root flare. Is it visible and solid, or is it buried and soft? Poke around the base with a probe. If major roots have decayed, you’ll find soft spots where there should be solid wood.

Soil movement around the base suggests root failure. If the ground is heaved or cracked on one side, the roots on that side might have broken or rotted. The tree is already leaning on its remaining anchors.

Root decay changes everything about a removal. A tree with a compromised root system is unpredictable. It might fall before you finish the cut. It might fall in an unexpected direction. The standard felling plan doesn’t apply.

What Decay Means for the Cut

A standard felling cut relies on hinge wood. You cut a notch on the side where you want the tree to fall. You cut from the back, leaving a strip of intact wood between the two cuts. This strip is the hinge. It controls the direction and speed of the fall.

If the hinge wood is rotten, there’s nothing to control. The tree falls wherever physics takes it. Maybe toward the notch. Maybe sideways. Maybe back toward the saw operator.

When I find decay in the hinge zone, I adjust. Sometimes I can cut lower or higher to find solid wood. Sometimes I abandon the felling plan entirely and take the tree down in sections from the top.

Severe decay might mean bringing in a crane. Hook the trunk above the decay, make the cuts, and let the crane control the fall instead of relying on compromised wood. It costs more but beats guessing.

The Bore Cut

On suspect trees, I use a bore cut to test the wood as I work. Instead of cutting straight in from the side, I plunge the bar into the trunk and cut outward. This lets me feel the resistance change as I move through the wood.

Solid wood grabs the chain and requires steady pressure. Rotten wood lets the saw sink in with almost no resistance. The saw tells you what the probes suggested.

A bore cut also lets you see the wood chips. Healthy chips are firm and consistent. Decayed chips are dark, stringy, or powdery. The chips confirm what you’re cutting through.

If the bore cut reveals extensive decay, I stop and reassess before committing to the felling cut. Better to pause and think than to drop a tree you can’t control.

Why This Matters

Tree work is dangerous. Falling trees kill people every year. Most of those deaths involve misjudging the tree, and decay is the most common misjudgment.

A decayed tree doesn’t announce itself. It looks fine until it doesn’t. The arborist who assumes every tree is solid will eventually meet one that isn’t. That meeting might be his last.

I spend more time reading a tree than cutting it. The assessment takes minutes. The cutting takes hours. But those minutes of assessment are what make the hours of cutting survivable.

Every tree gets the same respect. Tap it, probe it, inspect it, and think about what’s inside before the saw touches bark. The ones that look perfect might be rotten. The ones that look terrible might be solid. You don’t know until you check.

Reading Decay Before Making the First Cut