Hollow Doesn't Always Mean Hazard: What We Actually Look At Before Recommending Tree Removal

If you've ever had us out to look at a tree with a cavity, you've probably had this exact thought: "There's a hole in my tree. Shouldn't we just take it down?"

It's a completely reasonable instinct. But it's also one of the most common misunderstandings in tree care — and it's worth walking through why, because it can save you from removing a perfectly healthy, structurally sound tree.

Trees aren't solid rods. They're more like tubes.

Picture a paper towel roll. Now picture a solid wooden dowel the same diameter. Which one do you think bends and breaks more easily under the same pressure?

Most people guess the dowel is stronger. It's not — or at least, not by nearly as much as you'd expect. That's because when something bends, the material near the center barely does any work. Almost all the resistance comes from the material on the outside.

Trees work the same way. The outer rings — the living, actively growing wood just under the bark — are doing almost all the heavy lifting when it comes to standing up to wind and weight. The core? It's mostly along for the ride, structurally speaking.

That's why a tree can lose a significant chunk of its interior to decay and still be standing firm decades later. It's not damaged goods. In a lot of cases, it's just... normal.

Trees actually fight back against decay

This isn't luck, either. Trees have a defense system for exactly this situation, and it's been studied for decades — it's called CODIT (Compartmentalization of Decay in Trees). When a tree gets wounded or infected, it builds internal walls to contain the damage: walls that resist decay spreading up and down the trunk, walls that resist it moving inward, walls that resist it spreading sideways, and — most important — a wall that seals off all the new wood the tree grows after the injury from whatever's happening in the old wood.

So while the center of the tree might be dealing with decay, the tree keeps building strong, healthy wood on the outside, year after year. That's the whole reason hollow trees exist and thrive in the first place.

So what actually matters?

The question isn't "is there decay?" It's "how much solid wood is left, and where?"

There's a rule of thumb we use (borrowed from engineering research on tree mechanics) that says trees generally hold up fine as long as the sound wall around the decayed area is at least 30% of the tree's radius. Below that, risk starts climbing. That's why, when we're assessing a tree with suspected internal decay, we're not just eyeballing a cavity and guessing — we're using a mallet to sound the wood and listen for changes that indicate decay, combined with a full structural risk assessment (following ISA's Tree Risk Assessment Qualification methodology) to estimate how much solid wood remains and where.

But here's the part that doesn't get talked about enough: not all decay is equal

This is where it gets a little more nuanced, and honestly, where a lot of generic advice about "hollow trees are fine" falls short.

The type of fungus causing the decay matters just as much as how much wood is left. Some decay fungi break wood down slowly and predictably — the wood stays somewhat fibrous even as it weakens, and failure (if it ever happens) tends to be gradual, often with warning signs along the way.

Other fungi are a different story. Some cause what's called soft rot, which can leave wood looking deceptively solid on the outside while it's actually gone brittle inside — closer to the texture of wet charcoal than wood. That kind of decay is associated with sudden, brittle failure instead of a slow decline, and it's especially concerning when it shows up at the base of the tree or in the root system, since that's exactly where the tree needs the most strength to resist blowing over.

So the same "30% rule" that might mean "no concern" with one type of decay might warrant a much more cautious approach with another. This is exactly why proper diagnosis — knowing which fungus you're dealing with, not just that decay is present — is such a critical part of the assessment.

Cavities aren't just a structural question — they're habitat

Here's something that often gets lost in the "should this tree stay or go" conversation: cavities are genuinely valuable to wildlife. Woodpeckers, owls, songbirds, squirrels, bats, and all kinds of insects rely on tree cavities for nesting and shelter — and mature cavity trees are becoming harder to find as urban and suburban landscapes get cleaned up. If a cavity tree is structurally sound and not posing a real risk, leaving it standing isn't just about saving a tree — it's doing something genuinely good for the birds and animals in your yard.

At the end of the day, it's your call

We can tell you what the wood is doing, what fungus is present, how much sound wall remains, and what the engineering data suggests about failure likelihood. What we can't tell you is how much risk you're comfortable living with — and that's not a cop-out, it's just true. A tree next to a swing set carries different stakes than the same tree at the back of a two-acre lot. A homeowner who's watched a neighbor's tree come down in a storm is going to feel differently than someone who's never seen a tree fail. Risk tolerance is personal, and a good assessment gives you the information to make that call — it doesn't make the call for you.

The bottom line

A cavity in your tree isn't an automatic death sentence — for the tree or your peace of mind. Most hollow trees are mechanically sound and can safely stand for many more years, and some of them are doing quiet, important work as wildlife habitat while they're at it. But "probably fine" isn't good enough when it comes to something growing next to your house, your driveway, or where your kids play.

That's why we don't just look and guess. We measure, we identify what's actually causing the decay, and we give you a real answer — not a knee-jerk one — about whether your tree needs monitoring, mitigation, or removal. Then the decision is yours.

If you've got a tree with a cavity that's been worrying you, we're happy to come take a proper look.