My tree is hollow inside. Does that mean it will fall?

A hollow tree is not automatically doomed — many hollow trees stand safely for decades. What matters is how much sound wood remains in the trunk wall, not the size of the cavity itself.

The engineering behind it:

• Trees are strongest in their outer wood, much like a hollow steel pipe or bamboo resists bending better than you'd expect
• A widely used arborist guideline: a tree may have acceptable strength if the sound wood shell is at least 1/3 of the radius (roughly when the hollow is less than ~70% of the diameter)
• But this is a rule of thumb, not a guarantee — open cavities, multiple defects, lean, and heavy canopy on one side all change the math

Higher-risk warning signs:

• An open cavity or seam (not a closed-over hollow)
• Hollow combined with a lean, large cracks, or major deadwood
• Fungal brackets at the base or on the trunk (active decay)
• The hollow is at a branch union or near the base where loads concentrate
• A target below — house, driveway, play area, road

What to do:

1. Don't panic or fell it reflexively — hollow alone isn't a death sentence.
2. Have an ISA Certified Arborist perform a risk assessment. They can use a mallet, probe, or instruments like a resistograph to measure remaining sound wood.
3. Options may include crown reduction to lower the load, cabling, monitoring, or removal if the wall is too thin.

See Is My Tree Dangerous? and Signs a Tree Might Fall.