How a hollow tree can stay standing
The living tissue that keeps a tree structurally sound runs near the outside of the trunk, not through the center. Heartwood, the dense core, is mostly dead anyway. So internal decay does not necessarily mean a tree is about to fall.
Arborist guidance cited by East Hants Arborists puts the rough threshold this way: a continuous shell of sound wood covering about 30 to 40 percent of the trunk cross-section can still carry significant wind loads. A 16-inch-diameter tree, for example, might retain a shell of roughly 2.5 inches of solid wood around a decayed column and still be structurally functional, according to Ohio DNR training material. The shell is what matters, not whether there is a hollow at all.
A cavity taking up more than one-third of the trunk diameter is a common threshold for concern, especially if the tree leans toward people or property, according to Ryan Lawn and Tree.
Warning signs that change the picture
A hollow alone is one data point. The following signs, especially in combination, push a tree from "monitor" to "get an arborist out now":
- The hollow is at or near the base of the trunk, not up high
- The hollow sits at a major limb union
- Fungal fruiting bodies (mushrooms, conks, shelf fungi) on the trunk or roots
- Cracks or splits in the bark running vertically or at the hollow
- Sudden or new leaning since the last time you looked at the tree
- Root heaving, meaning the soil around the base is lifting or cracking
- Large dead limbs overhead
- The tree moved noticeably in recent wind in a way it did not before
Any one of these on a hollow tree near the house warrants a professional look. Two or more together is urgent.
Where the tree could land matters as much as the hollow
A hollow oak in an open field with no house, driveway, or play area underneath it carries far lower risk than the same tree hanging over a roof. Target risk is part of the calculation. A tree that is hollow but stable and sits away from any structure can often stay. A tree with a modest hollow directly over an occupied space deserves more scrutiny.
Do not fill cavities. Filling traps moisture and can accelerate decay or cause cracking, according to East Hants Arborists.
What to do next
For a quick first read, use the TreeNerd free tree-check tool. Answer the questions about what you see and it will give you an initial risk read.
If the tree is near the house, a car, a play area, or anywhere people spend time, get a certified arborist on-site. Photos and online tools cannot replace someone who can tap the trunk, probe for decay, assess the root zone, and look at the crown from the ground. A written assessment also helps if you ever need to document the condition of the tree for insurance or a neighbor dispute. TreeNerd lists certified arborists in your area who can do that inspection.