Tree Cavity Care: Why Filling Hollows Does More Harm Than Good

That water-filled cavity in your river birch isn't a problem to fix—it's a wound the tree is managing internally. Concrete, foam, and tar all eventually crack, trapping moisture and accelerating decay.

Trees Heal from Inside Out

Drilling drain holes spreads decay into healthy wood. Surprisingly, water in the cavity can actually inhibit certain decay fungi by limiting oxygen.

Practical Management

Add a mosquito dunk to the water twice each summer to prevent breeding. Otherwise, leave the cavity alone. The tree will gradually compartmentalize the damage on its own timeline.