Q: I just purchased a home with two very old Japanese maple trees in my front yard. I noticed that one of them has a hole in the base of the trunk large enough for a small adult fist to enter. One of my friends suggested I fill the hole with concrete mortar. I didn’t want to do anything without at least asking your thoughts.
A: Tree experts rarely advise filling cavities in trees. Your Japanese maple has already “walled off” the damaged area, surrounding it with dense cells to prevent the further spread of rot. If you fill the hole with concrete it will eventually give boring insects and diseases a more perfect hiding place than they’d have ordinarily. Expandible foam is sometimes used by arborists but only in special situations.
A bigger concern is that the cavity not be a reservoir for rainwater, where mosquitoes could breed. If the interior holds water, investigate installing a metal “roof” over the hole so it is kept as dry as possible. If that is not possible, place a mosquito larvicide like Mosquito Dunks in the hole occasionally during the summer. For an expert opinion on further steps to take, hire a consulting arborist to visit your landscape.