Q: In a recent tip, you recommend not filling or drilling tree cavities. An arborist has recommended inserting a pipe at an angle through the cavities in a large oak so that any water will exit the tree. Can you clarify this and make any other recommendations for this problem?
A: Current arboricultural thought is that drilling a cavity to drain it causes more problems than allowing the cavity to fill with water. Trees compartmentalize all wounds and openings with a thick, internal layer of hard-to-damage cells. Inserting a tube will break through the cell barrier the tree has already made, forcing the tree to further “wall off” the damage done by the drill. Standing water actually protects the wood because rot fungi can not grow rapidly in an oxygen-poor environment.
On the other hand, water-filled tree cavities are very good homes to mosquitoes. If the cavities are far up the tree where you can not treat them for mosquito larvae, an argument could be made to drain the cavities. However, if they are near the ground, you can put a brand name, round mosquito larvicide in each one.