Q: I have a 15-foot yellowwood tree that has a beautiful shape but a forked trunk that collects water. Is there any danger that the sitting water there will rot the wood? Should I fill the space with silicone or something to prevent water from collecting? What can I do to save this tree? — Polly van de Velde, Chapel Hill, NC
A: You have the right idea: fill the hole with something flexible yet rotproof. The fix should be temporary, though, because as the tree ages and its limbs and bark expand, they will eventually fill the hole on their own. So your filler will come out sooner or later in any case. Silicone is very long lasting, and some of it could become trapped in the cavity. Expandable polyurethane foam, the kind that comes in a spray can and is used for weather-stripping your home, is better. When it sets, the foam will still be spongy (so it will give and crumble as it weathers and the tree expands — a good thing). Because it will eventually break down in sunlight, you should paint the foam filling to delay its demise. Use a bark-colored paint with a matte finish, and the filling will be nearly invisible. The main reason for all this work, however, is to eliminate not rot but a potential breeding ground for mosquitoes. It is also possible that if the expanding water were to freeze solid it could open a split in the tree. The little pool of water won’t foster decay, though. Living wood is totally saturated already. Putting water on it can’t make it any wetter. What does make wood rot is decay organisms, and water can actually exclude them. Logs stored at the bottom of freshwater lakes have been preserved for decades in perfect condition.And this is why the exact wrong thing to do would be to drill a hole or cut a drainage channel. Cutting through the bark and into the bare wood will open an easy path to the healthy interior for all sorts of decay organisms.