Q: I have a river birch tree from which I had to cut a large section near the base. I covered it with pruning sealer but after a year I noticed decay has left a big hole. I cleaned it out and covered with a new type of sealant. But I still have a hole that fills up with rain water. How do I prevent the rainwater from getting in?
A: I don’t think you can successfully keep water out of the cavity. I’ve seen concrete, expanding foam and tar used…but they all harden, crack and eventually fail, leading to further rot.
Arborists tell us that trees heal wounds “from the inside out” so drilling a hole at the bottom of the cavity to drain the water just spreads damage into the healthy part of the tree. Water can actually protect tissue from further decay.
My suggestion is to put a note on your calendar to drop a preventative mosquito dunk (click for sources) in the hole twice each summer and leave the tree alone.