Can I put compost or dirt in it, or would that damage it?
Hole in Trunk of 50 Foot Willow .
Certified GKH Gardening Expert
Doing so would bring in a multitude of problems, such as pest and fungal infections. It would not be wise to do so. If you still plan to use this, you will want to water with 1 part peroxide to 3 parts water often to fight off any infection. Really, though, I do not recommend doing this.