Trench will drain the pool so you can retrieve objects "stuck in the tar" including tar itself. But growths will re-spawn even if you drain and fill the pool. You have to apply the standard practice of parking workbenches around the pool. I did all of the above and a local lox decided to smash the workbenches and claim the pool for him/herself. I watched from a nearby rock as the lox and re-spawning growths contested ownership.