I have an old rainbird head that has been surrounded by birch roots so that it is no longer possible to unscrew it. I can take off the top but that is about it. As this section has become a garden, I don't need a lawn sprinkler there and would be just as happy to block it in some manner. The popup sprayer has become loose so for now I put a rubber ball on it which I weighed down with blocks - some water still gets out.

this is on the edge of the zone, so I assume a pipe terminates at this head and if I were to fill the head with caulk or such, would it make a decent seal? Anyone tackle a problem where you want to cap off a head but the head cannot be removed?