See here how to correctly close a Plantex® RootBarrier ring with Plantex® tape.
Superimpose both ends of the root barrier sheet and glue them together with a double-sided tape like Plantex® Tape. Apply a 2nd strip before removing the top protective paper.
Fold the superimposed joints into a Z-form (as indicated) and apply a 3rd strip before removing the top protective paper.
After the fold is complete, remove the remaining protective papers and press heavily together. The joint will now be permanently root-proof.