You can put Bricks or Rocks in to the planter base. If no brick or rock, you can use soil or sand in bags instead.
Note: For hedge wall which its height ≥ 79 inches, If the windy speed in your area is under Beaufort 8(Knots 34-40/Gale Label) , just fill the planter base with add-weight-things completely, it’s ok.
If the windy speed >= Beaufort 8(Knots 34-40/Gale Label), we can remove the hedge from the hedge wall, then put down the hedge in the ground or move them into the house. The hedge is easy to uninstall and reinstall.