Place a small bead of wood glue down the side of one of the end pieces (5-1/2" x 5-1/2") and adhere it onto the bottom and side piece, creating a corner. Repeat this step on the other end of the planter. Place a small bead of wood glue onto both the final side pieces and end pieces and press gently to complete the box.
While the glue is still a little bit wet, shift your box as needed to ensure the pieces match up how you want them. Once your pieces are in place, use a clamp on each end to secure the box. Let the box dry for at least 12 hours. When dry, use a few finishing nails on each corner to reinforce the box.