Actually it was the other way around. My Pergola had seen its better days so I replaced the beams and the perpendicular boards. That was a project all on its own. Just the pre-staining of the boards took several evenings. But when it was finally done, I was left with a decent pile of already stained cedar 1×4 cut-offs. So after thinking what I could build with them, the result was these planter boxes.
I first went to orange and picked up the plastic ‘liners’ and made the whole project to fit around them. The top frames are mitered and biscuited together. The vertical members of the case are two boards biscuited together to make a corner that is 2 inches wide across both faces. The rest of the case is done with pocket screw joints. The paneling boards are rabbeted on the top and bottom to fit against the frame and then glued and pin nailed onto the frame. The supports holding up the plastic liners are cedar 2×4s that were left over from another project and pocket screwed and glued to the frames on three sides. I had put in pocket holes to hold the top frame on but unfortunately there was no way to use them. So I ended up having to face screw the top frames in place.