So my wife, helpful as always, picked up some cedar planks for like $1 a board. She asked me if I could make flower boxes from these bowed boards and this is what happened.
Step 1: I Used
She bought 3 8' planks and this is the best of them.
1x8 cedar planks
1 1/4" deck screws
drill, driver, miter saw
Step 2: Cutting and Screwing
The planks are about 7 1/8" wide so I cut the end pieces 5 5/8". I had some scrap pieces about 1" square so I used those for the corners. Line them up to the sides, clamp with the sticks on the concave side of the workpiece and screw down one side. Now I kept tightening the clamp until the cedar either straightened out or split. FYI only 1 didn't split. Clamp the board to tighten the split and I made sure there was 2 screws on each side of the split then screwed on the sides using the same method. As far as the bottom, I only screwed it to the sides and left the ends bowed for water drainage.