Hi Paul,
I haven't completed the bottoms of the planters yet, but I will probably be making the bottoms from 1/2" wbp plywood, which will rest on the top of the three feet which are just visible in the bottom photo, these are short pieces of Iroko which are stepped and screwed into the bottom of the sides, all the wood used is Iroko, about 40mm x 35mm section.
The two planters were made using different methods, although you need to look closely to spot the difference.
With the first one I made the sides are glued and biscuit joined, this was a slow method, but gave the best results, the second one was glued and pinned vertically, but I only did about two rows like this because I was struggling pinning with the different hardness of the pieces of wood, I completed the rest of the planter using glue and 6mm dowels, this wasn't as successful as the first method because the drilling for the dowels was a bit off and created some gaps between the ends of the pieces.
I have enough pieces cut for a third planter, so I think I will go back to the glue and biscuit method.