A friend mentioned putting in a large planter box ON TOP of the concrete, and that this would be sufficient for grass and light plants (no large trees or anything). Of course I'd want to put in some drainage and even automatic sprinklers. I figured I'd use some nice retaining wall blocks (e.g. Allan Block), perhaps 3 high (or ~12" in height) so that it looks nice and finished. The back of the planter box would be against a typical wooden fence that separates the yard from the neighbors. I also thought I'd install some nice pressure treated planks of wood along the bottom of the existing fence to the same height as the planter box to protect the fence against moisture, rot, etc. and make it a more permanent installation.
So that's my plan in a nutshell. I think with 12" of soil grass/sod will be no problem. Small plants should be fine too. I believe the retaining wall blocks are meant to be used on dirt, and that the bottom of the first row is usually partially (a few inches?) below the dirt, naturally this keeps the block in place. However on concrete this is not an option. I'm concerned about the bottom row sliding out since it would just be sitting on top of the concrete. I've entertained ideas of drilling holes in the concrete and hammering in some kind of anchor, such as rebar in the middle of the blocks to keep them secured. Seems like it would be a lot of work though 'cause I'll probably have 30-40ft of wall.
So, can I use retaining wall blocks on concrete? Do I need to secure them, or will the weight of the blocks plus gravel inside them keep them in place?