My wife wants me to build her a raised bed garden and I'm probably going the cinder block route. My problem is that we have very little yard space but we have a large unused concrete RV pad on the side. Can I build the garden on the concrete? Would I need to add something to retain water?

Would a concrete base make a difference in the required height? How deep would I need to make it? I was planning on a 2 cinder block heigth, which would be 1.5 feet.

Thanks.