Creating new raised beds.
I have a (really)small town courtyard and I want to build some raised beds.
My main query is about whether to build on top of concrete, which covers the area? I plan to build them about 2.5 ft/76cms high with the hope of having a couple of smallish trees with shrubs and perennials. Would I need to make holes in the concrete for root growth or not? I have not decided whether sleepers or rocks would be better to make the beds ? Help!! Jocelyn.