Posted: 17/10/2012 at 12:47
I set up a community raised bed garden this year: 13 beds, ech 4' x 8' and 3' high. They were constructed directly on to an uneven grassed surface. The ground could not be used for allotments becau of heavy metal conamination from an old gasworks very nearby. So I put down inside the boxes old thin plywood sheets and then weed-supressant membrane, then 6" well rotted manure and finally 2' of screened topsoil. This first season we've had no problems from the grass underneath, and now the worms are starting to make an appearance.
So personally, I would just build on top of the grass, so long as it's adequately barriered and has sufficient depth of growing medium