We have recently needed to build a retaining wall to prevent the garden sloping off into our neighbor's garden at the rear. We had to back-fill the gap created in building the wall with rubble and stone that was unearthed whilst digging the footings. The stone and rubble has been flattened and compacted for drainage and we plan to plant bushes or high shrubs to add privacy around the perimeter of the wall but obviously we need to replace soil to the area on top of the stone.
To what kind of depth would we need to replenish in order to plant the shrubs? At this moment, the types of shrubs and bushes are undetermined, but to create a bed suitable, I am unsure how much soil would there need to be to grow successfully?