What depth of soil for raised beds
Hello. We're planning on building raised beds this weekend, and my question is how deep should the topsoil be.
We'll have 2 lots of beds; each one 5m long X 0.4m wide and 0.3m high.
For the base, the plan was to use leftover builder's rubble to improve drainage and bulk out the bed. The beds won't be for vegetables, but for perennials and maybe small shrubs. I'm envisaging tulips, daffs, hyacinths, then allium, lilies, echinacea, lavender, phlox, black-eyed susans, maybe peonies, sedum and grasses. You get the idea.
How deep should the topsoil be? And how much compost would you include in this? I fear it's going to get expensive... We can't transplant any soil from elsewhere.