I am using purchased topsoil, top grade. How deep under the maximum depth for the roots should I dig for drainage or fill in with soil? How much space on either side of blackberry bush do I need for soil? I will be planting erect branches to wire, unless you can suggest another way.
Depth and Width for Blackberry Roots
Certified GKH Gardening Expert
I would work in the topsoil at least 15 cm, 30 cm if you can, but 15 cm down will be enough. In terms of how wide to incorporate top soil, I would go .75 - 1 meter on all sides from where the plant will be planted. Your method of trellising sounds good, just remember that you will need to remove older cane as the plant grows, so just make sure that you will be able to do that.