In the process of doing up the garden. I had planned to put some plastic edging, the ones with spikes along this area to put some flowers etc in but I got them and they are rubbish. I have decided to go with 6 inch treated timber. I’m planning on running a string line and putting pegs in and screwing the timber to them. My question is, do I need to dig up the area or can I lay directly on the grass (levelling if necessary). I will be putting membrane down obviously. Thanks in advance.