cactus since they have shallow roots. The other ones I put an old coconut liner in the area I was planting in - the tire is sitting up right and we put a chain through the tread to hang it with. The coconut liners keep the soil from washing around the inside. If you're using larger full size tires I would think you could still use the large coconut liners and just set it inside the tire hole when it's laying flat. IF you're going to build layers - I honestly haven't figured that part out but logically, I think you'd probably need to just set a pot inside and balance it. Maybe some slats of wood would work for support. I could not find anything in the above post to explain how they did it!! Hope this helps some!!