I'm not sure I'm understanding how cob planters will hold up. Cob isn't designed to handle water. The only reason why it works as a building material is if it has good boots and a good hat. So the roof needs to extend far from the house and the base of the walls is usually about a foot high made with stone. I would look into treating it with linseed oil or tadelakt
RE: Alternative, Non-Plastic Gardening Containers