Root bound trees are simply plants that have grown too big for their containers. Healthy growth will cause a plant to develop a root system that is too big to be contained. Occasionally, a tree may be put into a container that is too small to begin with in order to make transport and planting easier. This will also cause the tree to become quickly root bound. It's been known for rushed or incompetent landscapers to forget to remove the burlap root cover from new trees when planting. A tree whose roots are "bound" by some kind of barrier can appear under nourished or under watered and stunt their growth. Even plants growing outside in the ground can become root bound if their roots are caught between several solid barriers, like foundation walls, footers or pipes.