Are there any good rules about calculating the diameter of a rhododendron root system? I do understand that these roots will not go down vertically more than about 12 inches. They do spread out horizontally near the surface. I need to size a container to the predicted adult size of the plant, so I need a guideline on the root size, based on height or width of the part of the plant that is above the ground.

In a related question, what happens to a rhododendron if the planter is too small? Will it just reduce its growth? Can rhododendrons be raised in a bonsai fashion and ultimately end up being miniature versions of the normal plant?