I have to agree with Shawn that how deep the roots grow, depends a lot on culture. In the redwood root infested areas, which are mostly clay soil, with automatic sprinkling 2x/week, I was lucky to get roots 3" deep. In 8-10" deep pots with saucers (to hold any drip), saucers filled with half/half mix of planting mix and fine redwood mulch, and 2x/week watering, I usually got roots going down to the bottom of the pots, and sometimes out the bottom or circling around.
