How deep do the roots of Dwarf Southern Magnolia tree go? I understand they are quite shallow. Is this correct?
Well yes and no. Magnolias generally have their feeder roots spread out not far beneath the soil, usually out to the edge of the drip line (where the rain drips from the leaves), though they may spread out further. They do not like root disturbance either, and that's something to bear in mind if you want to plant anything under its canopy, but there will be other, stabilisation roots that will grow in order to keep the tree upright and to help it resist wind - these usually go down into the ground. How far into the ground is hard to say - any tall plant in a windy position, for instance, will produce these roots where it needs them, and as far down as it needs to, so the large root depth can vary.