Rita Carter, of Barden Ridge in New South Wales, sent Don an email asking how to grow a garden on rocks.
Don showed an example of a garden planted on a rock shelf at the back of his house. Basically it was done by barrowing in a thick layer (about 150mm or 6″) of good quality soil and then planting lilly pillies, tree ferns and other mixed ferns. However, the key to the success of this garden was the installation of an irrigation system, complete with spray heads and a timer on the tap.
Note: check water restrictions in your state for use of irrigation systems.