Root over rock Ficus.

The root over rock tree above is a world renowned Ficus Sycamores by Charles Ceronio.

Root over rock Ficus is a very natural style here in South Africa. Birds eat the seeds and the seed loaded droppings find their way to little hollows and crevices in rocks. Here they germinate in ideal conditions; food, warmth and enough moisture. But soon the trees get to big and the tree sends out roots to find more of the mentioned. In nature these trees send out roots over the rocks - it is rarely a case of soil being washed away and the roots are then exposed.
Here is one of mine progressing nicely:

Notice that the tree is not planted on top of the rock- that would be unnatural.
Another one I am working on:

The soil was washed away so I could inspect the roots. After rearranging some roots it was covered up again.

Root over rock by Pieter du Plessis.
Here follows the root over rock series done by Pieter. He plants the trees/cuttings/seeds he wants to use in 60-90cm plastic drain pipes to develop as long roots as possible. When he removes them he wash of all the soil (one third gravel, one third garden loam and one third compost) and wrap the tree in wet newspapers to keep the roots moist. He emphasize to place the rocks as found in nature and to find a natural hollow where a seed would have germinated as in nature.

The one root into a groove.

He doesn't use the plastic wrap or foil method- the roots are pushed into the grooves by elastic bands and cable ties.

Polystyrene, rubber bands and cable ties to push the roots as close as possible to the rocks.

Cable ties are used to pull the polystyrene as tight as possible.

Some more cable ties.

Putting the rock in a plastic container and filling it up with the above mentioned soil mix.

....and lastly he uses a plastic piece of lining to fill it up further. After six weeks he will remove the plastic to expose the roots gradually. After a year the roots will be fully exposed.

Normally it is not difficult to keep a rock upright in a grow out container - but trying to keep a rock upright in a bonsai pot, is not so easy! The easiest way to solve this problem is to use something like Rockcrete - it is as hard as .....rock!

Holes can be left in the Rockcrete to fasten the base to your pot. With this one I am not going to use a pot - the base will be covered by a little soil - just enough for the roots to survive.

Some more examples: