(Disclaimer: As with any bonsai endeavor there is NO QUICK way to produce a root over rock bonsai) This procedure requires thought of what you want to achieve. There are some aesthetic considerations. A tree overlooking a stream calls for a flat and shallow stone where the water has washed the soil from the roots. A tree perched on a mountain requires a tree that sits on a stone where the roots drip over the stone and merge with the soil; the appearance of the soil eroding from the side of the stone also gives the impression of the tree clasping to the side of the mountain. In both cases, the concept drives the style of planting and requires very different stones and planting material
Variables to consider:
Stone color…as this plays into your pot selection
Placement of the material on the stone
Placement of the branches and the angle of the branches
The texture of the bark to the texture of the stone
What do we need to start this project? First of all, a stone – it does not need to be craggy, and the shape is not important other than to be agreeable with your design. Secondly, you’ll need appropriate bonsai material. Finally, you’ll need an accumulation of materials to hold the tree to the stone. A key ingrediant is “muck”, which is a mixture of clay and fines sifted from akadama or kanuma, water, long fiber sphagnum moss (holds the mixture together and provides structure much like rebar in concrete). You’ll also need some raffia or string to bind the roots to the stone, and finally some plastic wrap or aluminum foil to hold the roots against the stone.
Timing for this technique is early spring to increase your chance of success (the tree lives). An important goal is to match the tree to the stone. Regardless of the forms mentioned earlier, there’s no getting around a large tree on a small stone; the effect that you get is of a full-grown tree without the rock. However, use of a small tree and large stone gives creates enormous perspective.
Some more details: the stone bottom must be flat - either chisel the bottom flat, cut the bottom of the stone so that it is flat, or find a stone that has a flat bottom
Assembling the project:
1. Wash off the roots gently, and then divide them into bunches and position the tree on the stone. It is critical that the tree roots are to stay moist at all times.
2. Prepare the much and apply where the roots will rail down the stone
3. The muck mixture cannot be too wet as it will serve as the cement for the roots that will grow to grasps the stone.