Recently there have been some fabulous examples of root over rock styles at bonsai shows throughout the UK and some excellent examples were present at Heathrow bonsai show this month. This technique of planting the bonsai over a rock or even clinging to a rock is not an easyone to perfect and to be convincing the roots have to look as though they actually clasp the rock before going down into the soil of the pot.
Another key factor is that the bonsai tree and roots need to be in harmony with the piece of rock, and the more weathered the rock the better. The rock needs to be ideally light in weight but this is not easy and it should be durable and not liable to disintegrate. It should have a range of textures and be non uniform in shape to add interest. The rock must also be in scale to the size of the tree to again present a convincing finished image. Very good results can be seen with Junipers and Acers and in particular Acer buergeranum, the Trident Maple.
With regard to the techniques used to create a root over rock style, there are many, some grow the trees in plastic tubes (tree shelters) to develop long fleshy roots that can then be laid over the rock and wired or bound in place. Others start with a seedling positioned over a rock and over time continue to work the roots and expose them little by little to harden them off. The main critical factor with this style is to ensure that the roots are well protected during the cold weather as the roots are exposed and not protected by soil and if they are not fully woody they can be easily damaged in extremes of weather temperatures. Sphagnum moss can be used to cover the exposed roots to reduce them drying out. Another technique is to grow the bonsai tree in tufa and the end result is more of the second root and rock style in that of a clinging to a rock style.
There are manyarticles and bonsai books containing more detail on how to go about creating this sytle, here are a few references I have found useful.
Here are some photographs of bonsai root over rocks that appealed to me…