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Developing root-over-rock bonsai is a simple but time consuming technique. The roots of the bonsai must be grown in such a way that they grip the rock or stone securely for both the stability of the tree and the integrity of the piece.
Attempts to tie the roots in place over the rock have a tendency to be poor. This is often the result of the ties marking the roots and causes scars that will take many years to fade; if at all. This photo series illustrates what I consider to be a better way of achieving a realistic rooting over rock.
This English Elm (Ulmus procera) was acquired along with a number of cheap bare-rooted saplings in the Autumn of 2001. Rather than just plant them straight into the ground to thicken up the trunks, I decided to try and create some very small (mame) root over rock compositions.
The bare roots of the trees were draped over the rocks and held into position using clear plastic sheeting, (shrink wrap is ideal), with the fine roots protruding freely below the bottom of the rock and the plastic. The plastic was held securely in place using tape.
Horticultural sand was then fed into between the plastic sheet and the rock; however small the gap. This is necessary to give the roots a degree of moisture and protection from the cold while still encouraging strong rootgrowth into the soil below the rock. The rock and bottom roots were then planted temporarily into containers and given protection from any late Spring frosts. Occasionally water was sprayed into the gaps between the plastic and the rock to help keep up moisture levels. It should be noted that the ideal time to carry out this procedure is early Spring just before the rootsystem becomes active.
Two months later and the trees had leafed out and were growing vigorously. Once I was satisfied that the trees would survive the transplanting, I planted them into the ground ensuring that the tops of the rocks were planted above the soil line; this is necessary to ensure that the trees cannot root directly into the soil above the plastic sheeting. By forcing the trees to only root out through the base of the plastic sheeting, the root sections growing around the rock thicken and start to take the shape of the rock.
Through the remainder of 2003 and the Spring of 2004, the trees were allowed to grow freely to encourage thickening of the trunk and the root system.
At midsummer of 2004, the trees were trunkchopped to start the process of creating taper and movement in their trunk.
The longer these trees spend growing on in the ground, the better their adherence to the rock they are being attached to; unfortunately the tree pictured in this article needed to be moved prematurely and out of season to make way for some new benching. The ideal time to lift the tree out of the ground is Spring; though lifting in Autumn can be done successfully, it is not necessarily recommended.
Here is the tree and rock after lifting from the ground in the Autumn 2004. (The shallow rootsystem below the rock is a result of planting on top of a buried tile).
It is possible to see in this image how the tree is well rooted below the rock. However, at this point there is no way of knowing in what condition the roots under the plastic are in.
The ground soil is now washed away from the rootball and the rock and tree is planted into a bonsai pot.
Carefully, the plastic sheeting is cut away from the rock.........
......And it appears that the roots have adhered to the rock. I then proceeded to cut and clean away the remainder of the sand and the tangle of fine roots that had been under the plastic sheeting.