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This article is intended as a follow-on to an article first published at Bonsai4me.com in 2004 'Development of Root Over Rock Bonsai'. The first article traces the successful building of an English Elm bonsai and describes in detail how to grow your own root-over-rock bonsai.
This second article traces the partial development of another tree, an Acer campestre/Field Maple. The tree is far from finished and its appearance leaves a lot to be desired at the time of writing (Sept 2006). However, this series of pictures illustrates the technique of developing a root-over-rock bonsai well and also demonstrates the use of sacrificial shoots to thicken the trunk of a future bonsai.
Summer 2002: This Acer campestre started life as a pencil-thick cutting taken around 2000. One of a number of cuttings I started at the time, it was planted out into the garden to thicken up the trunk.
October 2004: The tree was lifted from its growing bed and offered up to the rock it was to be grown over.
Always leave repotting and root work until the Spring!
The tree in this article had its roots exposed and worked on out-of season (during September and October). It is worth mentioning that out-of-season repotting/root work (that is, at any time other than early Spring) is not recommended unless you are certain of the strength of the tree species, the strength of the individual plant and the appropriate aftercare required for the transplanting to be successful.
However good my care regime is, the tree will be weak in the Spring. However, there can be advantages to this in some cases. When the first shoots appear on this tree next May, they will lack the usual coarseness and long internodes, typical of Field Maple. Perfect for starting new branches on a shohin or mame-sized bonsai.
The piece of rock I used was found locally in an old quarry; more exotic pieces of rock can be used but I felt that a true English native tree like a field maple should be planted over some locally sourced stone or rock.
To ensure that the roots remained in position during this operation, before being wrapped in clear plastic, I roughly tied them into place with some strands of raffia; these would rot away with a few months and not cut in and mark the root surface.
Finally, the roots were secured well against the rock surface and the tree was ready to be planted into the ground. The tree itself didn't look very promising and a prime objective was to encourage a branch low down on the trunk (marked by an arrow in the above image), to grow strongly and thicken the base of the trunk.
This 'sacrifice' branch would be allowed to grow freely for a few years until the base of the trunk had thickened sufficiently. The sacrifice branch would then be removed.
July 2005: Planted back into the ground since the previous October, the tree has shown no ill-effect from being planted over the rock. The tree is seen here at the same angle as the previous image; the sacrifice branch is the only branch/shoot that had not been pruned. This encouraged it to start growing strongly.