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'Free' grafting, where a new branch or shoot is made using a scion completely separate from the donor plant (prior to grafting), is a difficult technique to master. Free grafts have a failure rate, even when carried out by experienced nurserymen; for the amateur, the failure rate can be high. Alternatively, approach and thread grafting techniques utilise a scion to make a new branch or shoot that is still attached to the donor plant (often the same plant that also receives the graft) and the scion is not separated from its donor until it is has successfully grafted in its new position. The fact that its donor supports the scion until the graft has taken, makes approach and threadgrafting a much safer and easier technique than free grafting, even for the beginner.
Before opting to graft new branches onto your tree, consider whether it would easier and quicker to simply hard prune your bonsai to prompt budding from the trunk. Very hard pruning of most deciduous and broadleaf trees during the Winter will encourage back budding from the trunk in the Spring. However, if the branch structure of your bonsai is already well developed and ramified, and therefore hard pruning is not an option, thread or approach grafting can be a very useful technique to employ.
This is article explains the process of approach grafting. Though similar in principle to threadgrafting, approach grafting is more difficult to carry out and accomplish successfully (though it is not beyond the means of any enthusiast). I would recommend familiarising yourself with the principles of threadgrafting prior to attempting approach grafting.
Approach Grafting for Bonsai
The technique of threadgrafting sees the scion (the new shoot that is to be grafted) threaded through a hole made in the wood of the tree that the scion is to be grafted to. In approach grafting, the scion is pinned against the edge of the wood (bark and cambium layer) until such time that the scion grafts (or merges) to the wood of the tree.
Approach graft of a new branch at the side of a trunk-chop on an Acer palmatum bonsai. This will not only create a new branch in the middle of some scar tissue that would otherwise remain bare, but also help speed up callusing or the cicatrisation of the existing large wound.
Approach grafts should be seen as an alternative to threadgrafting; both techniques will fulfill similar aims, however both techniques have advantages and disadvantages over each other.
As can be seen in the image of the Root Over Rock Field Maple (Acer campestre) bonsai above, both techniques can be used in similar situations as and where they are best suited.
The graft on the left (a) is an approach graft; the graft on the right (b) is a threadgraft. Both techniques have been used to add new ‘roots’ to the tree.