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'Free' grafting, where the new branch or shoot is made using a scion completely removed 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
(very 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 the scion is supported by its donor until the
graft has taken makes approach and threadgrafting much safer,
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 during the following Spring. Very hard
pruning of most deciduous trees during the Winter will encourage
back budding from the trunk.
If the branch structure of your bonsai is already well developed
and ramified, and hard pruning is not an option, thread or approach
grafting can be a very useful option.
For further details about approach grafting please see 'Approach Grafting for Bonsai: Creating new branches by approach grafting'
Threadgrafting
Threadgrafting is the easiest way of attaching new branches where they are missing on a bonsai. The basic principal of threadgrafting is that a hole is made through the trunk (or another position where a new shoot is needed); the scion (new shoot) is then threaded through the hole and fixed into position. As the scion and the trunk continue to grow they swell and are forced together; eventually grafting together.
The only possibility of the graft failing is if the scion is separated from the donor too early, before it is successfully grafted into its new position. Though it may seem harmful to the tree, drilling a hole through the trunk only damages a small area of live wood at the entry and exit points. The graft does look a little ugly while the actual graft is taking place and will need to be in position for one to two years. However, afterwards, the resulting wounds are minimal and very hard to detect.
This method is suitable for all deciduous and broadleaf trees but not for coniferous species where complete defoliation of the scion will result in its failure.
Prepare
the tree to be grafted by allowing a number of long shoots to
develop. Suitable shoots to be grafted (used as the scion) will
be long and supple enough to be bent over and threaded through
the trunk. The scion can be from the same tree or from another
plant growing in a different pot. However, for the scion and
the tree to be grafted, they must be of the same genus, ie an
Acer and another Acer or an Elm and another Elm.
Theoretically, thread grafts can be made at most times of the
year but midsummer is best as growth of the scion and healing
of the graft will begin immediately and strongly.
Remove all leaves and petioles (leaf stalks) from the scion branch. Take great care not to damage the tiny buds in the leaf axils as these will be required to leaf out when the graft has been made.
Drill a
hole through the trunk; it is safest to use a thin drill bit
to make a pilot hole followed by progressively larger bits until
the hole through the trunk is very slightly wider than the scion
shoot to be threaded. Though too large a hole will increase
the amount of time it takes for the thread graft to successfully
take, too small a hole will cause damage to the scion or the
new buds on the scion.
Start the drilling from the exit side of the trunk so that the
final position of the graft will be in exactly the place you
intend the new branch to be. The position of the entry hole
does not need to be precise. However, bear in mind that once
the graft has taken and the donor side of the scion is removed,
a small scar will be left. For this reason, try to make the
entry hole at the back or side of the trunk where it will be
out of sight of the front, if at all possible.
Though not essential, try to make the exit hole higher than
the entry hole. The scion shoot will still be apical; if the
shoot faces upwards, the side to be the new branch (at the exit
of the hole) will bud out and grow more strongly than the entry
side.
If the entry side is apical, new shoots on this side will have a tendency to grow more strongly and will need to removed to make sure the threadgraft is able to grow strongly.
It is always important that the threadgraft is encouraged to grow as strongly as possible; the faster it grows, the sooner the branch thickens and the graft takes.
Thread the graft through the hole you have made, slowly and carefully. Particularly with soft-wooded shoots, try to pull the shoot through rather than push it, if possible. This helps to stop the shoot buckling as it goes through the hole.
To ensure
the new grafted branch will have a short first internode, position
the graft so a bud (node) is a short distance from the exit
hole.
Leaving a long distance to the first bud will mean that in the
future, the first secondary branch will be a long way from the
trunk.
To fix the
graft in position, insert a thin piece of wood (from elsewhere
on the tree for instance) into the hole alongside the threaded
shoot so it is firmly wedged in position. If the threadgraft
is able to move within the hole, the union will take longer
to graft together. Finally, seal with cut paste.