Thread Grafting Bonsai.

'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.

Encourage strong growth of the threaded scion. Feed the tree well to ensure strong growth. Remove any new growth on the entry side of the thread graft to encourage maximum growth on the exit side. Do not prune the threadgrafted as this will slow thickening. There is no reason why the new branch cannot be gently wired if necessary.

At first the new threadgraft supports itself entirely.

As the graft and the graft hole thicken, their cambium layers are forced together and start to merge. As they merge, the threadgraft begins to be supported by the trunk as well.

With the extra energy from the trunk, the exit side of the scion starts to grow and thicken faster than the entry side, eventually producing a pronounced increase in diameter. This indicates that the scion is being fully supported by the trunk in its new position and can start being removed.

Successful threadgrafts can be seen above on a Hawthorn, Acer palmatum 'Bloodgood' and Acer p. 'Katsura'.

Severing the Threadgraft

The amount of time for the scion to reach the stage where it can be severed depends on many factors. Some species take more quickly than others, in as little time as 2 or 3 months for Ficus species (if the graft is made just before a strong flush of growth), or 2 growing seasons for slow-thickening species such as Hawthorn.

Timing of the removal or separation of the scion; it does not particularly matter at what time of year the threadgraft is separated once the threadgraft has taken but I prefer to carry out this work during the growing season while the tree and grafted branch are active and able to respond to the changes in sap-flow.

Do not detach all of the redundant side of the graft at once; the scion will still receive a little energy and be supported from the entry side of the branch. Detach the scion from the parent plant but leave a length of the donor branch in position so that the scion can slowly become accustomed to being entirely supported by its new parent trunk.

 Over the course of 3 or 4 weeks, slowly shorten the donor side until it is finally removed.

Finally, the old entry hole can be pruned flush to the trunk and allowed to heal.

Failure of a Threadgraft

Threadgrafts are most likely to completely fail if enough time is not given for the two cambium layers to join together. It is important to be patient.

There are occasions where the threadgraft is unknowingly separated too early and the leaves of the new branch droop and/or fall. This can also occur if the donor side of the thread graft is reduced in length too quickly.

In these cases, do not assume that the new branch will fail completely. I have had instances where a newly separated branch has lost its leaves but has already grafted just enough to slowly recover and issue a new set of leaves in the future.

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