'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.

Advantages and Disadvantages of Approach Grafting over Thread Grafting

One of the principle advantages of approach grafting is that the foliage does not need to be removed from the scion in order to carry out the graft. This makes it suitable for coniferous species such as Pine/Pinus and Juniper/Juniperus.

Approach grafts are also easier to apply in situations where the wood to which the scion is to be attached is thick in diameter, making drilling a hole difficult or impossible. And because the trunk does not have to be drilled, unlike with threadgrafts, several approach grafts can be applied close together.

Four field maple seedlings approached-grafted to the back of a much larger field maple bonsai

The trunk of the field maple above is approximately 4” in diameter. Four field maple seedlings have been attached to the back of this trunk to create new surface roots and an improved nebari; this would not be as easy to carry out with 4 threadgrafts in such a confined area. Using approach grafts in the above situation also means that the thick trunk does not have to be repeatedly drilled nor will the trunk have 4 threadgraft-exit scars being created in full view of the front of the tree.

There are of course, disadvantages with approach grafts that must be taken into consideration. Firstly, approach grafts are not as ‘clean’ as threadgrafts. Rather than the simple entry and exit hole of the threadgraft, an approach graft requires that a strip of bark be removed in order that the scion is attached. Even after the graft has taken and the area has calloused and healed over, the scar can on occasions look unnatural and therefore be visible.
Whereas when a threadgraft is severed, there is only a small entry hole to callus over, the strip of damage to the bark can take longer to heal.

Successful approach grafts are also more reliant on the tree species having a thick bark/cambium layer and strong callusing/healing characteristics. Approach grafting is less suitable for tree species with thin bark, slow healing/callus formation characteristics and in particular, a susceptibility to the cambium layer dying back around the edges of wounds.

Finally, if all goes wrong and the graft does not take, a threadgraft at worst will leave you with two small holes/round uro in the trunk whereas a failed approach graft can leave a nasty and visible scar.

In summary, while approach grafts can be used to great effect and are fairly straightforward to carry out, it much harder to know when and where to utilise them. Great consideration has to be given to the tree species to be used, the position of the graft, the visibility of any resulting scar tissue and the health and vigour of tree in question. It is important that both the scion and tree to be grafted are growing strongly to maximise callusing, healing and minimise the time taken for the graft to take and for any resulting scars to heal over.

As a general guide, I would suggest considering approach grafts for all Acer/Maple species (red leaved varieties are much less suitable due to reduced vigour), Ulmus/Elms, Ficus/Figs, Pinus/Pines and other similar coniferous species with thick cambium/bark such as Picea/Spruce, Cedar/Cedrus, Larix/Larch and Chamaecyparis/Cypresses.

To gain experience with approach grafting, Trident, Mountain maples or Ficus species are by far the best subjects to start with due to their superior growth rates and callusing/healing abilities. I would also suggest practicing on ‘potentsai’, that is, developing trees being grown in training pots that are exhibiting vigorous growth rather than slower growing specimens in bonsai pots.

The Process of Applying an Approach Graft to an Acer Palmatum (Japanese Maple) Bonsai to Produce a New Branch

The subject of these images is an Acer Palmatum or Mountain Maple Bonsai with a thick trunk that has a ‘blank spot’ where a branch is needed but no new shoots or buds have previously appeared.
A second purpose of this particular graft is that the new branch will speed cicatrisation of the existing large wound that resulted from the removal of a large branch 2 years previously.

The first part of this process is to mark exactly the point where it is intended for the new branch to 'emerge' from the trunk when the graft has finally healed over. In this case I have used a black marker pen.

With a very sharp knife, the cambium layer and bark is cut away along the intended path of the graft. Try to take care to make a channel just large enough to accept the scion without damaging the bark of the scion itself.

The wood within the channel is then removed; I have used a Dremel in this case but hand tools are more than adequate for the job. It is important that the channel is deep enough for the entire scion to be seated at or just below the level of the trunk, where possible. This will eventually produce a cleaner scar and make the initial fixing of the scion easier.

I am using a scion or 'donor' shoot from the same tree. This is a shoot from a different part of the tree that has been allowed to grow long enough to be bent into the grafting position.

A branch from a separate tree can be used, however bear in mind the practicalities of keeping a separate tree in position while the graft is taking. The scion must be of the same genus as the tree to which it is to be grafted; it is normal that the same species and/or variety is used to ensure that the bark and foliage of both scion and grafted tree are the same. However, it is not unusual for the foliage of one Pine or Juniper species to be grafted to another

It is paramount to the whole process of approach grafting that the scion will be fixed very firmly into position.

Secure fixing of the scion ensures that the graft does not move around and break while healing. It also ensures that the scion cannot be 'rejected' or pushed away by the cicatrisation/callusing of the trunk. Be wary that even after the graft has fully taken, it will be some years before the scion is held strongly in place by the new scar tissue. Securing the scion manually now will save the accidental break of the graft in future years.

There are a variety of methods for securing the scion. Avoid using steel but copper, brass or aluminum are suitable. I like to use ordinary aluminum or copper bonsai wire. However tacks, pins or screws can also be used. Circled above are two 1.5mm pilot holes I have drilled so I can insert a staple made from 1.5mm aluminum bonsai wire.

The staple is then placed into the holes...........

.........and carefully hammered down to firmly secure one end of the scion. A second aluminum wire staple is used to secure the scion within the channel. In the future, when the surrounding area has fully healed, I will remove any visible parts of these staples but it does not matter (and is simpler and easier) if the staple is simply swallowed up by scar tissue.

Finally, the entire graft is sealed with cut paste/wound sealant. The scion will now be left to grow as strongly as possible to encourage faster healing. Notice that the leaves on the entry side of the scion have been removed. For the faster grafting you want to encourage as much growth on the exit side (the new branch side) of the graft; to do this remove all leaves and any new buds or shoots that appear on the entry side.

On this particular tree I would estimate that the graft will be ready to separate in late Summer next year.

When the area (a) has healed over you can see that the point of emergence of the scion from the trunk will be exactly where I had originally intended it to be.

As with threadgrafting, notice that the scion has been positioned so that the first internode is very short and the first node or leaf-joint (b) is close to where the new 'branch' emerges from the grafting point. This is very useful for future branch construction, ramification and division.

The staple at (c) holds the scion firmly in place; when the graft has taken, the base of the scion at (c) will be separated and the staple removed.

Severing the Approach Graft

As with thread grafts, when the exit side of the approach graft (b) is noticeably thicker than the entry side (a), you can tell that the scion or graft is being supplied with nutrients from not only the original pathway along the scion, but also via the grafting point.
Again, as with threadgrafts, sever the scion a distance away from the point of grafting to allow the tree opportunity to adjust its sap flow and reliance on the grafting point itself to support the scion.
Reduce the length of the scion slowly over the course of a few weeks until such time that it can be removed entirely.

Timing

Approach grafts can theoretically be carried out at any time of the year; but the timing is best decided according to the precise needs and risks of the individual graft you are carrying out.
In general, I much prefer to carry out approach grafts during the growing season as callusing and wound healing of both the scion and reciprocating branch or trunk is immediate.
At this time there is no (or very little) danger of the edges of the channel (into which the scion is introduced) dying back as can happen with some deciduous species during dormancy. Where dieback of the edges of the channel occurs, it can take much longer for the graft to take.

How long does an Approach graft take?

A commonly asked question with regard to approach grafting is ‘How long does the graft take and when can it be severed'?
There is no definitive time period that I can state here; so much depends on the vigour and growth rate of the scion and the limb to which the graft is attached, the time of year that the graft is made and the type of tree that is being grafted.
For instance, a very vigorous Ficus grafted during the growing season might successfully graft and be severed within just 2 to 3 months whereas a slow-growing coniferous tree might take 3-4 years.
However, for the majority of grafts made on fast growing deciduous and coniferous trees during the growing season, successful grafting and separation can be expected within one and two years. Typically the Acer approach grafts I have illustrated in this article were taken at midsummer and will be ready to separate during late Summer next year.

Approach Grafting New Roots for Better Nebari

This article illustrates how to approach graft a new branch onto a bonsai, however, as has already been illustrated above, approach grafting is a particularly useful technique for improving the trunkbase/nebari of bonsai. For further details of this aspect of approach grafting, please refer to "Approach Grafting New Roots for Better Nebari"