As the shoot grows and fattens, the cambium layers of the shoot and the trunk are forced together and a join or graft is made.
Threadgrafts can also be used to attach new roots to a bonsai. With the English Hawthorn (Crataegus monogyna) below, a new root is required to improve the nebari (rootspread) of the tree and this is provided by thread grafting a young Hawthorn seedling.
For more details on the subject of threadgrafting, please visit:'Threadgrafting Bonsai'
A second useful and related article is ‘Approach Grafting New Roots’; both threadgrafting and approach grafting new roots are equally viable techniques and should be seen as techniques that can be used individually or in conjunction with each other.
Pictured here is the rootball at the time of collection. The tree was planted into an inorganic soil mix with a little added sphagnum moss.
The nebari and rootspread is developing well though the lateral surface roots are very straight and untapered; this will be improved over the years by selective pruning to increase ramification of these roots.
When the tree is repotted it becomes obvious that the areas either side of one of the old thick taproots is ugly and bare.
The seedling used for a threadgraft must be of the same genus as the parent tree but it is also preferable that it is of the same species. Different species (such as Acer campestre (the field maple) and Acer palmatum (Japanese maple) will graft together but the bark colours and rate of growth are ill-matched. This will cause the graft to look artificial in future years.
It is important that threadgrafting is carried out before the seedling starts to bud out in the Spring as at this time the leaf buds swell and are easily dislodged as the seedling is threaded.
The seedling is bare-rooted and any side branches are removed.
A hole is drilled through the trunk of the bonsai.
Though it is preferable that the hole for the threadgraft is made from the entry side to ensure it's position with absolute accuracy, this tends to be difficult if the tree is already potted up.
Try to make the exit hole for the graft in a position where it will be hidden from view (facing the back of the tree).
Use a wood drill bit that is slightly larger than the diameter of the seedling so the seedling can be introduced through the hole without damage. However, making the hole too big will increase the time needed for the graft to take.
If a large diameter drill bit is needed for the seedling to fit, use thinner bits to make an initial pilot hole and gradually widen the hole with larger bits until the correct diameter is reached.
Ensure that the drill bit is fixed very firmly in the chuck; wet wood has a lot of 'drag' and there is potential for the drill bit to come out of the drill and become stunk inside the trunk.
After drilling, the seedling is threaded through the hole....
.............until it is finally in position.
As can be seen in this picture, the seedling has been selected for matching bark colour and the way the taproot naturally bifurcates (splits into two) creating a more natural surface root
The roots of the seedling are covered with soil and the holes in the main trunk sealed with cut paste to encourage rapid healing.
The seedling will be encouraged to grow strongly this year; the greater its growth, the faster the graft will take.
When the seedling and the main tree have visibly joined and healed together, the upper part of the seedling will be gradually pruned down until all that is left are its roots growing into the main tree.
Patience is necessary; at the very earliest this separation will take place in a year's time.
The main tree and the threadgraft July 2005
Update: December 2006. By August of this year, the threadgrafted shoot had grafted tightly into position. The top section of the threadgrafted seedling was pruned away leaving its base as part of the main tree.
The Hawthorn bonsai itself; December 2006. Here is another example of threadgrafting roots; this time on an Acer campestre/Field Maple.
Acer campestre, as with most Acer species, exhibit strong root growth. Though there are three strong lateral/surface roots that are being to develop some division and taper; there are also two large gaps in between these three surface roots.
With many maples, new roots could be encouraged by simply scoring the bark of the trunk, however, if new roots were to emerge, they would require many years of strong growth to thicken as much as the three existing surface roots.
Instead, three young, pencil-thick Acer campestre saplings are thread-grafted into the gaps in the nebari. These will take much less time to become nearly as thick as the existing surface roots
In this image the three saplings (new roots) have been threadgrafted into position.
A couple of reed-sticks have been lodged across the roots to ensure the new roots keep their positions when they are covered back over with soil.
Notice that the exisiting larger roots have been pruned to ensure a smooth transition into the smaller secondary surface roots; just as you would ensure a smooth transition between the branch sections of a bonsai.
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