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Threadgrafts are by far the easiest and most reliable grafts available to the bonsai enthusiast. Normally used to create new branches on a bonsai, a young, pliable shoot is threaded through the trunk of the tree.
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.
The roots of the Hawthorn immediately after collection. As is fairly typical of this species with its deep-growing tap roots, there were few fine roots in the rootball that I collected.
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.
3 years later, the same rootball is pictured after rootpruning.
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.
There are a number of ways of introducing new surface roots; some owe much to luck. Threadgrafting a new root is a relatively quick and sure-fire way to improve the nebari.
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 stuck 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.