Thread grafting is a grafting technique used to add branches to a bonsai by "threading" a long, flexible shoot through a hole drilled through the trunk, exiting in the direction that the new branch will be directed. A small sliver of wood is used as a wedge to press the shoot tightly into place to help insure that the cambium of the trunk and branch come into contact with each other.

As the shoot grows, the graft union forms. Within a year or two it is possible to remove the portion of the shoot still attached at the entry side of the trunk. You'll know the graft has taken when the exit side of the shoot is twice as large as the entry side.
Thread graft refers to the resulting graft

Hello,
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.
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.
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.

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