Thread and approach grafting: replacing a missing branch or root

Grafting is how a tree with a gap in its design gets that gap filled, without waiting fifteen years for a bud to appear in the right place. Thread grafting for branches and approach grafting for roots use the same principle — two cambium layers held in contact until they become one — and both depend almost entirely on doing the work at the right moment.

What the technique is actually doing

Both thread and approach grafting rely on the same thing: two live cambium layers held in close contact for long enough that the tree cannot tell them apart. The cut faces of the scion are pressed against the cambium of the receiving trunk, callus tissue forms on both surfaces, and within six to ten weeks they have bridged. The reason the technique works so reliably, when done correctly, is that both partners are still attached to their own roots and are therefore being fed throughout — unlike a detached scion, which has to survive on stored reserves alone.

Thread grafting a branch through a trunk

The hole is drilled with a bit one size smaller than the scion, at a shallow angle so the shoot travels through a longer distance of cambium. Once threaded, the wedge emerges on the far side and is cut back to a single bud. The cut faces on the wedge align with the cambium at the exit hole, and the section between entry and exit is in contact with the cambium along the length of the drilled channel. On a 5 cm pine trunk, a 2.5 mm shoot threaded through a 60 mm channel has about 150 square mm of potential contact, which is why success rates are high.

Approach grafting a root

Root grafting follows the same logic, with the donor being a seedling of the same species planted next to the trunk. A notch is cut in the side of the trunk below soil level, the seedling's root is laid into it, and both are pinned and buried. The seedling then grows normally for a season above ground, feeding its root into the trunk. The following spring the seedling's top is cut off below the graft and the new root is left in place. This is how a tree with a one-sided nebari is brought back into balance — three or four root grafts can add 6 to 8 cm of spread in two seasons.

The mistakes that lose grafts

Cutting the scion with a blunt blade, so the cambium is crushed rather than sliced, is the most common failure. Doing the work after the buds have opened is the second: the scion's reserves go into leaves rather than into callus. The third is separating the graft too early — the scion must be left attached to its own roots until new growth from the graft has extended at least 15 cm, which on a pine means the second flush. The fourth is letting the graft dry out: the entry and exit holes must stay sealed with tape for the full eight weeks.