Update: I had a go at the method
@Paulpash suggested earlier on in the thread and here is what happened in the two years since for this tree.... I got one root from the trunk and a few more roots grew straight up from the tops of the bigger roots that were unusable.
The root from the trunk:
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The rest of the repot:
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I also tried scoring the bark and painted hormone on it, them packed it with sphagnum moss:
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I've taken cuttings so hopefully I will have some root grafts ready for next repot.
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Was good to have a go at the experiment to see what would happen and there you go.