Archimedes Plutonium schreef
I am questioning an assumption.

I assume that all shoots emanating from
a grafted tree of its original rootstock is to be pruned off and
suppressed so that the graft becomes the sole plant above ground.

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Yes. That is pretty much the point of the graft
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Is that assumption in error? Or can a tree be allowed to express itself
without the graft being exclusive expression above ground?

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As a rule the rootstock will overgrow the graft, as the rootstock will be of
tougher stock than the graft, that being the reason for using it in the
first place.
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I wonder if there is an advantage to not cutting off all rootstock shoots.

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Depends on what you want and how much time you are willing to spend.
PvR