Answer: You need to check the tree regularly and remove those shoots from the roots below the graft just as soon as you see them. If you can, when they are very small, tear them off with a little twist so they come off completely at the base. If already too big for that, use a sharp knife or bypass pruners to trim them away as fully as possible. They are suckers coming up from the rootstock and they will potentially overwhelm the grafted-on top portion of the tree. They are undesirable because they steal nutrients and moisture and energy from the roots that should be going to the Colonnade top. You might be able to root one of the suckers, but it would grow into whatever tree the rootstock is(usually selected because they are well suited as rootstocks, not necessarily as bearing apple trees) rather than another Colonnade.
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