Answer: The recommendation about planting high is based upon the fact that soil can settle after a while and the tree or shrub can sink, which exposes the bark on the trunk to wet soil. This can result in rot after a few years. Planting high will eliminate this problem. And, since your tree is grafted, if you bury the graft, you'll end up with shoots emerging from below the graft, which will compete with the tree you originally purchased. For best results, plant the tree or shrub at the same, or slightly higher soil level than it was growing in the nursery pot. The graft whould be fine when exposed to your winter weather. Hope this clarifies things for you.
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