Answer: You can prune the canes as severely as necessary for size and shape, and trim the remaining lateral canes back to 2 or 3 buds. I've cut mine down to 3-4' from the soil without problem. It isn't necessary to prune them down that far, though. You can simply prune the main canes down to just below where you want them to be on your trellis. Hope this answers your question.
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