I am sorry for this delayed reply to your gardening question. We are transitioning to a new Q&A system and your question was unfortunately lost in the transition. I have discovered it now and hope this reply is not too late to be of help.
You can prune it back as far as you want. If need be take it back to a foot or two high. The best time to do this is late winter but now is also okay. As it regrows, start to do some light trimming periodically to keep it compact, as it will tend to be vigorous and lanky in response to the major pruning job.
Thanks for the question. Please stop in again soon!
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