Answer: Cut back in early spring just as new growth starts. For any plant, the general guideline is to remove no more than 1/4 to 1/3 of it in any growing season. Removing too much "shocks" the plant and reduces its ability to photosynthesize. So, I'd trim just the top couple inches, unless the lower plant is full of brown and woody dead material, it may be worthwhile to trim back further to reinvigorate it. Good luck!
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