Answer: The simple answer is that it never needs to be pruned, especially if it has plenty of room to grow. After all, in the wooded mountains of China and India where this plant grows naturally, there are no little elves that come around and prune these plants every year!
If, however, you decide that you absolutely have to prune your plant, the best time is from late March through early May, before the leaf buds open and begin to expand. Pruning in early spring will encourage additional side shoots to form. This should translate into more flowers in July and August, as flower buds will form on many of these new shoots.
You may be surprised to learn that it can be cut back to stubs no more than two or three feet tall without risk of injury. While you can use hedge shears to perform this major surgery, my recommendation would be to selectively remove individual shoots to different heights with hand pruners or a small, folding pruning saw. This technique will result in a smaller plant while preserving a more natural form.
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