Answer: These plants bloom only on new growth, and can become quite large and ungainly over time. In colder climates winterkill helps to prune them back, in warmer climates most gardeners will trim them back to within a foot or so of the ground each spring to stimulate vigorous new growth. The new branch tips can be pinched back a time or two as they come in to help keep the plant dense. I think regular pruning as described would help.
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