Answer: That is unusually large for this plant, and it is normally self supporting. I am wondering if you may have planted it in too shady a location or overfertilized it with nitrogen. Either or both of these would contribute to excessive legginess and weak stems. Generally the plant is not pruned, but you can certainly dead head it and possibly try trimming it back by about half. If conditions are unsuitable though it will repeat the same performance.
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