Answer: There are several reasons why this may happen. One is that the plants are in too much shade, or are overfertilized, or are overwatered causing them to grow too lushly. Another possible cause is that the plants need to be divided to encourage more vigorous, sturdier growth. One approach to controlling the sprawl is to trim them off short in early summer. This keeps them a bit shorter and supplies many tip cuttings that can be rooted as well although it can also delay flowering a bit. I hope this helps.
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