Answer: Based on your description, a possible cause might be that they have become crowded and need to be divided. This is usually done every four or five years in order to cull the oldest rhizomes, discard any diseased rhizomes, and to rework the soil where they are planted. Since this is done so infrequently, it is a good idea to do a thorough job preparing the planting bed, adding liberal quantities of arganic matter, (compost is ideal) and ensuring that drainage is good. This is usually done during the summer, after the bloom period, so it could be done now.
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