Answer: The more plants you need, the smaller the divisions you would probably try, and/or the more frequently you would divide these. Assuming they are growing well for you, they can be divided annually if desired. The smaller the division, the more care you should take to ensure a viable portion of rhisome is attached to the foliage -- but even very small pieces will grow. Also, the smaller you make them the more carefully you will need to monitor them after planting, particularly making sure the soil does not dry out. Some gardeners in fact prefer to grow their divisions in containers for a while so the divisions establish a good root system prior to replanting. In my experience the divisions should be several inches across for ease in handling and aftercare, but you could conceivably just cut each clump into quarters and replant, similar to handling say larger divisions of a Siberian iris. Be sure they do not dry out while you work on them. Keep in mind that this plant can be left in place to form essentially a turf substitute if you like that look so you would not have to divide it unless you want to. I should probably also mention that the usual time for dividing these is early spring as this eliminates the risk of frost heaving due to seasonal oscillating temperaturess. I hope this helps.
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