Best time to separate them is spring or fall. I prefer early fall and a good mulch before winter.
The plant does not have to have any certain number of fans to be divided. You can divide it at whatever size you like. Each plant is called a fan. The fans multiply and form a clump. When you divide the clump you just want to make sure you do not damage the crown (part where the roots are attached to the rest of the plant). You want to make sure this crown is not below the soil when you plant it becuase if it stays wet it can rot. Just make sure you keep a piece large enough to make a new plant for yourself when you divide them.
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