Marianne, they produce lots of tubers, some of which invariably get left in the ground after harvesting. These then sprout forming new plants. The rule with these lovely beauties is to ensure you have every piece of root and tuber out before spring. Alternatively, if you are not using the bed for something till later in the year, any tubers left will identify themselves by sprouting, then you can dig them up.
I am a total fan and recommend them to all.