Just according to how fast the plant multiplies, and how many fans I have remaining once the clump is divided. If the plant is going down in the bog area, I will normally only plant three fans. If the plant is going back in one of the upper dryer beds I will normally plant 5 fans back if I have that many left. If they are small fans I might plant them in pairs, if they are large mature fans I will probably plant them three fans and in both cases I have learned to separate them in a triangular pattern with some distance between them. I have learned that by leaving some distance between them it becomes much easier to divide the next time and maybe be able to dig just a double or single fan without having to dig the entire clump.