Question: When should I divide my hardy mums?
Answer: Don't be tempted to divide your mums in the fall. They respond best to being split in the spring, just as they're starting to grow. Dig up the plant. Discard the center of the clump and replant the vigorous outer roots. Don't repeatedly remove sections from the edge of the clump to give away, or you'll be left with just the old, woody center, which will grow and flower, but weakly.
To make more mums, you can also take cuttings in spring. If you spread some gritty soil among a clump of mums in the fall, you may well find that in spring you can detach new shoots with a few new roots already developing. Pot them up , care for them for a few weeks, then plant them in the ground. Cutting without roots can be stuck in fresh, sterile potting soil on a warm windowsill. Keep them moist and they will take root.