I have a mass of mums that originated from a fall planting about 4 years ago. Presently, it is about 1 meter in diameter with no voids. Eventually, it’s mere size will necessitate dividing; however, do I risk the centre dying as with hostas if they are not divided?
Hardy mums, as opposed to some that come in pots from the florist, grow by stolons, or underground stems. So, mums will eventually die off in the middle, as the live plant moves further from the centre each year. To keep mums bushy and relatively compact, pinch the growing tips back till around early July. This encourages side shoots and therefore a more compact form. Mums should be divided about every three years in the spring. So, next spring, divide your mums, and replant in a sunny, sheltered spot, preferably the south side of your house. Water and mulch well. If you end up with more than you need, pot the rest up and give a pot to a friend for her garden. She will thank you!