Answer: Healthy mums are "good spreaders" as you are seeing. They can be divided in early spring to control size and spread, and the new plants are healthier if the old woody centers are discarded and the new plants are started from the more vigorous outer edges. They should also be pinched back or lightly sheared about every ten days from now until mid July. This will keep them bushy and dense and as a result help produce more blooms in the fall.
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