I have a row of small mums in my garden. Chrysanthemums should be pinched every two weeks or so through mid to late July, for height management. This should keep them full and bushy. One consideration you should make prior to pruning, is flower bud development. If your plants start growing early in the season, they often will set flower buds in the spring for summer bloom, then again in the late summer for fall bloom.
Chrysanthemums set flower buds in relation to short days and long nights.
These conditions occur naturally in the fall and spring. If the plants get a jumpstart on growing such as we had this year, they can bloom twice. Enjoy the bonus flowers, then pinch back as the blooms fade. Avoid pruning past July or you may interfere with flower production. The Arkansas Cooperative Extension Service offers its programs to all eligible persons regardless of race, color, national origin, religion, gender, age, disability, marital or veteran status, or any other legally protected status, and is an Equal Opportunity Employer.