Hi Nancy, If you don't cut mums back in early spring, then they are more likely to produce a premature and disappointing period of poor bloom in summer and a lackluster season of poor bloom in fall. ... If you pinch too late, you may delay their flowering until after their normal blooming time
Nancy, I'm in zone 6 and I start trimming back in May. Some years I have to cut them back quite a few times, but not later than the middle of July.