Prob September on shorter stems. Here in cornwall the season is long and the light is good so maybe that's why it works for me. However ideally the cutting back would be earlier but the wet season has made everything grow so much. I have sedum pasties pride, new to me, that was cut back 3 weeks back and I expect to see it flower in 6 weeks. We will see. I regularly cut argyranthemum, cosmos. Etc back right now to usher in flowers right through October and beyond