Don't know but cutting chunks off established blooming Knautia macedonica plants, through the summer, never seemed to affect the show.

I did also deadhead them. That started with the idea of them continuously blooming (it worked). But I got to realize that without strenuous deadheading, at least in their favoured locations, they'd be seeding all over the place.

I just don't use perennials like that any more.

yes you will but it sort of depends on timing. If you keep yours dead-headed, no problem - chop away.....but once the seedheads start to form then it will consider it has done it's job (making more potential babies) and will not send out more flowering stalks. I have loads of this and regularly shear it back because it is a martyr to powdery mildew - hedge trimmers are your friend.

Many thanks!
CMK