Posted: 06/07/2016 at 19:38
My Centaurea flower in May. I regularly deadhead them back to a side flower shoot which prolongs the flowering season but eventually theyvget straggly and floppy. I then cut them to the ground, they reshoot and produce a 2nd burst of flowers in August.
I guess you could Chelsea chop them in April but the idea of the CC is to get sturdier, bushier plants and I dont think it would necessarily have that affect on Centaura. I grow mine through flower supports which helps keeps them upright.
I don't grow Achillea so can't advise on those,