u/Basic-Hospital-7703

I made a mistake. Everyone said Gregg's Mistflower was aggressive and I foolishly thought they were exaggerating. The first year, it was so tame and and grew into a cute clump that I really liked the look of. So I planted more...

In winter it all froze back but in spring the monster was unleashed! Its taking over everything! I plan to split it and maybe move it as well, to a more contained garden bed. But before I do that I want your tips: how do you control its spread?