pot alway more vulnrabal -did you protec?thik inslation around an over? mine in open ground but stil cover over winter

If its Bonariensis you are talking about, it seems to survive winter OK tucked away in the ground. some establish themselves as big clumps. There is a garden centre at Harlow, with rather sandy light soil, disturbed a lot by rabbits in the show garden. The clumps are huge. They seed amazingly well into the rabbit scrapes!!! then a few get transferred...ahem. In my garden these seedling seldom seed in open soil. Only between the bricks at the edge of the path. Cut them back but leave some stem so you know where they are if in the garden. If you want to leave in a pot, sink it in the ground.