Living in ny I would trim the leaves right down to ground level near the end of fall, not before. Once they reproduced so abundantly and the roots just peaked from the surface of the ground then i would trim the leaves only half of the height so that when the leaves wither and drop to the ground they give protection to the root system through the winter. Then in the spring i'd removed them as soon as there is no more possibility of frost.
If your root system begins to peak at the surface of the ground they are in need of thining out. They are a strong flower and you can remove at the roots of at least 5 inches or longer and trans plant in another area or you can leave them and watch how much they double every yr./ or you can just add a mix of top soil and regular soil to cover the roots that are peaking up from the ground. I hope my answer helps.
