you can trim or not trim them, cut them back or not, but if you cut them all the way back, you will lose the flowers for this year. i would remove the worst of the leaves, leaving the best, and fill in with something else, as was suggested. you can even fill in with white ribbons on short bamboo stakes. the stalks of flowers will still come up and bloom as long as you don't chop everything to the ground. fertilize carefully.