I have Calla Lilies planted out in a well drained, full sun area. They normally overwinter very well; however, a particularly bad frost hit them this year and the plants have reduced to mush. Some new growth was coming through but appears to hit the existing mush, which then causes it to turn itself at the tips and not getting much further really. What should I do with them? Should I cut the plants right down to ground level now to allow new growth through?