As the lotus leaves begin to turn brown, the lotus is beginning to go dormant for the winter. Do not cut off the leaves; allow them to die back naturally. At this point, it is safe to cut them off above the water line. You do not want water getting down the hollow green stem and having it get into the air channels of the tuber, if this happens you risk drowning the tuber, which will cause it to rot. Make sure the potted lotus is below the ice. Allowing the tuber to freeze will kill the plant.