Daylilies (Hemerocallis) are beautiful plants that have large blooms that typically bloom for just one day from early morning until the evening. They have tuberous roots, which are winter hardy and can survive underground during the cold weather months. They also grow fleshy roots during this time, which is essential to water absorption. You can do a few things, though, to help the tubers get ready for winter.
Leave the green foliage on until it withers and turns yellow, at which time you should cut it back so that about six inches remain above the soil. The plant must absorb as much sunlight as it can, converted it to sugar and stored it in its tuber. The tuber will continue to grow fleshy roots underground during the winter using up some of the sugar while leaving enough to grow healthy plants in the spring.
Mulch over the daylilies, especially in USDA hardiness zones 6 and colder. Mulch will help insulate the tubers underground and therefore keep them warm. Snow also insulates the tubers, but by mulching in the fall, you are maintaining soil temperature sooner when it is warmer and less heat is lost.
Water your daylilies a few days before the first freezing frost. This will give the tubers one last water before the cold weather arrives.