Prune day lilies by removing dead blossoms and trimming back stalks as they cease to flower or produce new buds. Pull or trim leaves when they yellow or turn brown, and in fall, prune back any remaining leaves to no more than four inches from the ground.
Day lilies flower in summer and provide blooms for several months before going dormant in fall or early winter. To keep plants healthy, deadhead blooms and trim back stems regularly.
Remove old and wilted flowers by grasping the base of the flower between your thumb and forefinger. Pinch off the flower, leaving the stalk and remaining buds intact. Throughout summer, remove stalks that no longer flower or produce new buds. When all flowers have finished blooming in fall, use clean, sharp shears to cut all remaining stalks down to the base.
Depending on local temperatures and the variety of day lily, leaves may remain green into late fall. As soon as dead leaves turn yellow or brown, remove them completely from the plant. In mid to late fall, prune back any remaining green leaves. Dispose of all pruned plant material away from the plant bed so it does not house insects or disease over winter.
After you prune day lilies for winter, cover the plant bed with one to two inches of mulch. Do not feed or water day lily plants during their winter dormancy. When growth resumes in spring, resume regular watering.