The [American Hemerocallis Society] (http://www.daylilies.org/AHSFAQsNew.html#care) advises cutting back old foliage on daylilies (Hemerocallis spp.) after blooming ceases, but daylily gardeners also trim plants after the foliage dies back in fall. Sometimes they wait until spring. The timing of this simple sanitation measure is determined by the dormancy patterns of the cultivars in your garden -- and what daylily predators lurk in your soil or shrubs.
Daylily Sanitation Basics
Daylilies are herbaceous perennials, hardy depending on cultivar or hybrid, in U.S. Department of Agriculture plant hardiness zones 3 through 9. Plants die back to their crowns -- thick white areas at the base of the green *fan of leaves. Daylilies are susceptible to crown rot, so sanitation, on the ground and in the fan, is vitally important. Dead leaves on the plant after bloom ceases and litter that accumulates on the ground should be removed.
Unless you plan on gathering seed pods, remove faded flowers daily and snip spent scapes -- the branches that hold the flowers -- close to the base of the fan. Whenever you cut, sanitize blades with a half-and-half solution of rubbing alcohol and water between plants.
Dormant vs. Evergreen
Choosing a time to cut back your daylilies can also depend on their growth habit. Old-fashioned tawny daylilies (Hemerocallis fulva, USDA zones 3 through 9), lemon lilies (Hemerocallis lilioasphodelus, USDA zones4 through 10) and many of their early hybrids bloom once and enter dormancy in late fall.
More modern hybrids, known as evergreen, semi-evergreen or remontant, hold on to their foliage until a protracted freeze, or they bloom repeatedly as long as spent blooms and scapes are promptly deadheaded. For these, prune in late fall in northern growing zones or delay pruning until spring in southern zones where winter freezes are rare or non-existent.
The Second Flush and Propagation
Along about mid-August, after the heat of summer abates, many daylilies get a second flush of growth. This is a good time to cut withered foliage back to 3 to 4 inches tall and water daylilies well. With deep watering, foliage will grow in aggressively, and remontant varieties will get a second wind for reblooming. August and early September are also prime weeks for division of large clumps that have declined in bloom vigor. Cut fans back before dividing to ease handling and reduce the load on divided roots.
Winter and Critter Considerations
Provided you've kept the ground clean and pulled rotten leaves, you can allow faded leaves to stay on the plant until spring when they'll need to be pulled off the plant, making way for new green leaves. If you live in USDA zone 6 or above or have evergreen varieties, this is a fall labor-saving strategy. If, however, you live in a colder climate where snow and piles of daylily detritus might form handy habitats for rodents and insect larvae, you should cut those leaves back to a few inches above the crown after the first frost. The leaf remnants will fall away in the spring, and those critters will have to search elsewhere for winter habitat.