Hemerocallis is the scientific name for the daylily, translating as "beauty for a day." Daylilies got this name because a single bloom only lives for one day (see Reference 1). Luckily, there are several blooms on each stem of the plant that allow this type of lily to give you a three- to four-week bloom period depending on the variety. Some daylilies, such as the tiger lily, grow in large, spreading patches, but most hybrids of daylilies grow in clumps that can become very dense and need to be thinned out to keep the plant vibrant and healthy.
Appropriate Division Seasons
The division of daylilies can be done in early spring — the beginning of April — before the growth season starts or early in September to allow for some growth and stabilization before the dormant season begins. Either time is acceptable, although many gardeners prefer to do this type of maintenance in the late summer. Daylilies are one of the simplest flowers to divide and can handle the rough treatment when done properly, no matter which season you choose.
Reasons for Thinning
Many times you will know it is time to divide your daylilies when you notice fewer blooms and your flower stems take on that tall, leggy look. Usually you will see that the middle of your clump or patch has less foliage and flowers. Plan on dividing your daylilies every four to five years, suggests the United States National Arboretum (see Reference 2). Keeping this schedule will help your plant remain vigorous and encourage strong growth.
You may also choose to expand your garden by thinning out your plants and using them in other areas of your garden or lawn. Daylilies are also a great gift to give to friends or neighbors so that they can enjoy them as well.
Whether you thin your plants in late summer or early spring, the first season’s growth after the division will usually have fewer flowers than you are used to seeing, but you should notice thicker stems and abundant foliage. Daylilies are very strong, tough plants and you will notice the difference by the second growing season when blooms should be back to their original number.
If you thin out your daylilies and divide them in the late summer or early fall, make sure that you supply them with a 4- to 6-inch layer of straw or other organic mulch. This will help protect the vulnerable roots through the cold season, according to the Iowa State University Reiman Gardens (see Reference 3). Remove the extra layer of mulch in early April before new growth begins.