For the healthiest growth and most abundant bloom, you should divide daylilies every three to four years. They can be divided in early spring growth starts, but it is more common to divide them in late summer. Daylilies have a tough root system that can make them difficult to divide. Although plants can be divided in place by using a shovel to chip off pieces of tops and roots it can be quite a chore if they have been growing undivided for a number of years. The preferred method is to dig up the entire and then use two garden forks pushed down into the clump back-to-back and then pried apart. This method preserves more roots than cutting them into sections with a sharp spade.
The size of the sections that you pry apart can vary from large clumps to individual fans. It pretty much depends on how many new plants you want – and how soon you want to have to divide them again! For ongoing, good garden display, each section should be about 4”-7” in diameter.
Another method, although it is probably messier depending on where you work, is to use a stream of water from a garden hose to wash the soil from the clump. Then “worry” the clump by hand or roll it around on the ground in order to separate the divisions. Prior to the starting the process of dividing the daylily you may also want to cut off the top one half to two thirds of the foliage. This makes handling the clump much easier. Once divided replant as many new divisions as you need to fill the planting space and then call the neighbors and share any extra divisions.