You may decide to divide your peonies and daylilies when you need to control their size or if your plants are not yielding blooms like in the past. You can divide the peonies and daylilies at the same time in September. You can propagate more flowers for yourself or give some away to friends, families or neighbors who will appreciate their beauty. Dividing in late summer will give the plants enough time to become established before the big winter chill comes along.
Peonies do not need regular division as some other perennials do and according to the American Peony Society, peonies should not be divided until the plant is at least 3 to 5 years old. Peonies can be divided anywhere from early September until early December, just keep in mind as autumn progresses the ground will become harder and it will be more difficult for you to dig up your peonies. Division of peonies needs only to be done if they are not growing or blooming well or if you need more ground space in your garden. This plant lives a long life and many gardeners do not divide peonies unless it is for propagation purposes.
Daylilies are best divided in late summer or early spring to allow them plenty of time to rejuvenate. Most types of daylilies can be divided every four to five years according to the United States National Arboretum and they will produce more blooms if they are divided occasionally. After four to five years, daylilies will develop a large fibrous root system that should be controlled by division. You may notice that the daylilies are taking over your flower garden or flower bed due to their fast and expanding growth. Many times you will be able to tell when it is time to divide because you will notice that you are not getting as many blooms as you used to. They can also be divided if more room is needed for other plants or if you want to propagate your daylilies.
If you divided your peonies and daylilies in the late summer, the Reiman Gardens specialists from Iowa State University suggest mulching the plants before winter and removing it in early April when spring comes. Use 4 to 6 inches of hay or other compatible mulch on top of your new shoots. This should help to keep the new, vulnerable plants and roots protected from any winter damage