The best time to divide your peonies (Paeonia spp.) or daylilies (Hemerocallis spp.) is when they experience a decrease in the amount and size of blooms, a direct result of an overcrowded root system, or when you need more plants for your garden. Overcrowding causes competition in the roots for nutrients and water and when the plants do not get enough, they reduce number of blooms or stop blooming altogether. This generally does not happen until plants are at least three to five years old.
Time of Year
Late summer into early fall is the best time to divide peonies and daylilies, giving the plants time to establish before winter. Dividing and transplanting too late into the fall can cause the plants to die of winter-kill or disrupt the spring blooming cycle. Daylilies can also be divided in the early spring, just as the leaves are beginning to emerge from the ground. Spring divisions of peonies rarely bloom the same year, resulting in no blooms until the following spring. Few or no blooms should also be expected of daylilies divided in the spring.
Depending on the variety, peonies are hardy in U.S. Department of Agriculture plant hardiness zones 2 through 8 and have large 5- to 8-inch single or double blooms in shades of pinks, reds and white. Because they are spring bloomers, dividing peonies in the late summer to early fall avoids interrupting the spring bud formation and bloom cycle for the following year. To prepare plants for division, cut the foliage and stems back to 3 to 4 inches above the ground. Carefully dig around the clumps, avoiding damage to the roots, and lift the clump from the hole. Gently shake to remove any excess dirt from the roots and using a sharp knife, divide the clump into several large pieces. Leave three to five “eyes” or buds and plenty of healthy roots in each new clump.
Daylilies are staple perennials for gardeners in USDA zones 3 through 9 because of their beauty, ease to grow and hardiness. Heirloom and native varieties grow more rapidly than newer hybrids, requiring division every two to three years, whereas the slower-growing hybrid daylilies need dividing every three to five years. Dig each clump far enough out from the plant to avoid damaging the rhizomes or root system. After lifting the clump from the hole, remove the loose soil from the rhizomes by shaking gently. Separate the plant, leaving at least three fans per clump, by gently pulling by hand until the divisions separate from the main clump. Plants receiving regular watering do not need the foliage cut back after dividing. In all other instances, cut the leaves 5 to 6 inches above the roots to help reduce stress from transplanting.
Dig holes large enough to comfortably hold the root systems of your new plants, making sure not to plant too deeply. Set peony eyes no more than 2 inches below the soil surface. Peonies planted deeper usually fail to bloom for one or two years after transplant. Barely cover the rhizomes of the daylilies with 1/2 inch of soil or less. Covering too deep can smother daylily roots, causing stress or death. Firmly tamp the soil down around the roots and water well to remove any air pockets and settle the soil. Apply 2 to 3 inches of mulch around the plants to help keep even moisture in the soil while plants are establishing new roots.