Why Divide Perennials?
Division is the easiest and most common method of increasing perennials. Most perennials respond well to division, which is done to keep rampantly growing plants in bounds, as well as to keep older plants looking good as the clump-forming center dies. Dividing is a surefire way to expand and strengthen your stock.
Multiply by Dividing
Division is good economics and lots of fun, if you observe the rules of propagation: when to divide (spring or autumn), where to plant (sun or shade), and how to divide plants. The best seasons for dividing are autumn and spring. Autumn planting allows plants to establish while the soil is still warm enough for good root growth. Spring planting is better for perennials not fully hardy or for those that dislike wet winter conditions. Do not divide perennials into very small pieces, as they will need several years to mature.
Time to Divide
Divide perennials when they are growing but not blooming. Divide spring- and early summer-blooming perennials in late summer to early fall.
| Plant | When to Divide | How to Divide | Where to Plant |
|---|---|---|---|
| Aster | Spring | Divide regularly. Each division becomes a mature, flowering plant in one season. Cut to ground after bloom in late October and enrich soil with compost or manure to several inches. | Full sun/light shade |
| Astilbe | Early spring, before foliage unfolds; also late summer | After first killing frost, cut plants to ground or leave in place for spring clean-up. Divide every 3 to 4 years. Dig up crown. Shake free of soil. Rinse roots around crown with water. Cut through center of crown with knife. Should have 3-4 "eyes" (buds) in each section. Replant sections at same height as original crown. | Dappled shade/sun if kept watered |
| Chrysanthemum | Spring | Divide roots. Stem cuttings. Take cuttings in early spring or late winter. | Sun/light shade |
| Coneflower | Spring/fall | Uproot clump. Cut in segments with knife. | Sun |
| Coral Bells | Spring/early fall | Remove clump with fork. Divide using young, outer parts of the crown. | Sun/light shade |
| Coreopsis | Spring/fall | Fall pruning. Cut off at ground. Lift with spade or fork. Divide with spade or knife. | Full sun |
| Daylilies | Spring/early fall | Divide every few years in spring or fall. Dig up clump; trim foliage to 5-8" of crown. With knife, pull clump apart so each division has at least one "fan" of leaves. Replant. | Sun |
| Dianthus | Spring or after bloom | Divide with knife or pull apart. | Sun |
| Ferns | Spring | Divide clumps when growth begins in spring. Cut through clumps with knife, leaving separate crown that can grow new fronds. | Shade |
| Hosta | Early spring; also summer and late fall | Dig up clump. Pull rhizomes apart. Separate with knife, leaving abundant roots on each cluster. Each division must have at least one bud. | Shade/light sun |
| Iris | After bloom (July/August) | Dig out clump with spade. Use knife to cut apart rhizomes. Divide when iris cluster is full. When replanting, barely cover rhizome. | Full sun |
| Lavender | Early spring/fall | Break apart clump. Use knife or spade to divide. | Full sun |
| Rudbeckia (black-eyed Susan) | Spring | Divide roots with knife after lifting with fork or spade. | Sunlight/shade |
| Sedum | Spring or after bloom | Mid-April separate new growth. Divide with knife and pull apart. Set high. | Sun/part shade. |
| Thyme | Mid-spring/fall to early summer | Cut heavily in spring to woody stem. Also cut up to October, but no more than 1/3 maximum. Divide (they break apart easily). | Full sun/partial shade |
| Yarrow | Spring and after bloom | Root division. Divide in half (or more if it has some age on it) with knife. Divide or replant every third year. Set plant high when replanting. | Full sun |
| Yucca | Spring/fall | Use sharp knife or garden fork to break offsets from main plant. | Sun |
Divide midsummer to fall bloomers in early spring. In Chester County, do most of your divisions in spring to give new plants time to get established before cold weather arrives.
Division Techniques
To divide, lift the plant with a spade or fork, being careful not to damage the roots. Shake off the soil and tease the sections of the clump apart. Hose soil off, if necessary.
If the clumps have thick, entwined roots and resist separation, use an old butcher knife, an ax, or a sharp spade to cut the clump into pieces. Place your cuts so that each division has a lot of roots. If you have a plant in which the center has died out, cut away the woody center and divide the outer ring of healthy growth.
Replanting Divisions
Replant those divisions that have strong, healthy root systems. When dividing in spring before plants start putting out new growth, replant the divisions immediately. If the plants have already produced full stems and leaves, remove 1/2 to 2/3 of the foliage before replanting. This is done because the reduced root system is now unable to replace the amount of water lost through the larger top plant growth. Replant slightly higher to allow the plant to settle, firm the soil around the plant, water well, but do not wash soil from around roots. If you can't replant immediately, keep the divisions moist and protected from the hot sun and drying winds.
