Dividing perennials is an excellent way to expand a flower border. It rejuvenates older plants which may have begun to die out in the center, and the new clumps are sturdy enough to become healthy, mature plants in the first year.
Dividing also can help prevent an existing bed from being taken over by one plant and it's fast, easy and inexpensive. If you have a new space that will require about 20 plants to fill in, for instance, you might consider buying only five perennials and filling the rest of it with annuals until the perennials get large enough to divide.
When and How to Divide
It's best to divide plants when they're still healthy and looking good, but division rejuvenates even older, failing plants. The mild weather of fall and early spring is a good time to divide perennials -- when they're using their energy to establish roots rather than for top growth. Another benefit of spring division is that growth is still very low to the ground, so you don't have to worry about breaking stems. In general, it's best to divide spring-blooming flowers in fall and summer- and fall-bloomers in spring.
If you divide plants later in the season, cut back the foliage to reduce heat stress and encourage new growth. Warm, windy days will cause the foliage to wilt, no matter how moist the soil. Work in the cool hours of the day if possible and try to avoid windy days. It's best to divide a few days after rain or watering so the root system is moist and doesn't fall apart.
Some plants are fibrous or have a loose crown and root system, making it easy to pull them apart. A flat spade, pitchfork or knife can be used to break clumps into halves or quarters, while trying to leave the crown sections of the plants fairly intact. It may be easier to remove the entire plant before dividing. Keeping the clumps fairly large reduces stress and aids in establishment. If you'll be moving a lot of plants or sizable ones, consider using a wheelbarrow half filled with compost to fill any holes left behind; the compost will provide renewed, fertile soil for both the old and new plants.
Place plants at their original depth in a hole at least as wide and deep as the root spread so the roots aren't tipped up or curled back against each other. After planting, mulch with a 1-2 inch layer of grass clippings or straw and water them generously. When new growth appears, plants are on their way to establishment and can be watered less frequently to encourage roots to grow deep into the soil.
Specific Varieties
Perennials, easy to divide: aster, beebalm, black-eyed susan, boltonia, coneflower, daylilies, goldenrod, hosta, sedum, spiderwort.
Perennials, difficult to transplant because of long tap root: anemones, balloon flower, candytuft, columbine, Euphorbia, foxglove, gas plant, geranium (perennial), milkweed, oriental poppy.
Perennials, difficult to transplant because of woody stem: Baptisia, butterfly bush, lavender.
Perennials, woody-stemmed ones that may propagate by branch-rooting -- separating a side shoot or branch and covering the stem with soil to create a new plant while still attached (cut once established): Artemisia, leadplant, Russian sage.
Grasses, easy to divide: feather reed grass, dropseed, sedge, switchgrass.
Grasses, difficult to divide because of woody crown or tight-clumping: little bluestem, Miscanthus.