Most perennial plants need periodic division to maintain their vigor and achieve maximum flower production. This may need to be done annually, as with hardy chrysanthemums, but is usually only necessary every three to four years. Some perennials, such as babys breath (Gypsophila paniculata), should never be divided. 

The time of year when perennials are divided is a major factor in determining the success of this procedure. Plants that bloom from mid-summer to the fall, like chrysanthemum, aster, or coneflower, are best divided in the early spring, before much new growth has begun. Perennials that bloom in the spring or early summer, such as peony or creeping phlox, should be divided in the fall, or after the foliage dies. The exceptions to this rule of thumb are iris and daylilies, which should be divided immediately after flowering. 

To divide a perennial, first remove the plant from the soil by digging around and under the entire plant and lifting it carefully from the soil to avoid as much root damage as possible. Dislodge soil adhering to the roots by hand or with a gentle stream of water from a hose. Remove and discard diseased parts and cut back the top of the plant (stems, shoots and leaves) to about six inches. Divisions are usually taken from the outer perimeter of the plants, as this younger area tends to produce more vigorous growth. The plant can be divided by carefully breaking it apart by hand or by cutting with a heavy sharp knife. Divide the plant in such a way that each new division has three to five "eyes"- buds that will produce new shoots.

Replant these new divisions as soon as possible. Rework the soil if necessary to improve drainage and structure. Dig a hole of adequate size, allowing room to spread out the root system of the division at the proper depth. Water well and protect the plant from the sun on bright, warm days. A winter mulch is needed for divisions that are replanted in late summer or fall to help prevent frost heaving.