Timing

The time of year when perennials are divided is a major factor in determining their success. Species that bloom from mid-summer to fall, are best divided in early spring, before new growth has begun. Perennials that bloom in the spring to early summer should be divided in the fall, or after the foliage dies. Exceptions are iris and daylilies, which are divided immediately after flowering.

Preparation

To divide a perennial, first remove the plant from the ground by digging around and under the entire plant and lifting it carefully from the soil avoiding root damage. Shake loose soil off the roots gently. Remove and discard diseased parts and cut back the top of the plant (stems, shoots, leaves) to about 6 inches.

Dividing

Fibrous rooted plants often can be divided by hand or by using two forks back-to-back. Divisions usually are taken from the outer perimeter of the plant, as this younger area tends to produce more healthy and vigorous growth. Plants forming a woody center or that have solid roots can be divided by using a sharp knife or a spade to cut through the crown. Divide the plant in such a way that each new division has at least three buds that will produce new shots.

Replanting

Replant 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 when planting. Take care to replant 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.