Dividing Most perennials can be divided, and in fact need periodic division to maintain vigor and maximum flower production. This may need to be done annually, as with hardy chrysanthemums, but is usually only necessary every three to four years. 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.
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. 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.