Most perennials can be divided as soon as they are through blooming for the year. Perennials need to be divided when:
- the flowers are smaller than normal
- the stems fall over easily
- there are a large number of underdeveloped shoots
- the center of the clump is hollow and dead
- the bottom foliage is sparse and poor
Most perennials need to be divided every three or four years, although a few species, such as chrysanthemum, Monarda and Anthemis, do best if divided every spring.
Here’s a good rule of thumb to help you determine when to divide your perennials: Divide spring-blooming plants should be divided in the fall, and summer bloomers should be divided in early spring.
To divide a perennial, first dig around and under the entire plant and lifting it carefully to avoid root damage. Gently shake the soil from the roots or rinse the soil off with a gentle stream of water from a hose. Prune the top of the plant to about six inches and remove damaged or diseased sections. Divisions should be taken from the outer edges of the plant since this younger growth will produce more vigorous plants. Some plants can be broken apart by hand, but if necessary, use a sharp knife.
For best results, make sure each division has at least three vigorous shoots. Small shoots will take longer to flower. Small divisions taken from a number of perennials can be planted together in an "nursery bed" where they can be nurtured to flowering size, then moved to the main flower bed.
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