Dividing Overgrown Perennials
The easiest way to get more perennial plants is to divide established plants that have become overgrown and thick, usually in three to five years for most perennials. Popular examples are hosta, chrysanthemum, daylily, black-eyed susans, and ground covers like ajuga or pachysandra.
The best time to divide perennials is in early spring when they first begin to send up green shoots. Dividing plants in the fall will work in many cases.
Dig up the entire clump. Use a square bottomed spade to carefully separate the clump into two, four, or more pieces. The best will be the pieces with vigorously growing shoots from the outer portions of the clumps. In all cases, make sure that the pieces have roots attached. Discard older woody central parts of the clump. Plant the new rooted shoots in the same way you would plant new seedlings from the garden center. Do not let the clump or pieces of the clump dry out too much before they are back into the ground.