The simple rules of thumb for dividing perennials are based on the season in which the plants bloom. Fall-blooming perennials, such as rudbeckia, should be divided in the spring, while spring-blooming plants are best split up in the fall. Those that bloom in the summer, like the purple coneflowers and 'Stella d'Oro' daylily, can be divided in either spring or fall. There are some exceptions, including Siberian irises, which should only be moved after they have bloomed, giving them time to store nutrients for the next year.
Dividing perennials is not difficult. Begin when the plants have a fair amount of leaf growth, which will allow them to recover after the stress of being moved.
1. Use a hand trowel or a small spade to dig out a small group of plants. The tool's blade should be angled straight up-and-down to minimize damage to the root system.
2. Pry the clump up delicately from below, and lift it out.
3. Shake free some of the excess dirt and divide the plants further, laying them out on a tray or a piece of burlap.
They should be replanted to the same depth that they were previously, about four inches apart. Firm up the soil around them and water well. Don't forget to refresh the disrupted area in your garden with a little well-rotted compost. Fork the compost in and smooth it out to the level of the surrounding bed.