Clumping perennials grow from a central clump – called a crown – which grows bigger each year producing offsets. They also develop an extensive root system that benefits from division every two to three years to prevent problems associated with overcrowding.
To divide clumping perennials, dig up the entire plant starting a good distance from the center to minimize root damage. Once you have dug up your clumping perennials, loosen the roots to determine where to make divisions. Often there will be obvious divisions; sometimes you will have to exercise your best judgement. Always make sure there is a healthy amount of foliage and root system for each division. The roots of clumping perennials tend to be thick and fleshy, so a soil knife, clippers, or other tool might be necessary to cut through the root system.
Hostas, daylilies, guara, astilbe, and many ornamental grasses are examples of clumping perennials.