When an established perennial produces fewer flowers, or the center of the plant looks sickly while the margins thrive, it could be time to divide the plant. Or even if the plant is healthy, perhaps you'd like to share it with a friend by splitting off a piece. For one reason or another, most gardeners will need to divide their perennials at some point.
The best time to divide plants depends on your region. In cold regions, early spring is usually the best time. The new divisions will have a more time to become established before the challenges of the long, cold winter. On the other hand, in climates with mild winters and hot summers, high temperatures pose a harsh threat, so fall may be a better time to divide, giving plants the mild winter to get established. Here are some guidelines for dividing perennials.
Prepare the Plant. Choose a cool, cloudy day to divide and replant. If the ground is dry, begin by thoroughly soaking the soil around the plant. Let it drain while you gather your tools: shovel, garden fork, pruners, and sharp knife. Then trim back the leaves or stems to 6 to 8 inches to make handling easier.
Dig In. You have two options when dividing perennials. You can dig up the whole clump, then separate it into several smaller clumps, or you can simply use a shovel to slice down and remove sections of the plant, leaving part of it intact. Generally, it's better to dig up the whole clump and carefully split it into sections.
Divide the Plant. Place the entire clump on a tarp in a shady spot, and check to see if any sections naturally split off. Some perennials have roots that are easy to separate; others are such a tangled mass that you'll need to tease or pry them part using two garden forks back to back. Still others have fleshy roots that you'll need to slice into sections. Prune away dead and damaged tissue, and make sure each section has a portion of roots and leaves.
Replant the Divisions. If you are giving the divisions away, place them in containers and pack moist soil around the roots. Water them and keep them in a cool, shady spot. Plant divisions as soon as possible. When replanting in the holes you've prepared, set the plants at the same depth they were in the original bed. Water the new divisions well, and keep them well watered throughout their first year.
Some plants don't need dividing and resent having their roots disturbed. These include monkshood (Aconitum), false indigo (Baptisia), bleeding heart (Dicentra), lupine (Lupinus), peony (Paeonia), and poppy (Papaver).
If possible, divide perennials just before a rainy spell is predicted. Cool temperatures and gentle moisture will help the plants recover from the stress of dividing and replanting.
Photography by Sabin Gratz/National Gardening Association