Photo of Tree Peony and article by Joyce Rapp, Goodhue County Master Gardener
There are three good reasons to divide perennials: to rejuvenate and stimulate new growth, to control the size of the plant and to multiply plants for your garden or share with others.
Spring and summer blooming perennials are best divided about 4-6 weeks before the ground freezes in the fall. Fall blooming perennials are divided in the spring just as new growth emerges.
Disturbing the root system challenges a plant’s ability to feed and hydrate itself. Choose a cloudy or overcast day to divide your plants. Water the soil well a day in advance. If there is abundant top growth, cut back the leaves by a third or tie the stems together for easier handling.
Minimize root damage by first removing soil around at the drip-line and then digging a trench around the clump by cleanly severing any roots. Remove the loose dirt around the roots and separate the plant into smaller divisions. Gently tease roots apart or cutting them with and a sharp knife or spade. Aim to keep the soil intact around the root ball. Densely rooted plants such as day lilies can be more easily separated by placing 2 pitchforks back to back in the center of the clump and pulling them apart.
Keep your divisions shaded and moist until they can be replanted. Ideally, prepare the transplant hole before you divide your plants to reduce stress on the root system. To avoid crowding, place a division into a hole that is at least as wide as its roots when spread out. Fill hole with soil and organic matter. Add mulch around your plants to maintain moisture.
If you must re-plant later, keep plants in the shade, at about 50% humidity and in cool temperatures. Place divided perennials you are planting later in a container. and cover with moistened newspaper, to prevent them from drying out. Whatever method you use, treat your divisions as new seedlings and keep them well watered until new growth appears.
This is one of three articles on perennials based on information by: Janet Macunovich: Ten Tips on Dividing Perennial Plants http://www.finegardening.com/how-to/articles/dividing-perennials.aspx and Perennials for Minnesota and Wisconsin (Don Engebretson and Williamson.)