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Dividing perennials

Susan H. Barrott

A perennial border packed with interesting leaf textures and colorful, fragrant blossoms is truly a treat for the senses. Unfortunately, overcrowded plants must struggle to compete for nutrients and water while restricted air circulation creates conditions favorable to many foliar diseases. Perennial plants vary widely in their growth habits and rate of growth. Some grow slowly and take a long time to become established while others grow so quickly that they soon threaten to take over the garden! Division is a useful technique to help keep your perennial border neat, healthy and in peak bloom.

The vast majority of perennials respond best to being divided in early spring, just as new growth is emerging. The weather is cool and there is usually adequate moisture in the soil. Root systems are full of stored energy to help the divisions recover from being cut apart and replanted. Very small leaves and shoots are likely to suffer less physical damage than fully emerged growth and will lose less water through leaf evaporation. Spring divisions also have an entire growing season to recover from the stress of division and to develop the strong root system needed to survive Minnesota's harsh winters.

There are two basic methods of dividing perennials. One is to cut them apart, the other is to pull or tease them apart. You will be able to determine which method to use by examining the type of root system and growth pattern of the plants you are dividing. Start by digging up the entire clump. Tough, dense roots like those of peony will need to be cut apart with a sharp knife or spade. Tangled, fibrous roots or clumps with small plantlets growing at the edges of a large clump can be separated by hand or by inserting two garden forks placed back-to-back then pulling them apart.

Discard the woody old centers of plants along with any sections that have soft, rotted root segments. The younger roots and offshoots growing at the edges of the clump have more "vigor"; they'll recover faster after division and are more likely to produce strong, healthy new plants. Each new division needs to have two or three new shoots and a good segment of healthy roots. Plant the new divisions at the same depth that the old plant was growing; water them in well and keep the soil adequately moist for several weeks while new roots are forming.

Division guidelines

Many perennials tend to die out from the center if not divided on a regular basis. The roots in the center of the clump become so densely overcrowded that they can't take up enough nutrients and water from the reduced amount of available soil. Plants that have died-out in the center, as well as plants that have fewer or smaller flowers than in previous seasons, need to be divided. This rejuvenates aging plants and may extend their life span in the garden.

The frequency recommendations listed are intended as general guidelines for maintaining the health and vigor of perennials under average growing conditions. Gardens that are watered, weeded and fertilized on a regular basis may have plants that need to be divided more frequently to keep them contained within their allotted space.

Divide in early spring: Every 1-3 years

Divide in early spring: Every 3-5 years

Divide in early spring: Infrequently, 5-10 years

Divide in late summer or early fall

*Can also be divided in early spring.

Do not divide

Divide only to propagate

H140P – 8/99

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