Master Gardener^sm Column for the Week of September 19, 2005
Carol Weaver
Brunswick County Extension Master Gardener^sm Volunteer

Most perennials decline with age and should be divided once you notice a dead center in the crown area with a ring of growth around it, blooms becoming sparse and smaller, or plants looking obviously crowded. Although many gardening books recommend dividing herbaceous perennials every 2-5 five years, instead of relying strictly on the calendar, consider a plant's shape and condition before deciding to divide it.

Do not divide perennials when they are in bloom. Divide midsummer and late-summer bloomers in the spring when growth is 2-3 inches high. Divide spring-blooming perennials in late August or September when the foliage starts to die back. Plants divided during an active growth period in the summer are slower to become reestablished. Some perennials can be divided following their flowering period even during the summer, including daylilies and bearded iris.

Transplants from a mature perennial clump’s center often grow poorly and bloom sparsely. Select only the vigorous outer edges of the clump and discard plants from the center. Divide the plant into clumps of three to five shoots each. Do not put all the divisions back into the same space that contained the original plant as more room is needed for the plants to spread and bloom well. Share extra plants with a friend, plant them elsewhere in the yard, or discard them.

Prepare the area for planting your new divisions before you lift the parent plant. Water the plants to be divided thoroughly a day before you divide them. Lift the clump with a fork or spade and then tease the roots apart with your fingers. Make sure each piece has its own root system.

When using a spade to divide, lay the root ball on its side and position the spade in the center of the root ball’s crown. With a quick jabbing movement, split the crown in half, and repeat the process until each piece contains three to five shoots. Replant the resulting divisions at the same depth as the original plant, making sure the crown is slightly above the soil level. This method works well for plants with fibrous root balls such as hosta.

As an alternative to using a spade, insert two pitchforks back to back into a clump’s root ball to divide it. By slowly drawing the handles away from each other, the root ball can be pried apart.

Some perennials, such as astilbe and liriope, form tough root systems that can't easily be divided with a spade or pitchfork. For these, try a short (8-inch long) handsaw to cut the root system apart. Each section should contain a piece of the woody root and growth points. When preparing to divide iris, cut the leaves to one-third their length. Dig the clump and hose off the soil. Cut rhizomes apart so that each section has at least one healthy fan of leaves and firm, white roots. Older rhizomes may seem firm but should be discarded since they have limited flowering capacity.

In addition to iris, canna and agapanthus are examples of rhizomatous species, while dahlia and some anemone species are tuberous plants. Once you get the rhizomes or tubers out of the ground, shake or wash off the soil so you can see their structure. Use a sharp knife or pruning shears to cut rhizomes and tubers into pieces that contain at least one growth point or dormant bud and are 2-4 inches long.

Replant only plump and healthy-looking rhizomes and tubers and discard those that are old, withered or diseased. Rhizomes should be planted no deeper than half their width. A 1-inch wide rhizome should be buried about 1/2 inch deep. Tubers should be planted in the soil with the growth point or dormant bud just peeking out of the ground. Arrange rhizome and tuber divisions 10-12 inches apart to give them room to expand.

To divide perennials, dig the entire plant and separate it into sections, or take divisions from the edge of the clump while it is still in the ground. Ornamental grasses have tightly tangled, tough root systems that are difficult to divide. Do not hesitate to cut through a large clump of roots with a sharp ax.

Woody plants are a little harder to separate. After you lift them, wash the crowns to reveal the growth buds. Cut through the crown with a sharp knife leaving at least one healthy bud and root, then discard the woody center growth.

You may have use even stronger measures with really tough perennials. Place one foot on the side of the clump to steady it, then stick a crowbar in the clump and lever out portions for replanting. If the root mass is very large, or tight and tangled, you can raise the clump 1-2 feet off the ground and drop it. This should loosen the root mass so you can pull the individual plants apart. This is not a good method for plants with brittle roots such as peonies.

Keep a pail of water nearby to soak any divisions that have lost the soil around the roots. Trim broken or straggly looking roots to stimulate the fibrous feeder roots to grow. Plants with large amounts of foliage may need to have some leaves pruned after they are divided since there are fewer roots to sustain a large leaf system.

Remove any weeds and dead leaves from the new plants before you replant them as they may harbor diseases and provide hiding places for slugs and insects. If you are unable to replant right away, put the smaller divisions in a pot in a shady spot.

Some perennials should be rarely divided or moved unless absolutely necessary. These include asclepias (butterfly weed), euphorbia, gypsophila (baby's breath), oriental poppies, and columbine.

Send your gardening questions or comments to: Brunswick County Master Gardener Column, P.O. Box 109, Bolivia, NC 28422, or call (910) 253-2610. Enclose a self-addressed stamped envelope if requesting information or a reply. Answers may be printed in this column.

For further information or assistance, please e-mail:
Charlie Spencer, Brunswick County Extension Master Gardener^smVolunteer

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Date Created 9/16/2005