Dividing Perennials

A certain amount of time and energy is required to create and maintain a visually appealing perennial garden. Results obtained from dividing perennials are well worth the time invested.

By dividing perennials, a gardener can control a plant’s size, rejuvenate plant growth, and increase the number of desired plants. Signs that a perennial needs to be divided include: a smaller-sized flower than usual; a hole or dead space in the center of the plant; less vigorous plant growth or the plant spreads beyond its intended space in the garden; and/or bottom foliage is sparse.

It is important to note that some plants do not respond well to division and others need to be divided only if you wish to increase the number of plants. A list of both types of plants has been included at the end of this fact sheet.

When Should Plants be Divided?

Certain sources recommend dividing spring- and summer-blooming perennials in the fall, while other sources suggest dividing spring-blooming perennials in April or May, right after the plant finishes flowering. Regardless of which method is used, dividing the plant when it is not in bloom allows all the plant’s energy to go to root and leaf growth.

Dividing fall-blooming plants in the spring when new growth is emerging is best. During this time of the year the weather is cool and there is usually enough moisture in the soil for adequate root growth and development. The root system’s stored energy will help the divisions recover from being cut apart and replanted. Fall divisions should be made between mid-September and mid-October, allowing for 6 weeks of root growth for plants to become established before the ground freezes.

Preparing to Divide

Plants that are to be divided should be well watered a day or two before you actually plan to divide them. Before you lift the plant, prepare an area where you plan to place the new divisions. Make sure that the area is well drained with good fertility and a pH between 6 and 7. You may wish to add some organic matter to improve soil texture and water-holding capacity. Dig the hole wide enough to accommodate the roots with room to spare and deep enough that the crown of the plant will be even with the soil surface.

When you are ready to divide the plant, cut back stems and foliage to 6 inches from the ground. Use a spading fork to dig deep on all four sides of the plant, pry underneath with the tool and lift the whole clump. Shake or hose off loose soil.

Dividing

Perennials have several different types of root systems and each root system needs to be treated differently when dividing.

Spreading Root Systems

Plants that have spreading root systems with many matted roots, such as grasses and tickseed, can be pulled apart by hand or cut apart with a sharp knife. Large plants may need to be separated by placing two digging forks back-to-back and prying them apart.

Divide the plants into individual clumps with three to five shoots. Discard the center of the clump if there is a hole or dead space.

Clumping Root Systems

Plants that have clumping root systems such as daylilies and hostas can be divided by cutting through the crown with a heavy, sharp knife. These can also be pried apart with back-to-back digging forks. Keep several eyes (buds) with each division. If you wish to have more plants, you can divide to one eye (bud). Remember these smaller plants may not flower the following year or two until they become better established. Plant the new divisions at the same depth the old plants were growing.

Rhizomes

Some perennials have root systems known as rhizomes. Rhizomes are stems that grow horizontally at or above ground level. Bearded irises are the most commonly known plants with this type of root system. Cut out and discard rhizome sections that have been damaged by insects or disease. Each division should retain a few inches of the rhizome and one fan of leaves that has been trimmed back by one half. Rhizomes should be replanted with the top showing just above soil level.

A partial list of perennials and their division requirements follows:

Divide in Early Spring

Every 1-3 Years

Aster

Garden Mums (Chrysanthemum, formerly Dendranthema x grandiflora)

Beardtongue (Penstemon)

Golden Marguerite (Anthemis)

Beebalm (Monarda)

Hollyhock (Alcea)

Carnation (Dianthus)

Painted Daisy (Tanacetum)

Carpathian Harebell (Campanula)

Perennial Fountain Grass (Pennisetum)

Coralbells (Heuchera)

Shasta Daisy (Leucanthemum)

Cornflower (Centaurea)

Spiderwort (Tradescantia)

Fernleaf Bleeding Heart (Dicentra eximia)

Tall Phlox (Phlox paniculata)*

Fernleaf Yarrow (Achilla)

Tickseed (Coreopsis)

Foamflower (Tiarella)

Every 3-5 Years

Astilbe

Gay Feather (Liatris)

Black-Eyed Susans (Rudbeckia)

Purple Cone-Flower (Echinacea)

Blanket Flower (Gaillardia)

Mallow (Malvia)

Catmint (Nepeta)

Sea Thrift (Armeria)

Daylilly (Hemerocallis)

Speedwell (Veronica)

Jacob’s Ladder (Polemonium)*

Every 5-10 Years

Cranesbill (Geranium)

Lungwort (Pulmonaria)*

Goatsbeard (Aruncus)

Meadow Rue (Thalictrum)

Hosta

Meadowsweet (Filipendula)

Japanese Anemone (Anemone x hybrida)

Oxeye (Heliopsis)

Lady’s Mantle (Alchemilla)

Siberian Iris (Iris siberica)*

Divide Late Summer or Early Fall

Asiatic Lily (Lilium)

Bearded Iris

Peony (Paeonia) (divide infrequently)

Do Not Divide

Baby’s Breath (Gypsophila)

False Indigo (Baptisia)

Balloon Flower (Platydocon)

Flax (Linum)

Bugbane (Cimicifuga)

Lupine (Lupinus)

Butterfly Weed (Asclepias)

Monkshood (Aconitum)

Clematis

Russian Sage (Perovskia)

Poppy

Divide Only to Propagate

Bugbane (Cimicifuga)

Tall Sedum (Sedum “Autumn Joy”)

Garden Peony (Paeonia)

Yucca

Red-Hot Poker (Kniphofia)

*May also be divided late summer or early fall.