Dividing Perennials- When... How... Why...
CLEMSON COOPERATIVE EXTENSION
The three main reasons for dividing perennials are to control the size of the plants, to help rejuvenate them, and to increase their number. Dividing and replanting keeps rapidly spreading perennials under control. Dividing will rejuvenate old plants, keeping them vigorous and blooming freely. Dividing perennials is an easy and inexpensive way to gain additional plants for your garden or to share.
When to Divide
In general, it is best to divide spring and summer blooming perennials in the fall, and fall bloomers in spring. By dividing the plant when it is not flowering, all the plant's energy can go to root and leaf growth.
Fall division should take place between early September in the uppermost Piedmont and mid-to late October on the coast. Allow at least four to six weeks before the ground freezes for the plants to become established.
If you divide in the spring, allow enough time for roots to settle in before hot weather. Spring division is ideally done in the early spring as soon as the growing tips of the plant have emerged. Spring divided perennials often bloom a little later than usual.
Never divide perennials on hot, sunny days. Wait until a cloudy day, ideally with several days of light rain in the forecast.
Most perennials should be divided every three to five years. Some perennials such as chrysanthemums and asters may need to be divided every one or two years or they will crowd themselves into non-flowering clumps of leaves and roots. Bleeding hearts and peonies may never need
to be divided unless you want to increase your stock.
Signs that perennials need dividing are flowers that are smaller than normal, centers of the clumps that are hollow and dead, or when the bottom foliage is sparse and poor. Plants that are growing and blooming well should be left alone unless more plants are wanted.
Prepare to Divide Ahead of Time
Water plants to be divided thoroughly a day or two before you plan to divide them. Prepare the area that you plan to put your new divisions in before you lift the parent plant.
Prune the stems and foliage to 6 inches from the ground in order to ease division and to cut down on moisture loss.
Lift the Parent Plant
Use a sharp pointed shovel or spading fork to dig down deep on all four sides of the plant, about 4 to 6 inches away from the plant. Pry underneath with your tool and lift the whole clump to be divided. If the plant is very large and heavy, you may need to cut it into several pieces in place with your shovel before lifting it.
Separate the Plant
Shake or hose off loose soil and remove dead leaves and stems. This will help loosen tangled root balls and make it easier to see what you are doing.
Perennials have several different types of root systems. Each of these needs to be treated a bit differently.
Spreading Root Systems
Spreading root systems have many slender matted roots that originate from many locations with no distinct pattern. Plants with spreading root systems include asters, bee balm, lamb's ear, purple cornflowers and many other common perennials. These can crowd out their own centers. Some can be invasive unless divided frequently. They can usually can be pulled apart by hand, or cut apart with shears or knife.
Large, vigorous plants with thickly intertwined roots may need forceful separation with digging
forks. Put two forks back to back in the center of the plant and use them to pry the pieces apart.
Divide the plants into clumps of three to five vigorous shoots each. Small or weak and woody divisions should be discarded. Discard the center of the clump if it is weaker than the outside edges.
Clumping Root Systems
The clumping root systems originate from a central clump with multiple growing points. Many have thick fleshylmage depicts the use of a heavy, sharp knife being used to pry apart thick fleshy crowns. roots. This group includes astilbes, hostas, daylilies and many ornamental grasses.
It is often necessary to cut through the thick fleshy crowns (the central growing area between the roots and the leaves and stems of the plant) with a heavy, sharp knife. You can also pry apart these roots with back to back digging forks.
Keep at least one developing eye or bud with each division. If larger plants are wanted, keep several eyes.
Rhizomes are stems that grow horizontally at or above the soil level. Bearded irises are the most common perennial with this Image depicts where to cut rhizomes during division.type of root system.
Divide irises any time between a month after flowering until early fall.
Cut and discard the rhizome sections that are one year or older. Also, inspect rhizomes for disease and insect damage. Damaged rhizomes should be trimmed and treated, or discarded if too badly damaged.
Iris divisions should retain a few inches of rhizome and one fan ofl eaves, trimmed back halfway.
Replant with the top of the rhizome just showing above soil level.
Dahlias are an example of perennials with tuberous roots. The tubers should be cut apart with a sharp knife. Every division must have a piece of the original stem and a growth bud attached. After division they can either be replanted or stored for spring planting.
Dividing Large, Tough Roots
If the root mass is very large, or tight and tangled, you can raise the clump 1 to 2 feet off the ground and drop it. This should loosen the root mass, and you can pull the individual plants apart. This is not a good method for plants with brittle roots such as peomes.
Plants that have very tough, vigorous root systems (agapanthus, red-hot pokers and ornamental grasses) may have to be divided with a shovel, saw or ax. You can also vigorously hose off soil to make the root system easier to work with.
Don't Divide These Perennials
Some plants resent being divided and it should be avoided if possible. These include butterfly weed (Asclepias), euphorbias, oriental poppies, baby's breath (Gypsophila), gas plant (Dictamnus albus), Japanese anemones, false indigo (Baptisia) and columbines (Aquilegia).
Lenten and Christmas roses (Helleborus) are very difficult to move when more than a few years old. Usually you can find tiny seedlings around the base. These are easy to move.
Lavender cotton (Santolina chamaecyparrus) and several other perennials are actually small woody shrubs and should not be divided. These include perennial candytuft (Iberis sempervirens), lavender, rosemary, southemwood (Artemesia abrotanum), and several other artemesias. These plants often have rooted layers (branches that have developed roots while touching the soil). The layers can be cut off the parent plant, dug up and replanted as though they were divisions.
Plant the Divisions
Never allow divisions to dry out. Keep a pail of water nearby to moisten divisions until they are planted. Trim all broken roots with a sharp knife or pruners before replanting.
Plant the divided sections immediately in the garden or in containers. Replant divisions at the same depth they were originally. Firm soil around the roots to eliminate air pockets. Water well after planting.
Fall-divided perennials should be mulched in the upstate the first winter to prevent heaving caused by alternating shallow freezing and thawing of the soil. The best winter mulch is loose and open such as pine straw, Christmas tree limbs or leaves.
When and How to Divide Some Common Perennials
Common Name Botanical Name How Often To Divide Season To Divide Method
Asters (Aster) Every year or two Spring Spreading root division. Replant outer growth and discard
the centers of older plants.
Astilbe (Astilbe) Every 2 to 3 years Early Spring or fall Spreading root division. Needs division for
Bearded Iris (Iris) Every 2 to 3 years Spring Rhizome root division.
Beebalm (Monarda) Every 3 years Spring or fall. Spreading root division.
Bellflower (Campanula) Every 2 to 3 years Spring or early fall Spreading root division.
Blackberry Lily (Belamcanda) Every 3 to 4 years after bloom is finished Rhizomes should be treated as iris.
Black-eyed Susan(Rudbeckia fulg;,da) Every 3 to 4 years. Early spring or fall Spreading root division.
Blanket Flower (Gaillardia wandiflora) Every 1 or 2 years Early spring. Spreading root division.
Bleeding Heart (Dicentra) Rarely needs division Early spring Fleshy root division. Be gentle with brittle
Cannas (Canna) Every 3 or 4 years Spring Fleshy root system
or as desired to
Cm:ysanthemurn (Chrysanthemum) Every year or two. Spring. Spreading root division
Cranesbill (Geranium) Every 2 to 4 years Spring or fall Spreading root division
Daylily (Hemerocallis) Every 3 to 6 years Spring, summer or fall Divide fleshy roots or as desired to increase stock. into segments with roots.
Garden Peony For plant increase September/October Divisions should have
rarely needs division. three to five well
Divisions may wait up developed eyes
to 3 years before blooming Plant peonies with
the eyes no deeper than 1" below surface
Garden Phlox (Paeonia) Every 3 to 4 years Early Spring or fall Spreading root (Phlox paniculata) division
Hosta (Hosta) Rarely need division Early spring or early fall. Fleshy root
will reach their best form if division. Divide
not divided too often, can be into clumps with divided as needed for plant increase. 1-3 eyes, a wedge
can be taken from an established plant, which will soon fill back in
Lambs-ear (Stachys byzantina) Every 2 to 3 years. Spring or early fall Spreading root divisions
Discard weak centers
Lily of the Nile (Agapanthus) When flowering slows Spring, summer or fall Fleshy clumping
due to crowding. roots are large and
brittle, do not divide unless needed to improve bloom
Lily of the Valley (Convallaria majalis) Divide for plant increase Early Spring Can be divided as as desired clumps or single "pips".
Ornamental Grasses Every 3 to 4 years Spring for most Very dense fibrous
grasses. Many grasses root system may
do not tolerate fall require the use
division of an ax or saw to divide.
Purple cone-flower (Echinacea) Every 4 years Spring or fall Spreading root division
Red-hot-poker (Kniphofia) Divide only for Spring or fall Fleshy clumping root
plant increase system, divisions may
take 2-3 years to bloom
Tall sedum (Sedum 'Autumn Joy') For plant increase Spring Clump divisions
Tickseed (Coreopsis) Every 1 or 2 years Spring or fall Spreading root division
to maintain vigor Discard weak center
Wormwoods (Artemisia) Every year or two Spring Spreading root division
for spreading wormwoods remove root division
like 'Silver King' and 'Valerie Finnis.' remove excess and dead center. Do not divide woody artemesias.
Yarrow (Achillea) Every 2 or 3 years or Spring or fall Spreading root division
when center dies out
Prepared by Karen Russ, HGIC Information Specialist & Bob Polomski, Extension Consumer Horticulturist, Clemson University. (New 06/99.)