Dividing Perennials- When... How... Why...

July 30, 2017

 

CLEMSON COOPERATIVE  EXTENSION         

 

Dividing Perennials

 

 

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.

 

Rhizome Division

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.

 

Tuberous Roots

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

                                                                                                                    best bloom.

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

                                                                                                                   roots.

Cannas                  (Canna)                 Every 3 or 4 years         Spring                Fleshy root system

                                                        or as desired to

                                                        increase stock. 

 

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.)

 

 

 

 

 

 

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