Division and Propagation Techniques
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.
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, southernwood (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 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.
Number 1 plants such as Iris should be planted with the roots just above the surface of the ground and the rhizome just on the surface.
Number 2 plants such as Peonies should be planted with the tips of the buds just below the surface of the ground - about 1 inch. Peonies will not bloom well if planted too deeply or if deprived of plant food.
Number 3 plants with leaves that spring from the crown should be planted with the crown just above the dirt line.
Number 4 plants with a fleshy root such as hollyhocks should be planted with the tap root straight down and the bud just below the surface of the dirt.