Coneflowers (Echinacea) can be divided. Photo credit: Lois Miklas
By definition, plant division is a form of vegetative propagation where each divided piece is capable of producing roots and shoots. This important gardening job is not as daunting as you may think; just follow a few principles that answer the questions: why, when, and how.
Why Divide Perennials
When you see that a robust perennial is beginning to crowd out surrounding plants, you know it requires division. It has sent up new shoots and grown new roots causing competition for light, water, and nutrients. This leads to a weakening of the overall plant with less, or smaller flowers and sometimes yellowing leaves. Another sure sign that you need to divide a plant is when its center begins to die out.
When to Divide Perennials
Don’t wait until the plant is going downhill; divide it while it still looks good. Divide the perennial when it is not in bloom, so it can focus its energy on root and leaf growth. Spring and autumn are the recommended times, as those are the seasons when plants tend to establish new roots. Some gardeners advocate dividing spring-bloomers in fall and all other flowering plants in spring. In areas where spring may be late and short, it is preferable to dig and divide in September, allowing four to six weeks for the plant to become established before the ground freezes. This also gives more time for the transplants to develop new root systems before the summer heat. It is not a good idea to divide your plants during the heat of summer. The exception is bearded iris that goes dormant in summer, making August the best time for its division. How often should you divide plants? The general rule of thumb is to complete this task every three years, but I only divide a plant after careful observation of its shape and condition, believing the old adage that ‘if it ain’t broke, don’t fix it.’ With a little planning, you can stagger your perennial divisions over several years to avoid becoming overwhelmed.
How to Divide Perennials in the Fall
Follow the step-by-step plan described below. You will need to consider the type of root system of each plant you are dividing.
Useful tools to have on hand include: shovel, spade, garden forks, large sharp knife, pruners, shears, and gloves. For tough root systems, you may want to use an eight-inch-long handsaw.
A day or two before you plan to divide your perennials, water them thoroughly. Choose a cloudy day, preferably with rain in the forecast—not hot and sunny—for the digging-up task. Before you begin, prepare a well-drained area where you intend to put the divisions. Make sure the hole is more than wide enough to accommodate the roots. Add some organic matter if you wish. Before digging up the plant that is to be divided, first cut down its stems and foliage to six inches from the ground. This will make it easier to remove and you will reduce moisture loss.
3. Digging Up
Dig about four to six inches away from the plant, at its natural drip line. At an angle, cut down under the clump from several points around the edge. Lever the plant out of the hole. Spray with a hose, or shake, to remove loose soil.
To divide the clump into smaller sections, you will need to treat each root system differently:
- Spreading Root Systems. These plants include tickseed (Coreopsis), aster, beebalm (Monarda), lamb’s ear (Stachys), and purple coneflower (Echinacea.) Their roots are intertwined and may be matted. Pull the smallest of them apart by hand or cut them into sections with shears or a sharp knife. You can forcefully separate large plants by inserting two forks back-to-back in the center of the root ball and slowly draw the handles away from each other. The divisions should each contain three to five vigorous shoots. Discard small, weak, woody divisions and the center of the plant if it is dead or weaker than the outside.
- Clumping Root Systems. Examples are astilbe, daylily (Hemerocallis), hosta, and many ornamental grasses. This type of root system originates from a central clump with multiple growing points. Divide by cutting through the crown with a sharp knife. Use back-to-back forks if necessary. Keep at least one eye (bud) with each division, or several if you want larger plants.
- Rhizomes. Bearded irises are the perfect example. Their roots (technically stems) grow horizontally at or above ground level. To divide rhizomes, cut them with a sharp knife, discarding parts damaged by insects or disease. Each section should contain a few inches of rhizome and a fan of leaves.
- Tuberous Roots. Examples of plants with tuberous roots are dahlia and canna lilies. In most of Pennsylvania, they need to be dug up in the fall and stored in a frost-free area until the spring. Use a knife or pruners to slice the root mass into sections, each containing at least one bud.
5. What Not to Divide
Do not divide shrub-like perennials such as lavender (Lavendula) and Russian sage (Perovskia). Their single woody bases are not amenable to splitting. Leave untouched butterfly weed (Asclepias) and other plants with a single taproot. Euphorbias, false indigo (Baptisia), baby’s breath (Gypsophila), lupine (Lupinus), clematis, and columbine (Aquilegia) are more examples of perennials that resent being divided.
6. Planting the Divisions
Do not allow your divisions to dry out. You may moisten them with water (keep a pail handy) until they are planted. Plant them as soon as possible in the prepared holes in the garden or in containers. You should plant them at the same depth that they were originally. Don’t forget rhizomes should have the top showing just above soil level. Water well after firming the soil. Because I divide my perennials in the fall, I use mulch to prevent the heaving caused by alternating freezing and thawing. A loose mulch such as straw is suitable for winter.
Don’t fertilize your new plants; give them time to grow new roots. Water when needed, being careful not to overwater as this promotes root rot and fungal growth. Provide temporary shade if there is unexpected hot sun.
Dividing perennials is an easy way to rejuvenate them and control their size. You will gain additional plants that you can use to start a new flowerbed, to fill in a space, or to give to a friend. Completing this task always gives the gardener a great feeling of accomplishment.