Divide & Multiply: Dividing Perennials

As we become more involved with gardening, most of us begin to add Daylilies perennials to our plantings. They grow well for a few years, perform spectacularly, then perhaps three or so years down the road, we notice that the blooms become fewer or the clump is beginning to take over the space of neighboring plants. Now we are faced with the prospect of dividing perennials! It need not be a daunting task, however, so let's explore some do's and don'ts about dividing perennials.

There are many reasons to divide perennials. Two reasons already mentioned were because of dwindling performance and space issues. There are other reasons as well, so I will list some of the most common reasons for dividing perennials.

Top Reasons For Dividing Perennials

Dwindling Performance: Perhaps you have a clump of daylilies or bearded iris that you planted about five years ago. It is large and healthy, but this summer you noticed that the clump is not blooming as well as it has in previous years, despite cultivation, fertilizing and watering. You realize that it is time to divide the clump and reset the divisions.

Reducing the size of the clump: If you have Rudebeckia growing in your yard, you can relate to how this plant can sometimes migrate and grow into surrounding plantings. You simply need to reduce the size of the clump to give your other plants room, or to make space for new plants.

Dividing for the purpose of obtaining more plants: It certainly is cheaper to divide perennials that are already doing well in your garden and to spread them around to new beds or other positions in your border than to buy new plants.

Moving: Yes, we do move to new homes, so it is good to bring some of our favorite plants along with us. If we have a really large clump of plants, dividing them will lessen the stress of moving them to a new location by reducing the size into manageable transplants rather than by bringing a monster-sized clump along with us to a new location. Smaller divisions simply suffer less transplanting stresses than large-size clumps.

How To Tell When A Perennial Needs Dividing?

Generally, if the performance of a plant has dwindled, if the center of a plant is becoming "bald" or the plant is flopping over, or if it has outgrown its bounds, it is time to divide the plant.

The Million Dollar Question: When Should Perennials Be Divided?

That depends upon the perennial and what climate you live in. Here in the southern Great Lakes region, a general rule of thumb is almost anything can be divided in the spring, period. This general answer will mean that your spring and summer blooming perennials will most likely not bloom well if divided in the spring, but they will grow a vigorous root system and survive. A more refined answer for our region would be that plants that bloom in the late summer to fall should be divided in the spring, while those that bloom from spring throughout the bulk of summer should be divided in the late summer into fall. Generally, that rule of thumb will work. If spring and early- to mid-summer blooming perennials are divided by no later than mid-September, most perennials will survive the winter into spring if care is taken to protect them throughout the winter. Anything planted after mid-September might not develop enough of a root system to pull through the winter, even with protection. Exception: Daylilies and Iris should be divided after they have finished blooming. Although neither will suffer from a spring dividing, they will lag for perhaps a season longer before bouncing back.

The Two-Million Dollar Question: What Should Be Divided?

Most perennials will benefit from a reduction in size, and this maintenance will help to maintain the performance and vigor of your plants.

The Three-Million Dollar Question: Which Plants Resent Dividing? There are a few perennials that resent dividing. Dividing them can result in a serious setback as they re-establish. For example, they might not survive the division due to long taproots that are broken off in the process, or they are actually small shrubs. Here is a list of those that divide poorly, should only be divided by those who are experienced, only under exceptional circumstances, or should not be divided at all:

Peonies Aconitum Lupines

Oriental Poppy Baptista Gysophila

Lavender Dictamnus Russian Sage

Bleeding Hearts Bugbane Balloon Flower

Asclepias Flax Gentian

Evening Primrose Santolina Candytuft

Columbine Hellebores Euphorbia

A Baker's Dozen Of Common Perennials:
When To Divide, How Often, & Other Comments

Plant Name When To Divide Additional Comments

Astilbe Early Spring or Early Fall: Every 2-3 Years Plant Outside Growth, Discard Center

Aster Spring: Every 2-3 Years Plant Outside Growth, Discard Center

Blanket Flower Spring: Every 2 Years Divide Clump Into Smaller Plants

Beebalm (Monarda) Spring or Fall:
Every 2-3 Years Divide Clump Into Smaller Plants

Bearded (German) Iris July to Late Aug:
Every 3-4 Years Discard Center Woody Rhizome

Black Eyed Susans Spring: Every 3-4 Years Divide Clump Into Smaller Plants

Chrysanthemums Early Spring:
Every 1-2 Years Plant Outside Growth, Discard Center

Coreopsis Spring or Fall:
Every 1-2 Years Plant Outside Growth, Discad Center

Daylilies Anytime, mainly after bloom: 2-3 Years Fleshy Root (Tuber) Segments

Hostas Spring or Fall:
Divide Infrequently Take Divisions With 3 to 4 Eyes

Creeping Phlox Spring: After Bloom
Spring or Fall Divide Clump Into Smaller Plants

Tall Garden Phlox Spring Or Fall:
Every 3-4 Years Divide Clump Into Smaller Plants

Ornamental Grasses Spring Only:
Every 3-4 Years Use Saw to Cut Through Clumps

Of course, there are many other perennials not even mentioned here. One good way to learn more about when, how, and how often to divide perennials is to get a good gardening book with region-specific information. For instance, a book geared towards gardeners in Southern California is not going to give the best information regarding when to divide plants in the southern Great Lakes region. Another good option is to contact your county or state extension agency for any publications they might have on when and how to divide perennials. Also, don't overlook the wonderful and often overlooked resources, such as that guy next door who has been gardening for forty-odd years or the guys and gals down at the landscaping and nursery place around the corner! Trust me, I have learned more about how to divide and multiply my perennials by talking to long-time gardeners and local nursery people than almost anywhere else! Above all else, don't let the process of dividing plants intimidate you. Chances are, you will do just fine! Nature is very forgiving, and most plants are very resilient, so, keep that in mind