We've all heard that many perennials should be divided or can be divided. But when is the right time, and how do you go about it? Taking a garden spade after a beautiful clump of flowers is intimidating! We'll discuss why we divide perennials to help you understand when to divide them and a few strategies for doing so successfully. 

Gardeners divide perennial plants for two main reasons: propagating them (getting more plants) and improving their health and vigor. For example, a patch of bee balm (Monarda spp.) may be divided in spring, and the new clumps planted elsewhere to give you more bee balm without having to buy new plants at the garden center. Stonecrops (Sedums) develop faded, open centers and floppy stems as the mother plant ages. Dividing it will give each new division room to grow, restoring vigor and growth. 

Perennials can get overgrown, and while in nature, they would merely keep spreading out while the center fades, in our gardens, we desire a more aesthetically pleasing look. For most perennials that benefit from dividing, a 3–4 year cycle is usually sufficient. You won't need to do it every year unless you are dividing for propagation purposes.

Of course, there are some perennials we don't divide. For the most part, plants with woody stems, like roses or rosemary, aren't divided. Plants with taproots are also not divided. Some, like peonies, we only divide if desired for propagation. They'll be fine left undivided otherwise.

Should I Divide Perennials in Spring or Fall?

Choosing to divide in spring or fall is based largely on when the plant flowers. As a rule, perennials that bloom in spring and early summer should be divided in autumn, and those that bloom in autumn can be divided in spring. 

Dividing perennials during a season when the plant is not flowering allows it to direct all energy toward new growth, both below and above ground. I prefer dividing in spring for another reason, and that's because I have a tender heart. Hacking away foliage and digging up my plants makes me feel bad for them (I know), but in spring, when they are just starting to wake up, it doesn't seem so bad.

Dividing in spring should be done when the plants are dormant or just breaking with new growth. When dividing in fall, do so 4–6 weeks before the ground freezes in your area (several weeks after your first fall frost). Getting them in the ground a month or a little more before the ground freezes lets them put out new root growth before winter.

Examples of perennials commonly divided in spring include artemisia, bee balm, delphiniums, ornamental grasses, sedums, and yarrows, along with many others. 

Perennials best divided in fall include peonies, alliums, globe flowers, Jack-in-the-pulpits, and other early spring-blooming plants. 

Many perennials like black-eyed Susans can be divided in spring or fall, whichever is more convenient for you. When figuring which season to divide, check this list from the University of Minnesota extension. It’s not exhaustive, but will get you started. 

How to Divide Perennials

Whether in spring or fall, try to divide plants on a mild-weather day. Avoid doing so in the middle of blistering hot, sunny weather. Keep the newly divided plant roots moist, and never let them dry out. It's normal for a newly divided plant to look a bit wilty for a few days. Keep it watered and talk nicely to it; it will perk up.

Dividing perennials can be thought of as four steps:

1. Preparing the plants for division.

2. Getting them out of the ground.

3. Dividing the root and plant material into new plants.

4. Replanting.

Preparing Perennials for Division

While I have been known to spy a clump of one perennial or another and decide to grab the shovel and divide it right there and then, it's better to prepare the plants a bit in advance. The division is quite traumatic, and they'll do better with some preparation.

Water perennials thoroughly a day or two before dividing. You'll be damaging some roots during the dividing process, and the plant should be at top water levels before doing so. 

Prepare the area where the new divisions will go. Once you've separated the rootball into new pieces, you want no delays in getting them back in the ground. New clumps can easily dry out in a matter of hours, so get them in the ground as soon as possible

For larger plants, prune the stems and foliage down to 6–12" from the ground. Of course, the exact size depends on the plant—don't remove every leaf and stem. While harsh, pruning the plant down will not only ease your task of division but reduce moisture loss while the plant reestablishes its root systems. 

Lifting the Entire Plant

Usually, when dividing perennials, we lift the entire plant. If your reason for dividing is to restore vigor and plant health, this is the way. Lifting is gardener speak for digging all around and then giving it a heave-ho, setting the whole plant and root ball next to the hole it came from so you can work on it.

Begin working your tool into the soil 4-6 inches away from the edge of the plant. Use a garden spade for larger clumps. Small plants can often be lifted with a hand trowel if your soil is loose. 

Gently work the spade into the ground all the way around the plant. You'll probably cut some roots. Once it is loosened, lift the entire clump. For large plants, you may want a helper. The idea is to take about as big a hunk as you can all in one go. Excess soil can always be returned to the hole. 

Dividing only a portion of a plant

Some perennials can be divided to make new plants without lifting the entire mother plant, a method best suited when dividing only for propagation reasons. 

The previous mention of dividing bee balm is a perfect example. As the original plant spreads, new stems and root mass will grow and the size of the plant increases. Instead of lifting and disturbing the entire plant, I grab a healthy chunk from the edge. 

A spade can be used to slice cleanly down and then lift a small portion while not disturbing the rest of the mother plant’s roots. Ensure the new portion has some stems and roots, and plant it immediately in its new home. 

Techniques for Dividing Different Root Systems

Once the plant has been lifted, give it a good wiggle and shake to knock off some of the excess soil. Brush soil off the roots with your hands as best as possible until you can see the root mass. You'll notice the roots have a pattern: spreading, clumping, or rhizome.

Spreading root systems have many roots running in no distinct pattern and from no identifiable central origin. Bee balm, rudbeckia, and many other perennial flowers have a spreading root system. These plants can be pulled apart by hand wherever they seem ready to split or cut apart with the soil knife or spade. 

Clumping root systems are identifiable by the central growing point or clump of roots which then go out in all directions. Hostas, ornamental grasses, and daylilies follow this pattern. Cut through the crown with a soil knife or shears, and keep one or more buds with each new division. 

Rhizome root systems, like those of bearded irises, cannas, and many hops, are easy to divide. Basically, you cut off a piece of the rhizome and its associated stem and replant it. It's similar to cutting a potato into pieces with an eye and planting them. Use hand pruners to make clean cuts. 

Whatever the plant's root type, your goal is to divide it with the least amount of damage and disruption possible. Tease and pull clumps apart by hand when the plant will let you, and cut and sever when necessary.  

Tools for Dividing Perennials

You likely already have all the tools you need in your garden tool collection. A garden spade of any shape will work well. Spend a few minutes sharpening your spade, and the process will be much easier. Tools should be clean, and everything ready to do the job. Have your mulch, compost, and stakes ready. 

Garden pathogens are easily transported on equipment, and you don't want to introduce them directly to an open wound in your perennial's root system. Sanitize tools in a 10% bleach solution for 30 minutes or rub down thoroughly with alcohol. For a 10% bleach solution, combine one cup of bleach with nine cups of water. I like to soak the portions which contact soil or plants (not the handle of the shovel) in a plastic tub I use for nothing else. Rinse the tools with the hose afterward to avoid splashing bleach on your pants. For smaller tools like pruners, I find it easier and faster to rub them down with alcohol between plants. 

A soil knife is the perfect hand tool for perennial dividing and can easily accomplish many tasks, like dividing a bit of mint all on its own. For larger root systems, you may need to use a pruner or knife but beware: the grit in your soil will dull it a bit. 

Lastly, a wheelbarrow for larger plants and some mulch will help you finish the job. Don't forget to water both the old plant and the new divisions well after transplanting.