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How to Divide Herbaceous Perennial Plants - Part 2

Updated on May 18, 2012

One of my gardens in July


Why to divide a mature plant

In my last article in this series, I covered the risks of dividing perennial plants, and when you should do so. So, if you risk killing the plant when you divide it, and the timing is so very critical, why would any sane gardener risk doing so?

In some cases, it’s a matter of whether the plant will bloom at its maximum potential or not; in some cases division is done to give the plant a kick in the pants and rejuvenate it; and in many cases the goal is to simply diminish the amount of real estate it’s taking up in the garden. Too much of a good thing is still too much, and a huge clump of any one plant will almost always be better enjoyed once split into several different, scattered clumps.

Iris 'Atlanta Belle'


A crowded plant that no longer blooms well

A good example of the first scenario—a plant that will bloom better if its roots are not crowded—is the notoriously fickle Tall Bearded Iris. All iris propagate by creating new little rhizomes as offshoots to their existing root structures. These baby iris will produce their own leaves and eventually their own flower spikes. If this process is allowed to continue uninterrupted, the rhizomes will force each other either up out of the ground, or else deeper and deeper.

Mature plants compensate for this by the natural cycle of life: as the oldest rhizomes are crowded out by their offshoots, they will, eventually, either wither by being forced completely out of the ground, rot by being forced too deep, or just be strangled out of existence. Stressed by the crowding they are also more vulnerable to disease and pests. These plants are best dug up, lifted from the soil and cut into several individual plants, then replanted further apart, in order to avoid those consequences. Crowded plants do not bloom as profusely, either.

Siberian Iris Clump


Big clumps with bare centers

Another reason for division is well illustrated by a mature clump of Siberian Iris. The offshoots from the mother plants tend to form on the outer side of the clump, and the original rhizomes in the interior begin to die for all the reasons noted above. Eventually, you will have a circular mat of dense plant material with a sort of male pattern baldness situation in the center. These plants can readily be dug up, and chopped with the blade of a shovel into several smaller chunks. Replant one in the original location and stick the others anywhere you want more iris, or call a fellow gardener and offer her some free plants. If done very early in the spring, the new plants won’t even realize they’ve been moved; they will simply wake up the next morning in their new location and grow, grow, grow.

You Will Need A Good Shovel - Here's A Great One!

The majority of herbaceous perennials can be treated this way to good result. Day Lilies, all of the many Campanula, Monarda, Rudbeckia and Echinacea species benefit from periodic division, as do Heleborus (they are poisonous, remember, so wear gloves!), all of the various daisies, perennial herbs such as chives and the vast multitude of mints.

There are some herbaceous perennials that require special care when dividing established clumps. Basically, look at the growth habit of the plant as it emerges in early spring to understand how best to divide it. If the plant is producing a nearly uniform mat of new growth, you can assume its roots are equally well-distributed and what you are looking at is, in essence, many, many plants all bunched together, connected by roots that are “holding hands”: either actually attached to each other, or so intermingled they might as well be. Splitting that kind of clump into pieces poses no risk of harm and can be done quite brutally (chopping through a mature clump of Siberian Iris is not a job for the timid. I have on occasion found a shovel inadequate to the chore and employed a small hatchet, instead. On at least one occasion I contemplated using a chainsaw.)

Emerging Campanula Clump


If, however, the existing clump has an area of larger, central growth surrounded by smaller patches of emerging leaves, that plant has replicated more discreetly, either by sending out special roots which develop into baby plants, or by seed. In such a case more care is needed, and each smaller plant should be dug up individually. These can then be relocated wherever you wish, and the mature plant left in place or moved as well, as desired.

Emerging Primrose Clump


Next in this series - Step by Step Instruction

In my next article in this series, How to Divide Herbaceous Perennial Plants - Part 3, I will take you step-by-step through the process of dividing an overgrown clump of daylilies, with lots of photos to show you exactly how to do it!

Copyright Roberta Lee 2012. All rights reserved.

(I am an artist and the author of the Suburban Sprawl series of novels as well as two nonfiction books. Find out more about my work at

Daylily 'Stella de Oro'



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    • DIYmyOmy profile image

      DIYmyOmy 5 years ago from Philadelphia, PA

      Thanks so much for the kind words! Yes, GinnyLee, I agree--plants want room to stretch out their roots and grow and respond well to being given that thru division.

    • GinnyLee profile image

      Ginny 5 years ago from Arlington, VA

      What a wonderful article! I found this while hub hopping and love it! You have a fan! I have always found that separating the perennials make them a lot healthier too. They seem to grow better after I divide them.

    • DIYmyOmy profile image

      DIYmyOmy 5 years ago from Philadelphia, PA

      Thanks so much for the kind words! I think the key to successful dividing is timing: doing it quickly, and in weather conditions the least stressful for the plant. So happy my article helped!

    • profile image

      leann2800 5 years ago

      This is very useful. I have never really been able to divide successfully. Thanks for sharing.