A step by step guide to dividing iris and other perennials

Most perennials, plants that come up every year, need to be divided at some point. They grow outward at their base, usually with new plants or stems coming up at the edge of the root mass. If not divided periodically, the root system becomes crowded and the plant has fewer blooms. This includes perennials like daisies, coreopsis, phlox, daylilies, peonies, and ornamental grasses. Plants that have only a single stem or which are woody like roses don’t get divided. I have never heard of anyone around here who has successfully divided a lavender or rosemary, but that doesn’t mean you can’t do it.

Dividing is a great way to expand your plant material since you can immediately replant the divisions in your garden. Or you can make friends and give the divisions away.

The trick to dividing is to get enough roots on each division, cut back the foliage to about half, to plant it well and keep an eye on it. You should water it as if it is a new plant from the store, enough but not too much. The division has the advantage of having grown in your soil, so it doesn’t have to adjust from the commercial lightweight potting soils to the heavier native soils in your garden. 

I usually divide my perennials in the spring, just as they start greening up. But late August is the suggested time to divide iris.

I have a friend who is a great gardener. She is very detailed oriented and know exactly when to divide her iris. So every 3 years she divides it. This is ideal, but few people, including me, are as attentive to a 3 year schedule as she is, though her iris are always beautiful. I often help her divide and go home with new colors to add to my garden.

They are quite tough to dig up, especially if you haven’t divided them in a while. You need to get a sharp shovel and start digging. Dig up around a big clump and take a look at it.  Iris grow off a bulb structure called a rhizome. Buds grow on these rhizomes and develop into a fan of leaves, which is the characteristic shape of an iris plant. These rhizomes become tangled and breaking apart the large mass takes muscle.

They are surprisingly resilient so you can also place your shovel in the middle of the clump and cut downward. Sometimes, I have found it necessary to jump on the shovel to get the mass to cut apart. Don’t worry, there will be enough new plants that any cut rhizomes can be thrown away.

You need to knock the dirt off so you can see the mess of rhizomes. Each of these cluster of leaves, called a fan, will become a new plant.

This is a clump - free of soil. You can see how the rhizomes are connected and where you can break them apart.

This fan now breaks easily off of its neighbor. Notice its strong roots emerging from the rhizome. It will easily reestablish in a new spot in your garden.

All of these fans came off one clump. Each should be planted separately. The flower stems should be discarded. They are round, not flat like the fans. The flower stem from this clump is horizontal in this picture. Not every clump will have a flower stem.

Usually there will be an old “mother” rhizome that the younger ones are attached to. This will have no roots or dead roots on it. Or sometimes, many holes where roots used to be. There is one of these next to the stem in the above picture. Discard this when all the young fans have been removed from it.

Even though there are live roots on this flower stem, throw it away.

I cut off the tops of these leaves in mid-summer because I planted them too close to the front of my garden. They blocked the sun and water from the back part of the garden. I cut them down even further for transplanting – leaving about 7” of leaf. This seems like a lot to remove but it makes it easier for the fans to grow if they don’t have to support so much leaf with their smaller root system.

To plant these, I dig a wide trench, add a bit of compost, gather up the roots like I was making a pony tail and lay them all out in one direction. I repeat this for all the fans I want in that area. This way the roots are all running in the same direction but not touching each other. This gives me right spacing because I put another fan at the end of those roots. You can also spread the roots out in all directions. Space the fans about 6-8” apart to allow for new growth, but close enough to give a good mass planting.

THIS IS IMPORTANT! Don’t plant them too deep. This seems counter-intuitive if you are used to planting tulip and daffodil bulbs deep, but the iris rhizome needs to be almost to the surface or it won’t bloom. Some experts even suggest having the rhizome exposed. However, I find it gets sunburned in our harsh sun, so I always cover mine with less than an inch of soil. After you have watered them in, if you find they are too deep, just go in and gently pull up on the fan until the rhizome is closer to the surface and gently tamp the soil back around the small roots.

If you are lucky, you will get blooms next year. If not though, they will give you joy the following year.