When to Divide Iris? Know When to Transplant!

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when to divide iris

The iris gets its name from the Greek word for rainbow, and with so many varieties of color, it’s no wonder. There are two basic types of iris, those grown from bulbs and perennial irises that grow from thick, fleshy roots called rhizomes. The perennial type is the one that we divide and transplant. 

Iris thrives in regions that experience dry, hot summers. This is the best time to divide and transplant them too. But that doesn’t mean you should divide your iris rhizomes every summer. Most perennial iris species are divided every three to five years. Also, if flower production fails, it’s time to divide and transplant. 

When to Divide Iris?

The choice of perennial iris plants is extensive and includes the German, Japanese, Louisiana, Siberian, and Yellow Flag iris. Some are bearded irises, like the German variety, while others are beardless but no less beautiful than the gorgeous bearded iris. 

So, when do you know that it is time to divide your bearded irises or other iris rhizomes? The three- to the five-year rule of thumb is quite vague. 

Signs That You Should Divide Your Irises

It’s more important to recognize when your iris bed becomes overcrowded. And the best indicator is that the number and quality of usually gorgeous iris flowers decrease and deteriorate.

It is also important to realize that if you don’t divide your perennial iris rhizomes, their health will likely be impacted. The rhizomes grow horizontally under the ground, storing food for the plant to grow and flower.

If you don’t split the rhizomes, they aren’t going to be able to produce enough food. If you do split them, you will have more flowering iris plants than you know what to do with. Enjoy them, give them away, or sell them for a small profit. 

When is the Best Time of the Year to Split Your Irises?

The answer to this question depends largely on where you live. But there are lots of leeways.

According to the University of Illinois Urbana-Champaign, the best time to plant iris rhizomes is from late July through September. But it all depends, even though they typically continental Illinois climate is warm. In some parts, it is quite humid and wet, and the iris prefers it to be dry. 

The Kansas State University’s Research and Extension unit and the University of Wisconsin Horticulture Division of Extension both reckon July through August is the best time for that part of the world.  

PennState Extension simply says you should divide “at the right time of the year, after flowering.” This, of course, is when the iris is dormant and there will be less chance of bacterial soft rot

How do you divide and replant irises?

how to divide and replant iris

It can be a messy job dividing iris rhizomes, especially if they are madly overgrown. But, as the horticulturists at Kansas State University’s Research and Extension unit say, there is no simple way. You are going to have to dive in and get your hands dirty.  

Here’s a brief rundown on what to do. 

Materials Needed

You don’t need many tools to divide and replant perennial irises. If you are already a keen gardener, these will already be part of your most-used toolkit. 

The essentials are:

  • Shovel and/or spade
  • Pruners and/or a sharp knife

Steps in Dividing Irises

Regardless of the type of perennial iris you have planted, as it matures, the rhizome will increase in size and produce more rhizomes. It stands to reason that you will need to remove, and either get rid of or replant the small fleshy roots. 

Now, this is a wonderful rule of thumb! A good iris rhizome should be as thick as your thumb! 

It should also have very healthy roots and at least one or two leaf fans. A fan of leaves forms above the rhizome and it can be quite short or several inches long. 

Don’t worry too much. If you are going to divide your iris, this is what you will need to do:

Water Well Before You Dig

Iris has a fairly shallow root system so you aren’t going to have to dig very deep. But to make your job easier, water well for a couple of days before you start digging. 

Break the Plants Apart

Once the soil is nice and moist you can dig the clump out with a spade or shovel. Then you can start to break the iris rhizomes apart. 

If you can’t do it with your hands, use your knife or pruners to do this. Also, trim the leaves to no more than 4-6 inches. The aim is to do this in a fan-like shape so that it creates a fan of leaves. 

What to Keep

Don’t even think you can keep every part of the rhizomes you are digging out. You need to keep the best parts and be sure to discard the bits that are furthest away from the fan of leaves.

When you cut the bit of the rhizome that you want to transplant, make sure that there is a little bump attached to the leaves. That’s the bit that will produce a new stem. 

Check the bits you have saved, and if there are any bits of the rhizome that are soft, cut them off. You only want solid, white, fleshy bits of root. At this stage, you can coat the edges of the rhizome with a fungicidal powder to prevent disease.  

How to Replant Your Iris Rhizomes

If you are dividing your iris plants then you’ve planted irises before. But let’s go over the process.

Start by preparing an area where you can plant them. Dig a shallow hole and add lots of compost and/or peat moss to improve the quality of the soil.

When you plant your iris rhizomes, make sure that the roots spread out facing downwards. Also, make certain that the top of the rhizome isn’t above the surface of the soil 

Planting iris rhizomes too deep can be a fatal flaw. 

When you have planted the rhizome, pack soil around and against it, and then make sure that your transplanted iris plants have enough water to grow. 

What happens if you don’t divide irises?

If you don’t divide perennial irises, they will probably become so overgrown they won’t produce flowers anymore. We also know that they become more prone to disease and root rot.

Can you divide iris anytime?

We have looked at the best time to divide iris, and you will see that summer is the best time. You certainly cannot divide your iris plant successfully in the cold, wet winter months.

Knowing when to divide iris will ensure you have the best possible display of iris flowers at any time of the year.

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