An overcrowded water lily is the most common reason for a disappointing flower display. If you’ve had a disappointing iris bloom this spring, it’s probably time to divide your iris rhizomes.
While most gardeners have already done root divisions, for Siberian and bearded lilies we will need to do a rhizome division. It might seem a little daunting at first, but it really isn’t hard to do (and it’s a very useful gardening skill).
When should you divide your irises?
Irises should be divided every 3-4 years.
The best time is six to eight weeks after the expected flowering period. You should use this timeline even if your irises missed their bloom time this year. So for lilies that would have bloomed in May, you can start thinking about dividing them in early August.
A word of advice though: avoid dividing irises too late in the growing season.
This is because you want them to have plenty of time to settle into their new space long before cold weather hits.
This group of irises must be divided.Depending on how early your winters are, you may be able to wait until mid-September to divide your irises.
These are the steps to follow for the correct division of the iris rhizome.
Step 1: Gather your tools.
You will need a sharp garden knife, garden fork or hand shovel and an alcohol swab to sanitize the blade when moving a rootstock to the other. The disinfectant wipe isn’t necessary, but it’s a good idea to keep it on hand if you suspect your iris rhizomes are suffering from bacterial soft rot.
A word of advice: I specified that the scissors or the knife must be sharp enough to cut the rhizomes without tearing them. This is very important as clean cuts are less likely to cause cracks in the rhizome, leaving the plant susceptible to bacterial attack.
Step 2: Trim the foliage from your irises.
Start by pruning the foliage to a height of about six to eight inches above the crown. If you don’t feel comfortable cutting that much, you can stick to cutting about a third off the top. If there are dead or damaged leaves, you can cut them even shorter.
The leaf of this iris is shortened for the division of the rhizome.This step is optional, but very useful for several reasons:
- short foliage is easier to maneuver than tall blades;
- short foliage means less moisture loss through evaporation by disturbing plant roots;
- cutting the foliage makes it easier to inspect for pests and diseases when dividing your irises;
- and finally, once you disturb the rhizomes, the tall foliage is likely to look limp and ugly (due to water loss in the leaves), so shorter foliage solves this problem.
Some gardeners prefer to cut the leaf blades in a straight line, while others prefer to shape the iris leaves into a fan-shaped structure, following the leaves’ natural growth pattern. How to prune the foliage is up to you, if you want to prune it.
Step 3: Dig up and inspect the iris rhizomes.
If the orris root clump has already solidified into a compact mass (which is more likely to happen in depleted clay soil), you will need a little leverage to lift it out of the ground .
Gently lift the rhizomes from the soil, but do not pull. This is where a trowel or hand shovel comes in handy. Don’t use a regular shovel, as it might be too big for the job and damage the rootstock as you dig.
This is what a rhizome looks like before dividing.Remove the rhizome structure and gently brush off any remaining debris or rinse with a garden hose. So you can see what you have left. Here’s what to remove to make sure you only keep the healthy rhizomes:
Discard any rhizomes that seem soft and slimy.
This symptom is often accompanied by a bad smell. (Trust me, you’ll know it when you feel it!)
This is most likely an indication of bacterial soft rot and you need to be very careful not to spread it from one rootstock to another during division. This is where the disinfectant wipe comes in handy. Clean your knife blade between cuts if you notice any signs of disease. It is not necessary to throw away the whole rhizome (especially if it is large), but to cut off the rotten parts. Throw the rotten parts, but not in the compost.
Discard the holed rhizomes.
This is a sign that the iris piercing is working (macronoctua onusta), a caterpillar that chews the leaf and burrows to the bottom of the rhizome. Get rid of these rhizomes and any caterpillar eggs or larvae you may find.
The rhizome in the center shows signs of iris borer damage.Step 4 – Separate the rhizome from your iris.
Start by removing the rhizomes that are already growing separately. Set them aside. They are ready to return to earth.
Then, for the remaining large rhizome structures, identify the sections that split or changed direction naturally. Cut larger pieces into separate sections, making sure each is at least four to ten inches long, has healthy roots and, if possible, at least a few leaf fans protruding from the surface of the rhizome. Leaves are important for the rhizomes because the rhizomes get enough energy through photosynthesis to start growing again.
This is what a rhizome looks like after spring division.At this point, you can choose to keep the rhizomes you’ve divided or share them with others.
If you don’t have enough room to move the new rhizomes, you can make another gardener happy by dividing the excess irises. In one of the neighborhoods where I lived, there were often iris rhizomes for sale at garage sales from back to school in the fall.