Have you noticed that your Iris blooms weren’t as plentiful this year as in years past? Do the iris rhizomes, the tuber-like root of the iris, seem to be crowded or there are dead spots in your iris patch? If your answer to any of these questions is “yes” than it is probably time to divide your iris patch. Every 3 to 4 years irises need to be divided to ensure that they perform at their peak and the cooler temperatures associated with late summer and early fall along with the return of regular rainfall conditions make it the ideal time to perform this task.
Rejuvenating your irises involves digging them up, removing the dead or diseased rhizomes and then replanting them and you’ll need a shovel and a sharp knife to properly perform these tasks. The first step is to carefully dig up the iris clumps to avoid damaging the rhizomes and then separating the rhizomes from one another. The knife can come in handy when trying to separate the rhizomes from one another but you’ll want to be careful not to cut yourself as the tubers can be tough to cut through.
A good rhizome is one that is as thick as your thumb and has at least 2 leaf fans to it. You’ll also want to wash any soil off and be on the lookout for rhizomes that have small holes in them as this is a sign that iris borers have made a home in your iris patch. You’ll want to discard any rhizome with these holes and any rhizomes that are soft or rotting. Additionally, any older rhizomes without a strong leaf fan should be discarded as well.
The next step is to take the remaining rhizomes and clip the leaf fan to a height of 4 to 6 inches above the rhizome. This helps relieve some of the stress on the plant that occurs due to its roots being disturbed and it is a signal to the plant that this is a time to concentrate on developing a strong root system rather than growing more leaves. Irises generally don’t need and actually perform better in soil that is not enriched but if you want to work a little compost into the soil where you are going to replant the iris rhizomes or scratch a little fertilizer in the soil that is fine too.
When you replant the rhizomes you want to make sure you do so just above the soil level. If you plant the rhizome too deeply the iris will not flower. You can plant the rhizomes 12 to 18 inches apart with the leaf fans planted facing away from you to ensure a good looking display. Water the newly planted irises in and you are finished. Finally, beyond the increase in blooms you will get next spring an additional benefit of dividing your irises is it usually gives you plenty to transplant to other places in your garden or to share with your family and friends.