Bearded irises (Iris germanica) are a star of the late spring garden. Their big floppy blooms come in a huge range of colors, capable of creating a rainbow of hues in the landscape. They are deer- and rabbit-resistant, and many varieties have a lovely but subtle fragrance. Though they are fairly easy to grow, it’s important to know when to cut back irises to maximize both the plant’s health and the appearance of your garden.
Why is knowing when to cut back irises important?
There are three primary reasons to trim back irises:
- While bearded irises put on quite the show for two to three weeks every spring, once their blooms fade, the plants look a lot less attractive. You’ll want to trim off the spent flowers to keep the garden looking its best.
- In addition to removing the spent flower stalks, disease prevention is another factor that plays into when to cut back irises. There are several common iris diseases that can be reduced or eliminated by proper iris care, including iris pruning.
- Lastly, knowing when to cut back irises is critical for preventing a dreaded pest known as the iris borer.
Let’s talk about each of these three reasons in turn and discuss how and when to trim irises to help them achieve peak performance.
When to cut back iris flower stalks
Deadheading is a daily or weekly task for most gardeners. Removing the spent flowers from some types of plants encourages the development of subsequent blooms. However, this is not the case with bearded irises. The majority of varieties bloom only once in the late spring, and that’s it for the season. Like peonies, even if you deadhead irises, they won’t flower again until the next year. There are a handful of bearded iris cultivars that bloom again in the autumn, but they are not the norm. That being said, you should still trim off spent iris flower stalks to keep the plants looking tidy and prevent botrytis and other diseases. The right time to remove the flower stalks is a few days after all the blooms have faded.
How to deadhead irises
There is no need to remove individual dead flowers as they fade. Instead, trim off the entire flower stalk at the end of bloom. To remove spent flower stalks from bearded iris plants, follow the stem all the way down to the base of the plant where it meets the rhizome (the thick fleshy root-like structure), and trim it off using a sharp pair of pruning shears. Cut the stem at an angle if possible. If you cut it flat, rain and irrigation water collect on the cut surface, which can lead to crown rot. Angled cuts encourage water to run off the side.
When to cut back irises to manage diseases
Unfortunately, bearded irises are prone to several different diseases. Bacterial leaf blight, botrytis, ink spot, soft rot, and leaf spot are among the most common. These diseases leave iris foliage marred with brown spots or streaks. Some cause the rhizomes to turn mushy. And they make the plants look downright ugly. What’s a gardener to do?
Since all of these diseases are encouraged by wet weather or damp, humid conditions, encouraging good air circulation is key to preventing these diseases or, at the very least, lessening their damage. Do not water irises in the evening and use drip irrigation or a soaker hose instead of an overhead sprinkler if possible. The drier the foliage stays, the better. But, since you can’t control Mother Nature, it’s inevitable that the foliage will get wet from time to time.
Immediately remove any iris leaves that show signs of disease (spots, streaks, or blotches) and throw them into the garbage. Cut them off as close to the rhizome as possible. Continue to remove discolored foliage as soon as it appears. I do this two or three times through June and July to keep the plants looking good.
How to limit iris disease by pruning the plants
In late summer, if the plants are flopping over and looking really disheveled, prune the entire plant back. Use a pair of garden scissors to trim each “fan” of leaves down to half of its height. This improves air circulation around the rhizomes and discourages rhizome rot. Prune each leaf blade at an angle to create a rounded fan shape and prevent rainwater from sitting on the cut surface and causing rot.
At the end of the growing season, after the leaves have been killed by a few fall frosts, completely remove all foliage. Most of iris diseases overwinter on the leaf debris. Getting rid of the foliage limits the presence of overwintering fungal spores.
When to cut back irises to prevent iris borer
Iris borers (Macronoctua onusta) are a species of moth larva that feed on the rhizomes of bearded iris. Their feeding causes iris foliage to turn slimy and yellow. Often there is a putrid smell around the plant (think rotten potato). The female moths lay eggs on iris foliage in late summer. These eggs sit on the foliage all winter long, and then they hatch in spring. The tiny caterpillars burrow into newly emerging leaves and begin to feed inside of them, working their way down to the base of the leaf and into the rhizome. By mid-summer, a single borer can carve out the insides of an entire rhizome, leaving only a stinky, smelly mess behind.
Iris borer larvae are chubby and pinkish white. If you divide your iris in the late summer, you’ll often find a few borers in the process. Come August, the borers burrow into the soil to pupate. A few weeks later, the adult moths emerge to breed and lay more eggs.
While you can use beneficial nematodes to control this iris pest, knowing when to cut back irises is a really easy way to prevent more borers. Since the eggs overwinter on the foliage, it’s critical that you cut back irises in the fall. Trim off the leaves all the way back to the rhizomes, and toss them in the trash or bury them. If you come across a soft, rotten rhizome, dig it out and throw it away.
One final way to up your iris game
As you can see, there are many reasons why learning when to cut back irises and how to do it properly can improve the health of your plants and the beauty of your garden. Another task to keep your iris plants in good shape is to divide them every few years. This important practice further serves to prevent disease and encourage better blooms. Here’s an article on how to divide iris.
Learn more about when to prune your irises in this video:
For more on growing beautiful flowering plants, please read the following articles:
- Cutting back hydrangea bushes
- When to cut back daylilies
- Purple-flowering perennials
- Shasta daisies
- How to grow daylilies
- When to cut back ornamental grasses