Q: My bearded iris didn’t bloom well this year. I suspect they are overcrowded. Will dividing them help and, if so, how and when should I do it?
A: Bearded iris (Iris germanica) are hardy, low-maintenance, spectacular-blooming, spring perennials that perform best in sunny gardens with light, loamy soil with a pH of 6 to 7. Erv Evans, NCSU Extension Associate, notes that in addition to overcrowding, “planting in excessive shade, using excessive nitrogen fertilizer, or planting the rhizomes too deep” may be responsible for poor flowering in bearded iris. Whether or not you are dividing the iris, cut the bloom stems off about two inches above the rhizome after the blooms fade. This prevents seed formation and promotes better blooms next season. Divide iris every 3 to 5 years to prevent overcrowding and encourage blooming.
The good news is that irises are easy to divide! Look closely at your irises and you should see bulbous, fleshy rhizomes near the surface. These form a network that stores food produced by the leaves. Each year underground offsets develop from the original rhizome, and it is this growth pattern that can lead to overcrowding.
Step One: Preparing the plant
You can divide iris at any time of the year, but the recommended time is 4 to 6 weeks after blooming. Identify the clumps you want to separate. Using a small shovel or a garden fork, loosen the soil around the perimeter, carefully lift each clump, shake off as much soil as possible, and then hose off the roots.
Step Two: Divide and conquer
Using a garden knife, cut off sections from the original rhizome. Check to make sure each division has white roots (a sign of good health), a minimum of one healthy leaf fan, and no evidence of borers or disease. Trim back broken roots and cut the leaves into a fan shape, leaving about one-third foliage on each division. Be sure to discard remaining pieces of rhizome, roots, and leaves.
Step Three: Planting the divided offsets
Choose a garden spot for the divided rhizomes and prepare shallow holes. In the center of each hole, make a mound of soil that is high enough to keep the new clump slightly above soil level. Evans suggests spreading the roots around the mound before filling with soil and watering. Replant the rhizomes at or slightly below the soil surface so that feeder roots can penetrate the soil below. For massed bloom, plant at least three rhizomes, 8 to 10 inches apart.
Bearded iris don’t require much attention, but to look their best, remove yellowing or diseased foliage. These plants need nitrogen, potash, and phosphorus in moderation. In the absence of a soil test, Evans recommends incorporating a ½ pound of low nitrogen fertilizer (such as 5-10-10) per 50 sq. ft.
This question is one of many received and answered by Master Gardener volunteers on the Buncombe County Garden Helpline. Got a gardening question? We’ve got answers! Give us a call at 828-255-5522.
Article written by Janet Moore, Extension Master GardenerSM Volunteer.
Bearded Iris for the Home Garden
by Erv Evans, Extension Associate, NCSU
How to grow impressive iris and avoid common problems such as bacterial soft rot, crown rot fungus, leaf spot, mosaic, and iris borer.