When to Divide Irises: Bearded Iris in Late Summer
When to divide irises depends on the type, and for bearded iris the answer is midsummer to early fall. Iowa State University Extension advises dividing bearded irises every three to five years because they crowd quickly and stop blooming well. July or August is the main time to dig.
The short answer
Divide bearded irises every three to five years in July or August. Clemson Cooperative Extension advises six to eight weeks after bloom, or July through September, but no later than six weeks before the first frost. Replant firm rhizomes with a fan of leaves at the soil surface in a sunny spot.
Why do irises need dividing?
Iowa State University Extension notes undivided bearded irises become overcrowded plus flower less. Crowded plants are also more prone to leaf diseases.
| Task | Timing | Source |
|---|---|---|
| Divide | Every 3 to 5 years | Iowa State University Extension |
| Dig | July or August | Iowa State University Extension |
| Last date | Six weeks before first frost | Clemson Cooperative Extension |
| Plant new rhizomes | Mid July to early September | University of Nebraska Lincoln Extension |
Source: Iowa State University Extension, Clemson Cooperative Extension plus University of Nebraska Lincoln Extension.
When is the right time to divide them?
Iowa State University Extension names July or August as the main window. Clemson Cooperative Extension advises six to eight weeks after bloom. University of Nebraska Lincoln Extension notes mid July to early September lets roots establish before winter.
How deep should rhizomes be replanted?
Clemson Cooperative Extension advises setting the rhizome just above ground level on clay soil. On sandy soil, the top can sit 1/2 inch or less below ground. University of Nebraska Lincoln Extension advises the rhizome sit level with or just below the surface.
How to: step by step
- Water the bed the day before digging.
- Loosen the soil around the clump.
- Lift plus shake off the rhizomes.
- Cut young rhizomes from old spongy ones.
- Save only firm pieces with a fan of leaves.
- Replant 12 to 18 inches apart in full sun.
Troubleshooting
Few flowers on an old clump: the clump is crowded. Divide every three to five years.
Rhizomes rotting after replanting: they were set too deep. Clemson Cooperative Extension advises setting rhizomes at the surface.
Sparse bloom the next spring: this is normal. Iowa State University Extension notes divided irises often bloom lightly the first spring.
For related reading see sanitizing pruning shears for cannabis, how to sharpen garden tools plus fall garden cleanup after cannabis harvest. Kind Seed Co also stocks cannabis seeds.
Sources
- Transplanting and dividing iris, Iowa State University Extension
- When and how do you divide bearded irises?, Iowa State University Extension
- Give your bearded irises some space, divide them, Clemson Cooperative Extension HGIC
- Culture of iris (G1741), Nebraska Extension
FAQ
How often should irises be divided?
Iowa State University Extension advises dividing bearded irises every three to five years. They crowd quickly and then bloom less. Clemson Cooperative Extension notes closer spacing gives a full display sooner but means dividing more often. Watch for clumps with bare centers or fewer flower stalks.
Can irises be divided in spring?
For bearded iris, the sources point to late summer. Iowa State University Extension names July or August as the main time. Clemson Cooperative Extension advises July through September. University of Nebraska Lincoln Extension notes this timing gives roots time to grow before winter.
Should divisions be watered?
Yes. Clemson Cooperative Extension advises watering divisions to settle the soil around the roots. In the absence of rain, it advises watering weekly to help roots establish. Iowa State University Extension also advises watering each plant thoroughly after replanting. Ease off once new growth appears.
Do irises need mulch?
Clemson Cooperative Extension advises a shallow layer of mulch that does not cover the rhizomes. Bearded iris rhizomes need to stay at or near the surface. University of Nebraska Lincoln Extension notes that in poorly drained soil the rhizomes should sit slightly exposed to prevent rot.
How far apart should iris divisions go?
Clemson Cooperative Extension advises spacing divisions about 12 to 18 inches apart in groups of three, with fans pointed away from each other. University of Nebraska Lincoln Extension advises at least 18 inches. Wider spacing means the clumps take longer to crowd plus need dividing less often.
