How-to Guide
How to Divide and Grow Bearded Iris
The short answer
Iowa State University Extension is unambiguous: July or August is the best time to dig, divide, and transplant bearded irises. Dig the clumps with a spade, wash the soil off the rhizomes with a hose, and cut them apart with a sharp knife so every division has a fan of leaves, a healthy rhizome, and several roots. Discard the old, leafless rhizomes from the center of the clump and anything showing disease or insect (iris borer) damage.
Cut the leaves back to one-third their height, then replant with the large fleshy rhizome just at or below the soil surface, 12 to 24 inches apart. Planting too deep is a classic cause of poor bloom. Newly divided irises bloom sparsely the following spring and are back in full bloom by their second year — and crowded, never-divided clumps declining in bloom are exactly why this midsummer job matters.
When to divide
Divide in July or August, a few weeks after flowering, when the plants are semi-dormant. Colorado State PlantTalk adds that bearded iris simply need this rejuvenation every three to five years — when a clump becomes congested and bloom drops off, it's time.
How to divide, step by step
- 1Dig the clump — Lift the whole clump carefully with a spade.
- 2Wash the rhizomes — Use a forceful stream from a hose to remove soil so you can see what you're cutting.
- 3Cut into divisions — With a sharp knife, separate the rhizomes so each division has a fan of leaves, a healthy rhizome, and several roots.
- 4Cull ruthlessly — Discard the old leafless rhizomes from the clump's center and every division with disease or insect damage — this is your chance to remove iris borer-damaged tissue.
- 5Trim the fans — Cut the leaves back to 1/3 their original height to balance the reduced roots.
Replanting
Set each division with the large fleshy rhizome just at or below the soil surface — never buried deep — with roots spread downward in well-drained soil and a sunny exposure. Space rhizomes 12 to 24 inches apart; close spacing means dividing again sooner. Water in well.
✔ Expect patience, not failure: Iowa State Extension notes newly divided irises bloom sparsely the first spring and reach full bloom in year two.
Why bearded iris stop blooming
- Overcrowded clumps that haven't been divided in years — rejuvenate every three to five years.
- Rhizomes planted or mulched too deeply, instead of sitting at the soil surface as recommended.
- Old, exhausted center rhizomes left in place at division time instead of being discarded.
- Borer- or disease-damaged rhizomes; inspect and cull when you divide.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- Iowa State University Extension — Transplanting and Dividing Iris — July or August is the best time to dig, divide, transplant bearded iris; wash rhizomes, cut with sharp knife; each division needs leaf fan + healthy rhizome + roots; discard old center and damaged rhizomes; cut leaves to 1/3; rhizome just at or below surface; 12-24 in spacing; sparse bloom year one, full bloom year two
- Colorado State University PlantTalk — Dividing perennials — Bearded iris need rejuvenating division every three to five years, best a few weeks after flowering