Plant bearded iris rhizomes in late summer: July or August in the Upper Midwest (Iowa State, Illinois Extension), or from six weeks after bloom through September in the South, October near the coast (Clemson). Finish at least six weeks before your first fall frost (Clemson). Plant shallow, with the top of the rhizome at or just below the soil surface (Iowa State, Clemson), 12 to 18 inches apart (Illinois Extension, Clemson), in at least 6 hours of sun.
Most garden irises are bearded irises (Iris germanica and hybrids), which grow from thick rhizomes and flower in late spring. They are tough, drought tolerant and deer resistant (NC State), but they rot if planted too deep, mulched over or kept wet (NC State). Clemson describes many other types too: Siberian, Japanese and Louisiana irises like moister soil, and Dutch irises grow from true bulbs planted in fall. This guide focuses on bearded iris, with notes on the others.
When to plant irises by region and type
| Area or type | When to plant |
|---|---|
| Upper Midwest (Iowa, Illinois) | July or August for dividing and replanting bearded iris (Iowa State, Illinois Extension) |
| Southeast inland (South Carolina) | Six weeks after bloom ends through September (Clemson), and no later than six weeks before the first fall frost (Clemson) |
| Coastal Southeast | Through October (Clemson) |
| Container-grown iris, any area | Spring (Clemson) |
| Siberian, Japanese and Louisiana iris | Summer and early fall (Clemson) |
| Dutch iris (bulbs) | October or November, 4 to 6 inches deep (Clemson) |
Our planting calendar shows your average first fall frost. Count back at least six weeks to find your last safe date for planting iris rhizomes (Clemson's rule).
How deep and far apart to plant irises
| Bearded iris rhizomes | |
|---|---|
| Depth | Top of the rhizome at or just below the surface (Iowa State); just barely beneath the surface (Clemson); in heavy clay, up to half the rhizome exposed (Clemson); Illinois leaves it slightly exposed |
| How | Make a mound or ridge in a hole about 4 in deep, set the rhizome on top and spread the roots down the sides (Illinois Extension, Clemson) |
| Spacing | 12 to 18 in apart (Illinois Extension, Clemson); closer than 12 in means dividing every 2 to 3 years (Illinois Extension) |
| Grouping | Groups of 3 with the leaf fans pointing outward (Clemson); Illinois suggests 3 to 7 |
| Sun and soil | 6 to 8 hours of sun (Clemson, Illinois Extension); well-drained soil (NC State); bearded types like slightly alkaline soil (Clemson) |
| Leaves | Trim divisions to a third or half their height (Clemson, Iowa State) |
Watering, feeding and mulch
- Water new plantings about once a week until they root in (Clemson), then sparingly: NC State says bearded iris prefers drier soil and overwatering can lead to rot.
- Feed lightly with a fertilizer low in nitrogen, such as about 1 pound of 5-10-10 per 100 square feet when preparing a bed (Clemson), and keep it off the rhizomes (Clemson). NC State fertilizes in early spring. Clemson warns that too much fertilizer is a common cause of poor flowering.
- Avoid fresh manure. Clemson says manure can encourage soft rot, so use only compost aged at least a year.
- Mulch lightly and keep it off the rhizomes (Clemson, NC State). In cold areas, Illinois Extension adds straw in late fall to prevent frost heaving of new plantings and removes it in early spring.
Dividing crowded irises
After three to five years, irises get crowded and flower less, and it's time to divide (Clemson, Iowa State, Illinois Extension). Dig the clump, cut apart healthy rhizomes that each have a fan of leaves, discard old, hollow or soft pieces, trim the leaves and replant (Iowa State, Clemson). Dividing is also the best time to check for iris borer damage (UMN).
Other iris types
- Siberian iris: likes moist soil but not standing water, tolerates light shade, and is planted in summer or early fall (Clemson). UMN says it tolerates iris borer better than other types.
- Japanese and Louisiana iris: need moist summer soil and full sun (Clemson). Clemson plants Louisiana rhizomes at least 1 inch deep and mulches them.
- Dutch iris: true bulbs planted 4 to 6 inches deep in October or November, blooming in early summer (Clemson). Time them like other fall bulbs; see when to plant tulips.
Iris borer, soft rot and safety
The iris borer is the worst insect pest of iris (UMN). Its eggs overwinter on old iris leaves, and in spring the tiny caterpillars tunnel into the leaves, leaving tan or water-soaked streaks, before reaching the rhizome by midsummer. By then they are pinkish and up to 2 inches long (UMN). Bacterial soft rot often follows, turning the rhizome mushy and foul-smelling (UMN, Clemson). UMN's main control is cleanup: remove and destroy old iris leaves and debris after a hard frost, crush borers in the leaves in spring, and discard infested rhizomes. Clemson also lists iris leaf spot as the most common disease. NC State lists bearded iris as low-severity poisonous and a problem for dogs, cats and horses, and contact with the sap or rhizome can irritate skin; call a doctor, poison control or a veterinarian if any is eaten.
Irises around the vegetable garden
Bearded irises like sun and dry feet, so they suit a sunny, well-drained border along the vegetable garden, away from spots you water heavily (GardenPal suggestion based on NC State's preference for drier soil). They bloom with peonies and alliums in late spring; see when to plant peonies, when to plant alliums and when to plant daffodils.
Frequently asked questions
When is the best time to plant iris rhizomes?
Late summer: July or August in the Upper Midwest (Iowa State, Illinois Extension), or from six weeks after bloom through September in South Carolina (Clemson).
How deep do you plant iris bulbs?
Bearded iris rhizomes go just below the surface, with the top at or barely under the soil (Iowa State, Clemson). Dutch iris bulbs go 4 to 6 inches deep (Clemson).
Can you plant irises in the spring?
Container-grown irises can be planted in spring (Clemson). Bare rhizomes do best in late summer.
How far apart should irises be planted?
12 to 18 inches apart (Illinois Extension, Clemson).
How late can you plant irises in the fall?
Clemson says no later than six weeks before your first fall frost, or through October near the coast.
How often should irises be divided?
Every 3 to 5 years, when clumps get crowded and bloom less (Clemson, Iowa State, Illinois Extension).
Enter your frost dates to see your first fall frost and your last safe date to plant iris.
Open itSources
- Virginia Cooperative Extension, "Companion Planting in Gardening" (SPES-620P)
- University of Maryland Extension, "Vegetable Succession and Companion Planting"
- Oregon State University Extension, "Companion Planting"
- Penn State Extension, "Herbs Make Good Plant Partners and Companions"
- University of Minnesota Extension, "Planting the vegetable garden"
- USDA Plant Hardiness Zone Map
- NOAA NCEI U.S. Climate Normals (frost/freeze data)
- Clemson Cooperative Extension HGIC, "Iris"
- Clemson Cooperative Extension HGIC, "Give your bearded irises some space: divide them"
- Iowa State University Extension, "Transplanting and dividing iris"
- Iowa State University Extension, "How deep should I plant bearded irises?"
- University of Illinois Extension, "How to divide and plant bearded iris"
- NC State Extension Gardener Plant Toolbox, "Iris germanica"
- University of Minnesota Extension, "Iris borers"
Pairings are summarized in our own words from extension-service guidance. Research support for many companion effects is limited, so treat them as good habits, not guarantees.