Plants · Flowers
Iris planting zones
Iris (Iris species and hybrids) is a perennial. By the type table below, at least one type lives through winter in the ground in 3a–10b; outside those zones, none of the types on this page is rated. Below the types are the dates its year turns on: each zone’s median last spring freeze and first fall freeze, from NOAA records.
Published October 24, 2026 · Cite this page
The types
Which iris to grow
| Type | Hardy in the ground | Notes |
|---|---|---|
| Bearded iris Iris × germanica, tall, border and dwarf bearded | 3a to 10b, then too little winter | Penn State rates German bearded iris zones 3–10. Clemson says Southern gardens can grow cultivars that rebloom in late summer or fall. |
| Siberian iris Iris sibirica | 3a to 8b, then too hot or humid | Clemson: among the easiest iris to grow in most regions; tolerates light shade and moist soil, but not standing water. |
| Japanese iris Iris ensata | 4b to 9a, then too hot or humid | Needs slightly acid soil, full sun and moisture in summer; it will grow in shallow water (Clemson). |
| Louisiana iris Iris fulva, I. brevicaulis, I. giganticaerulea and hybrids | 6a to 10b, then too hot or humid | Native to the Gulf Coast (Clemson). Texas A&M’s Travis County list includes Louisiana iris for moist soil. |
| Dwarf crested iris Iris cristata | 4a to 8b, then too hot or humid | A small native for light shade; plant the rhizome at ground level (Clemson). |
Zone ranges are a judgement: horticultural convention set beside what the sources below say, not a measurement. “Too little winter” means the type needs winter chill the warm zones do not give it. In the next table the freeze dates are measured, and the count of hardy types comes from these judged ranges; its last column counts back from a measured date by a source’s rule.
Zone by zone
Where iris lives through winter, and when the freezes fall
Each date is the median across every NOAA station the zone’s ZIP codes cite, 1991–2020 normals. The middle half of those stations falls within the range shown beside it.
| Zone | Types hardy in the ground | Last spring freeze | First fall freeze | Plant or divide by |
|---|---|---|---|---|
| 3a | 2 of 5 | only 10 stations with comparable freeze dates — too few to publish a date | ||
| 3b | 2 of 5 | May 18 · middle half May 15 – May 25 | September 23 · middle half September 17 – September 26 | August 12 – August 26 |
| 4a | 3 of 5 | May 15 · middle half May 10 – May 21 | September 27 · middle half September 22 – September 30 | August 16 – August 30 |
| 4b | 4 of 5 | May 11 · middle half May 5 – May 23 | September 30 · middle half September 23 – October 3 | August 19 – September 2 |
| 5a | 4 of 5 | May 9 · middle half May 1 – May 20 | October 2 · middle half September 26 – October 7 | August 21 – September 4 |
| 5b | 4 of 5 | May 4 · middle half April 27 – May 15 | October 8 · middle half October 1 – October 14 | August 27 – September 10 |
| 6a | 5 of 5 | May 1 · middle half April 24 – May 9 | October 14 · middle half October 5 – October 20 | September 2 – September 16 |
| 6b | 5 of 5 | April 25 · middle half April 17 – May 3 | October 20 · middle half October 11 – October 24 | September 8 – September 22 |
| 7a | 5 of 5 | April 19 · middle half April 11 – April 27 | October 23 · middle half October 17 – October 28 | September 11 – September 25 |
| 7b | 5 of 5 | April 8 · middle half April 3 – April 16 | October 30 · middle half October 25 – November 3 | September 18 – October 2 |
| 8a | 5 of 5 | March 30 · middle half March 25 – April 5 | November 5 · middle half November 1 – November 10 | September 24 – October 8 |
| 8b | 5 of 5 | March 20 · middle half March 13 – March 29 | November 12 · middle half November 7 – November 18 | October 1 – October 15 |
| 9a | 3 of 5 | March 5 · middle half February 24 – March 15 | November 24 · middle half November 18 – December 2 | October 13 – October 27 |
| 9b | 2 of 5 | February 12 · middle half January 30 – February 25 | December 8 · middle half December 1 – December 17 | October 27 – November 10 |
| 10a | 2 of 5 | 107 of 190 stations here record no ordinary freeze, so there is no freeze date to count from | ||
| 10b | 2 of 5 | 70 of 79 stations here record no ordinary freeze, so there is no freeze date to count from | ||
| 11a | 0 of 5 | 34 of 34 stations here record no ordinary freeze, so there is no freeze date to count from | ||
| 11b | 0 of 5 | 12 of 12 stations here record no ordinary freeze, so there is no freeze date to count from | ||
“Types hardy in the ground” is a judgement, from the type table above. The freeze dates are measured. “Plant or divide by” is Penn State’s rule for bearded iris, roots established four to six weeks before the first hard freeze, counted back from the median first fall freeze.
A zone does not tell you your own dates. Inside a single zone the median last spring freeze still varies by 34 days from station to station, so the box below gives the dates of the NOAA station nearest your ZIP code.
Your ZIP code
Your ZIP, your answer
Enter your ZIP code to see your USDA zone, how many of the 5 types on this page live through that zone’s winter and when its freezes fall, read from the table on this page, and the freeze dates of the NOAA station nearest you.
Your answer appears here.
Full details for any ZIP code: ZIP code zone finder.
Which iris for your zone
Penn State Extension says German bearded iris (Iris × germanica) thrives in zones 3 to 10, and that irises flourish where winter freezes and lets them go dormant. Bearded irises come in thousands of cultivars and are grouped by height, from miniature dwarfs to tall bearded; Penn State says the shorter ones generally bloom earlier.
Clemson’s Home & Garden Information Center says there is an iris for virtually every garden in South Carolina. In the South, it says, many tall bearded cultivars bloom in spring and rebloom in late summer or fall, and old irises such as white flag and Dalmatian iris are heat-tolerant and well adapted. Of the beardless types, Siberian iris is among the easiest to grow in most regions, Japanese iris needs slightly acid soil and summer moisture and will grow in shallow water, and Louisiana iris is native to the Gulf Coast.
Mississippi State Extension says Louisiana iris thrives in wet soils, that bearded iris is popular but often rots in heavy soils or when planted deep, that Siberian iris is more dependable in central and north Mississippi, and that dwarf crested iris is a shade-loving groundcover. UGA Extension says bearded irises need full sun and excellent drainage, while Siberian and Japanese irises grow in light shade and tolerate moist soil. Texas A&M AgriLife’s Travis County list for Central Texas includes bearded iris and Louisiana irises, noting the Louisiana types prefer moist soil.
Planting, and how high to set the rhizome
Clemson says the best time to plant bearded iris is from six weeks after bloom finishes through September, or October near the coast; container-grown iris can go in in spring. Penn State says the roots of new plantings should be established four to six weeks before the first hard freeze; the table above counts that back from your zone’s median first fall freeze.
Bearded iris rhizomes sit at the surface. Penn State digs a hole 4 to 6 inches deep, mounds soil in the center, sets the rhizome on the mound with its roots down the sides, and covers it so the top of the rhizome is just below or at ground level, adding that it is better too shallow than too deep. Penn State’s guide to dividing says the top surface of the rhizome should be just visible. University of Illinois Extension leaves the rhizome slightly exposed, and University of Wisconsin–Madison Extension leaves the top exposed or barely covered.
Clemson sets the rhizome on a ridge between two trenches with the top just barely beneath the soil, and says that in heavy clay up to half of the rhizome may sit above the soil. Louisiana iris is the exception: Clemson plants it deeper than other iris, at least 1 inch under the soil, then mulches with 2 to 4 inches of compost. Dwarf crested iris goes at ground level.
Penn State spaces bearded irises 12 to 18 inches apart, with the fan of leaves pointing away from the center of the clump. Illinois says planting closer than 12 inches means dividing every two to three years.
Care through the year
Penn State fertilizes with ½ cup of 5-10-10 around each clump after flowering, never on top of the rhizomes, removes the flower stalk after the last flower fades, and in late fall cuts the spent leaves to 3 to 6 inches. Clemson uses a fertilizer low in nitrogen, says it is better to underfeed than overfeed bearded iris, and warns that manure that is not well composted can encourage soft rot. Clemson says reblooming iris is more likely to rebloom with extra food and water after spring bloom.
Penn State says not to mulch on top of the rhizomes. Wisconsin and Illinois mulch newly planted iris after the ground freezes, to prevent heaving, and remove that mulch in early spring so the soil can dry out.
Dividing iris
Penn State, Illinois and Clemson divide bearded iris every three to five years, when flowering declines or the clump is crowded; Wisconsin says it may be in as little as two years. Illinois and Wisconsin divide in July or August, and Penn State in late summer, when the plants are dormant, which reduces the chance of bacterial soft rot.
Cut the leaves back to about a third of their height (Penn State, Illinois, Wisconsin) and lift the whole clump. Keep firm, healthy pieces with roots and a fan of leaves: Wisconsin wants each piece at least 3 inches long, and Penn State about as thick as your thumb. Discard old rhizomes with no leaves and any that are soft, smelly or bored. Penn State dips a knife in a 10% bleach solution between cuts. Wisconsin says transplanted iris typically bloom sparsely the first season.
Why iris does not bloom
- Too much shade. Clemson and Wisconsin say bearded iris needs full sun, at least 6 hours a day; Penn State says flowering suffers in partial shade.
- Planted, or buried, too deep. Clemson names it, and Wisconsin says mulch, soil and debris piling up can bury rhizomes over time.
- Too crowded. Clemson and Wisconsin say overcrowded clumps flower less until divided.
- Too much fertilizer. Clemson names it among the usual causes.
- Just divided. Wisconsin says transplanted iris bloom sparsely the first season.
Common problems
Penn State calls bacterial soft rot the most common disease: the rhizome turns mushy and smells awful. Remove and bag the whole plant, and do not compost it. Good drainage, planting close to the surface and little mulch keep it down. Clemson calls iris leaf spot the most common disease and says to remove all leaf debris in fall.
The iris borer is the most serious insect pest (Penn State, Clemson). Penn State says the caterpillars hatch on the leaves and tunnel down into the rhizome, leaving dying brown leaf tips, water-soaked streaks and sawdust-like frass, and opening the way to soft rot. Crush them in the leaf early in the season, dig and bag damaged rhizomes in midsummer, and clean up debris in fall after a hard frost.
NC State Extension’s plant toolbox rates bearded iris low in toxicity if eaten, says contact with the rootstock or sap can irritate skin, and lists it as a problem for cats, dogs and horses. It also says bearded iris is very resistant to deer.
Common mistakes
- Burying the rhizome. Penn State says it is better too shallow than too deep, and Clemson lets up to half of it show in heavy clay.
- Mulching over the rhizomes. Penn State says not to; it holds moisture against them.
- Leaving clumps undivided. Penn State divides every three to five years, when bloom slows.
- Planting bearded iris in wet, heavy soil. Mississippi State says it often rots there.
- Planting a Louisiana iris like a bearded one. Clemson plants Louisiana iris at least 1 inch deep, in moist soil.
We found no Florida, Louisiana or Alabama source for iris that we could read, and the Texas source we read, Texas A&M AgriLife’s Travis County bulb list, names the irises that suit Central Texas without planting dates. Southern planting advice on this page is Clemson’s, Mississippi State’s and UGA’s.
Questions
- How deep do you plant iris rhizomes?
- Bearded iris goes at the surface: Penn State sets the top of the rhizome just below or at ground level, Illinois leaves it slightly exposed, and Clemson lets up to half show in heavy clay. Louisiana iris is planted deeper, at least 1 inch under the soil (Clemson).
- When should I divide bearded iris?
- Illinois and Wisconsin divide in July or August, and Penn State in late summer, every three to five years. Penn State says new roots should be established four to six weeks before the first hard freeze.
- Why are my irises not blooming?
- Clemson names too much shade, too much fertilizer, planting too deep and overcrowding. Wisconsin says rhizomes can end up too deep under mulch and debris, and that newly divided iris bloom sparsely the first season.
- Which iris grows in wet soil?
- Clemson and Mississippi State say Louisiana iris thrives in wet soil, and Clemson says Japanese iris will grow in shallow water. Bearded iris needs good drainage; Mississippi State says it often rots in heavy soils.
- Is iris toxic to pets?
- NC State’s plant toolbox rates bearded iris low in toxicity if eaten, says the rootstock and sap can irritate skin, and lists it as a problem for cats, dogs and horses.
Sources
Freeze dates on this page are NOAA 1991–2020 Climate Normals, for every station the ZIP codes in each zone cite. Zones are the USDA Plant Hardiness Zone Map, 2023. The type table is a judgement, as labelled above. Growing advice is quoted from the extension publications below, each written for its own area.
- Penn State Extension, German Bearded Iris , by Susan Marquesen (Updated April 13, 2026). Written for: Pennsylvania (USDA zones 5b–8a).
- Penn State Extension, Dividing Irises , by Laura Spencer (Updated June 8, 2026). Written for: Pennsylvania (USDA zones 5b–8a).
- University of Illinois Extension, How to divide and plant bearded iris , by Ken Johnson (August 20, 2021). Written for: Illinois (USDA zones 5a–7a).
- University of Wisconsin–Madison Extension, Dividing Bearded Iris (Revised April 29, 2025). Written for: Wisconsin (USDA zones 4a–6a).
- University of Wisconsin–Madison Extension, Why Iris & Peonies Fail to Bloom , by Bruce Spangenberg (June 16, 2025). Written for: Wisconsin (USDA zones 4a–6a).
- Clemson Cooperative Extension, Home & Garden Information Center, Iris (HGIC 1167) , by Karen Russ and Robert F. Polomski . Written for: South Carolina (USDA zones 8a–9b).
- Mississippi State University Extension, Annual and Perennial Flowers for Mississippi Gardens (P1826) , by Eddie M. L. Smith . Written for: Mississippi (USDA zones 7b–9a).
- UGA Cooperative Extension, Flowering Perennials for Georgia Gardens (B944) , by Bodie V. Pennisi (Published with full review November 14, 2023). Written for: Georgia (USDA zones 7b–9a).
- NC State Extension Gardener Plant Toolbox, Iris × germanica (bearded iris) . Written for: North Carolina (USDA zones 6b–9a).
- Texas A&M AgriLife Extension, Travis County, Bulbs for Central Texas . Written for: Travis County (Austin), Texas (USDA zones 8b–9a).
Published October 24, 2026
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