When to Plant Iris Flowers by Hardiness Zone

Bringing joy to the garden with their ruffled, full-throated blooms and rigid, sword-like foliage, iris flowers are a landscape designer’s go-to plant for reliable, late spring color and season-long structure.

Filling the perennial gap between the tulip’s demise and the daisy’s rise, the iris holds a special place in many a gardener’s heart. Offering a sweet, light scent and a prolonged bloom time, irises have a familiar face and give off a vintage vibe.

The USDA Plant Hardiness Zone Map, which divides North America, Europe, and the rest of the world into 13 regions based on 10 degree differences in average low temperature, is considered the standard reference for agriculture and horticulture planting recommendations.

The map is color coded from the coldest and lowest numbered zones in the northern hemisphere to the warmest and highest numbered zones in the southern hemisphere.

If you’re looking at central Iowa, for example, you’ll see that it falls mostly into zone 5, which has average low temperatures of somewhere between -20 and -15 degrees.

So, if a plant is said to be hardy to zone 5, that means it can withstand winter temperatures that fall in that range during its dormant period.

Iris Hardiness by Species

Iris species have such broad differences in their growing profiles that the general hardiness zone recommendations for the genus are 3-10, but that doesn’t mean all irises can grow in all of those zones.

Knowing the major iris groups’ hardiness category is the first step in determining the best time to plant, so let’s look do a quick break out:

Dwarf and Dutch Irises: Zone 5-9

Irises in the dwarf (reticulata) and German (x hollandica) species are grown from bulbs rather than rhizomes. Members of these groups will not perennialize reliably in parts of the world below zone 5 or above zone 9.

German, Siberian and Japanese Irises: Zone 3-9

Irises in these species (germanica, siberica, and ensata) will not be damaged by severe winter temperatures during their dormant periods, but they are somewhat vulnerable to extreme summer heat.

Iris Germanica is one of the most popular species amongst those mentioned here and are usually what many gardeners think of when looking at irises in seed catalogs. They are also known as bearded irises.

Louisiana Irises: Zone 6-10

Native to the southern regions of the United States, Louisiana irises (ser. Hexagonae) will not survive harsh winters, but are highly tolerant of humid summers, hot sunshine, and high temperatures.

Aril Iris: 7-10

Aril iris species (oncocyclus and regalia) are native to the hot, dry climates of Central Asia and the Middle East. The cultivars of this iris thrive in mountainous or desert regions with arid conditions and can grow in sandy soil.

Take exception with aril hybrids, or arilbreds as they are known. These irises typically possess the cold tolerance of a parent iris from one of the other groups.

When to Plant Irises

Nursery grown iris plants can be installed at any point during the growing season, but for optimal growth, all irises should be planted or transplanted after the risk of extreme heat has passed and before the ground freezes. In other words, the best time to plant irises is in fall.  

However, with temperatures varying so greatly between zones 3 and 10, it can be challenging to determine exactly which month is best for planting irises in your region.

Frosty nights in Northern Canada occur significantly earlier than those in Florida, and that gives us a planting range between August and January.

To help make things easier and give your irises the best possible chances for optimal growth, here’s a zone by zone recommendation for the best time to plant them.

Keep in mind that most states and countries have multiple zones and subzones (a,b,c, etc.), and temperatures can vary wildly from north to south. It’s essential that you use your specific hardiness zone as a planting guide rather than your state if you are to get the timing right.

Here’s a quick grid that breaks it all down, followed by detailed information broken down by each hardiness zone.

Zone 3

• Best time to plant: early August to early September
• Best irises for zone: German, Siberian, and Japanese
• Recommended varieties: Iris siberica ‘Blue Moon,’ Iris germanica ‘Acapulco Gold,’ Iris ensata ‘Pink Frost’

Zone 3 has winters with low temperatures in the -30 to -40 range, and it includes the northernmost parts of Montana, Minnesota, Wisconsin, Michigan, New York, and Maine. There are also Zone 3 regions in Canada, and Alaska. Weather in these parts is characterized by freezing conditions, dry air, and high winds.

With an average last frost date between May 15 and an average first frost date of September 15, Zone 3 has a very short growing season and limited light. But the cycle of dormancy actually works quite well for many perennials, including iris.

Only rhizomatous irises can be grown as perennials in this zone. They should be planted in late August or early September in Zone 3 to ensure roots have a few weeks to establish before the first frost. Water regularly and evenly until the ground freezes.

Zone 4

• Best time to plant: mid August to mid September
• Best irises for zone: German, Siberian, and Japanese
• Recommended varieties: Iris ensata ‘Queen’s Tiara,’ ‘Iris germanica ‘Before the Storm,’ Iris sibirica ‘Butter and Sugar’

Zone 4 can expect the lowest winter temperature to be somewhere between -30 and -20 degrees. This hardiness category includes parts of northern U.S states like Montana, North Dakota, Minnesota, Michigan, New York, and Maine as well as the coastal regions of Canada and Alaska.

Regions in this zone can expect short growing seasons, high winds, volatile winter conditions, and a bit more moisture than zone 3. The average last frost date in Zone 4 is June 1 and the average first frost date is October 1.

Only rhizome irises can be grown in Zone 4. The best time to plant them is mid August to mid September, when overnight temperatures can be expected to fall somewhere between 40 and 50 degrees.

Zone 5

• Best time to plant: September
• Best irises for zone: Dwarf, Dutch, German, Siberian, and Japanese
• Recommended varieties: Iris reticulate ‘Eye Catcher,’ Iris hollandica ‘Casablanca,’ Iris ensata ‘Carol Johnson’

Zone 5 is a band through the middle U.S. states and includes the likes of Nebraska, Iowa, lower Wisconsin, upper Illinois, parts of New England and the mountainous regions in the central-west. It also covers some Alaskan and Canadian shorelines.

Regions in this zone can expect a last frost date to occur sometime around May 15 and a first frost date to occur around October 15. The average minimum temperature is somewhere between -20 and -10 degrees, and winters are considered more moderate than zones 3 and 4. 

In Zone 5, both rhizomatous and bulbous irises can be grown as perennials. Both should be planted in September at least 4 weeks before the expected first frost. Keep an eye out for late season swings in temperature as this zone is particularly unpredictable in fall.

Zone 6

• Best time to plant: September to October
• Best irises for zone: Dwarf, Dutch, Louisiana, German, Siberian, Japanese
• Recommended varieties: Iris fulva ‘Copper Iris,’ Iris reticulata ‘George,’ Iris germanica ‘Chasing Rainbows’

Hardiness zone 6 is known for being a mild climate with cold but not extreme winters and mild to very hot summers. On the coldest nights in this hardiness zone, temperatures can be expected to average between 0 and -10 degrees.

Included in this zone are portions of central U.S. states like Kansas, Missouri, Kentucky, Pennsylvania, and West Virginia, as well as some of the elevated regions of Oregon and Washington.

Residents in these regions can expect their last frost of the winter to occur near April 21 and the first frost of fall to occur around October 31. All but Aril irises can be grown in Zone 6. For best results, plant bulbs and/or rhizomes in late September or early October to allow enough time for roots to establish before a hard freeze.

Zone 7

• Best time to plant: October
• Best irises for zone: Aril, Dwarf, Dutch, Louisiana, German, Siberian, Japanese
• Recommended varieties: Iris hexagona ‘Dixie Iris,’ Iris stolonifera ‘Claudius,’ Iris sibirica ‘Lavender Bounty’

All residents in Zone 7 experience mild winters, but the summers can vary greatly across the map. Differences in elevation and climate mean differences in humidity and high temperatures.

Virginia, Tennessee, Arkansas, and Oklahoma are mostly in this zone. It also includes parts of Texas, New Mexico, Georgia, Alabama, and North Carolina. The average low temperature experienced in Zone 7 is between 0 and 10 degrees.

Based on a typical first frost date of November 15, gardeners who want to plant irises in Zone 7 should aim for early to mid-October.

Zone 8

• Best time to plant: March, October
• Best irises for zone:  Aril, Dwarf, Dutch, Louisiana, German, Siberian, Japanese
• Recommended varieties: Iris reticulata ‘Katharine Hodgkin,’ Iris korolkowii x stolonifera ‘Vera,’ Iris germanica ‘Autumn Circus’

For many flowering plants, Zone 8 is at the top of their hardiness range. Temperatures in this swath of land, which includes the bulk of Texas, Louisiana, Mississippi, Georgia, and South Carolina can be quite high. This region also includes much of California and Arizona.

In most years, the lowest temperature recorded each year falls somewhere between 10 and 20 degrees in Zone 8. That’s only because these areas are prone to experiencing occasional cold snaps during winter storms.

Zone 8 typically has its last frost on March 28 and its first on November 28. But because the ground never freezes solid, the typical rules do not apply. Rhizome irises can be planted at any time throughout the season. But bulb irises still need a fall installation to ensure their dormancy requirements are met.

Keep in mind, however, that irises do not like extreme heat and will root best when nightly temperatures average between 40 and 50 degrees. In Zone 8, March and October are considered the ideal months for installing rhizome irises. And October is the ideal month for installing bulb irises.

Zone 9

• Best time to plant: April, November
• Best irises for zone:  Aril, Louisiana
• Recommended varieties: Iris korolkowii ‘Regel,’ Iris Louisiana ‘Black Gamecock,’ Iris brevicaulis ‘Zigzag,’

The southern portions of California, Arizona, and Texas, as well as central Florida are included in Zone 9. The biggest frost risk occurs in the month of January, and minimal winter temperatures are usually between 20 and 30 degrees.

For most irises, temperatures in this part of the map are at the high end of their tolerance spectrum. While many varieties say they are hardy to zone 9, most will only do well with afternoon shade or shelter. This especially applies if you are growing them in desert locations like Arizona. If you want to grow Dutch, German, or Japanese irises in Zone 9, be prepared to baby them a bit.

You can expect more success with members of the Aril and Louisiana iris species. These species are native to warmer parts of the globe. Like all zones that do not experience frozen ground, irises can be planted any time of the year. They will however do best when temperatures are mild in spring and fall.

Both bulb and rhizome irises can be planted in spring or fall, but keep in mind they will not likely flower in their first season. The ideal month for installing all irises in Zone 9 is November.

Zone 10

• Best time to plant: November, December
• Best irises for zone:  Aril, Louisiana
• Recommended varieties: Iris giganticaerulea ‘Giant Blue,’ Iris susiana ‘Mourning Iris,’ Iris regeliocyculs ‘Dardanus’

In Zone 10, which includes the southernmost parts of California, Texas, and Florida as well as most of Mexico, the average cold temperatures will come in somewhere between 30 and 40 degrees. And the peak season temperatures are excessively hot.

The only members of the iris genus that can be grown here are from the Aril and Louisiana species. To give these irises the best chance at growing strong roots, plant during the months with the mildest temperatures. Typically, this is November or December.