Flowers in the iris genus are derived from nearly 300 species and have been cultivated into thousands of diverse, colorful varieties. They range in height from 6 inches to 3 feet and vary in hardiness between zones 3 and 11. Irises can be annuals or perennials, depending on your region and maintenance.
With firm, strappy leaves and full-throated, face-like blooms, iris flowers are easy to spot in the late spring or early summer landscape. Look for scalloped or sheared petal edges in a three-up, three-down bloom arrangement at the top of thick, sturdy stems. Some will even have fuzzy, caterpillar-like ‘beards’ near their centers.
Determining the likeliness of their return next year depends on where you live and which species you plan to plant. Read on for an in-depth look at the iris flower’s official plant classification and the difference between an annual, biennial, and perennial plant. We’ll also explore some situations where the lines might be blurred.
The Short Answer
Irises are technically perennial plants, which means they will go dormant in winter and return for at least one more growing season. Remember that this is true only for irises planted within their recommended hardiness zones or for out-of-zone irises brought inside to overwinter. When properly planted and cared for, an iris can be expected to return perennially for somewhere between 5 and 20 years.
What’s the Difference?
All herbaceous, flowering plants exist to perpetuate themselves in some way. And each falls into one of three life cycle categories: annual, biennial, or perennial. Before we determine which group includes the iris, it helps to understand the differences between them. Here’s a quick breakdown:
These plants complete their life cycle in one growing season, flowering continuously if properly deadheaded.
Plants in this category will complete their life cycle in a single growing season. This means a seed will sprout, grow, bloom, produce more seeds, and die in the same year.
Although seeds from some annuals can be collected, stored, and planted next year, they will not likely produce new plants without human intervention. Hybrids are unlikely to produce true-to-type seeds, meaning they don’t contain the hybridized DNA that made their parent plant beautiful and may revert to prior genetics once planted.
Annual plants will typically flower all season if properly deadheaded and are often selected to provide constant color. Zinnia, marigold, petunia, begonia, and impatiens are all true annual plants that many of us pop into pots and beds each year, accepting their short lifespan as part of the deal.
Sometimes, we treat a perennial plant like an annual, allowing it to die at the end of the season as if it were a one-and-done thing. Gardeners might pop some tulips into a porch pot and pull them out when they’ve finished flowering, or use a perennial grass as a container thriller, knowing it won’t survive the cold winter with its roots above ground.
Biennial plants complete their life cycle in two years, producing flowers and seeds in the second year before dying.
Plants in this category will complete their life cycle during two growing seasons. In their first year, they produce roots, stems, and leaves but will not bloom.
In the second year, they’ll produce flowers and seeds before dying at the end of the season. Those seeds will typically self-sow and produce new vegetative plants the next season, and the cycle will continue until it’s interrupted.
Examples of biennial plants include parsley, dill, carrots, bellflowers, and forget-me-nots. There are also many biennial weeds, including burdock, Queen Anne’s lace, and thistle. Biennial plants typically thrive in temperate climates, where seasons change predictably, and the highs and lows are not extreme.
Surviving for multiple growing seasons, perennial plants maintain woody growth or regenerate above ground each spring.
Plants in this group will live for at least two growing seasons. The leaves and stems above ground will die back completely after the first freeze, but their roots will persist and survive below the soil surface. Perennial plants will produce new leaves, stems, and flowers (if applicable) each spring.
While some perennial plants flower enthusiastically for just a few years (coral bells, delphinium, lupine, columbine), others can potentially flower for decades (peony, hosta, daylily, and yes, iris!). Flowers in this group typically reproduce through root expansion below ground and must be divided regularly if you want them to keep coming back.
The Iris Life Cycle
Selecting the right iris for your region is crucial for perennial growth; otherwise, it might not survive the extreme temperatures.
Irises vary significantly in hardiness, meaning not all varieties grow as perennials in all zones. For example, if you plant a heat-loving Louisiana iris in chilly Minnesota, you’re not likely to get a return on your investment. Louisiana species can only tolerate the average low temperatures of zones 6 through 10. Plant one in a cold northern state, and it will most likely freeze to death.
Conversely, a hardy Dutch iris from zone 5 to 9 will probably die from heat stress if planted in southern Mexico. So pay attention to the nursery tags and select the right iris for your part of the world if you’d like it to behave like a true perennial.
Of course, if you’re satisfied with just one season of blooms (which, for most iris varieties, is a fleeting period of 2 to 3 weeks), you may treat perennial irises like they are annual flowers and dispose of them at the end of the season. Irises lend themselves well to spring container arrangements because their foliage provides an enduring, spiky focal point. Plus, they flower just when the tulips are done.
To bring cold-sensitive irises back next year, dig them up at the end of the season and store them carefully. Alternatively, drag your container to a cool, dry location where you can monitor them throughout winter. Otherwise, they’ll meet the same late-season fate as the ‘real’ annuals in your yard.
Which Irises Are Perennial In My Zone?
To ensure the iris you’re planting will behave like a perennial, know your hardiness zone and carefully consider your species’ nursery tag. While the genus is hardy from zone 3 to 11, not all iris species will perennialize in all zones. Here’s a quick breakdown of major groups and their regional preferences:
German, Siberian, and Japanese Irises
Hardy perennial plants (German, Siberian, Japanese irises) prefer cold winters for storing energy to bloom annually.
Plants in this group (members of the germanica, sibirica, and ensata species) have the largest hardiness range. These irises will live perennially in zones 3 through 9 but remain annuals in frigid or tropical zones. They are partial to regions with cold winters and need the break to store energy for the next year’s bloom.
Dutch and Dwarf Irises
Dwarf and Dutch irises are picky about light and temperature, determining their suitability for various zones and climates.
With slightly less tolerance for freezing winter temperatures, plants in this group (members of the hollandica and reticulata species) will return year after year in zones 5 through 9. Plant them in Northern Michigan or Maine; they will die like an annual when the chill sets in. Conversely, Dutch and dwarf irises do not enjoy the hot afternoon sun. They often suffer an annual’s fate if planted in scorching locations.
Louisiana and Dixie Irises
Southern iris species like Louisiana and Dixie Irises thrive in marshy, humid regions but act as annuals in northern climates.
Irises in this group (hexagona) are native to marshy southern regions and enjoy lots of humidity. These irises thrive as perennials in zones 6 through 10 but only grow as annuals in colder zones. If you live in the north and want to feature one in your landscape, know that southern iris species will die at the end of the season unless brought indoors.
Irises are perennial plants by design. You can manipulate a species that’s annual in your zone with meticulous maintenance. To do the genus justice (and get more garden bang for your buck), choose plants for your zone, encourage their perennial potential, and allow them to grace your garden for many years to come.