How to Plant and Grow Iris

With over 200 species in this diverse group of plants, there is bound to be the perfect iris for your garden. In general, irises are low-maintenance and easy to grow. Their flowers come in almost every color and bloom times vary depending on the species; some irises bloom in spring or summer, while others bloom in spring and again in fall.

All species of iris boast flowers that are intricate and detailed. The three lower, drooping petals are commonly referred to as the falls. The three upright petals are called the standards. All parts of an iris blossom are colorful and can have a variety of patterns. Bearded irises are especially noteworthy for being available in a rainbow of colors and forms. The foliage of irises can also be quite striking, with long, lance-shaped leaves in an attractive gray-green color. Even when not in bloom, the upright foliage can add interesting texture to a garden.

Where to Plant Iris

For the best bloom, plant iris in a sunny location. If your climate is on the hot side, some shade is acceptable. Almost all irises need well-drained soil but there are some species that prefer more moisture than others. The ideal soil for irises is slightly acidic but irises are quite adaptable pH-wise.

There are lots of possibilities for planting irises in your landscape. They look lovely as an accent near an entrance area or gateway, along a fence or wall, or interplanted with climbers or evergreens. Wherever you plant them, don’t skimp—plant irises in groups or mass-plantings. Because irises grow over a wide range of zones, make sure to pick varieties that fit your local climate.

How and When to Plant Iris

Iris should be planted early enough in the late summer or early fall so that their roots are well established before the winter cold sets in. In cool climates, this can be as early as mid-July, whereas in southern locations planting in the early fall is fine. Space several plants 1`2 to 24 inches apart, depending on variety—irises have a tendency to spread into large clumps.

When planting iris, don’t plant them too deep—the tops of the rhizomes should be exposed and never covered by soil. After digging a shallow hole or trench, place the rhizomes inside with the roots facing downwards and spread the roots out in the hole. Backfill with the original soil and gently tamp it down. Water slowly but deeply to remove any air pockets. Keep the soil moist and water in the absence of rain so the roots can get established.

Iris Care Tips

 Light

 When it comes to sunlight, all irises need six to eight hours of full sun for the best blossoms and foliage growth. In too much shade, they may stretch for the sun and become leggy and prone to flopping over. Also, the plants will be less likely to bloom.

Soil and Water

Bearded and bulb-type irises need well-drained soil to thrive because they will rot easily in soggy conditions. Many of the iris species are native to rocky mountainsides where there is sharp drainage. If you are looking for a species to grow in moist soil, look for a Louisiana iris, Japanese iris, or a yellow flag iris. Because there are such differences in the growing conditions of irises, make sure to do a little research before planting so you can provide your particular iris with what it needs.

In terms of pH, irises prefer slightly acidic soil with a pH of around 6.8 but they are quite adaptable.

After irises are established, they don’t need watering except during extended dry spells.

Temperature and Humidity

Irises vary in their cold-hardiness. Siberian irises are especially hardy and don’t like extremely hot summers while others, such as Bamboo iris (Iris confusa) can even be grown in southern climates up to zone 11.

Irises are relatively tolerant of humidity.

Fertilizer

Irises don’t need much fertilizer unless they started out in soil that is poor in nutrients. About a month after the bloom, feed them with bone meal, superphosphate, or a fertilizer that is high in phosphorus. Do not use any fertilizer that is high in nitrogen because that will lead to excessive foliage growth and root problems.

Pruning

After the bloom, cut the stems close to ground level so the plant does not waste its energy producing seeds. But other than removing brown and dead leaves, do not cut back the healthy green foliage until late fall, then cut it back to about 6 inches. The plants need the foliage to build up energy in its rhizomes.

Potting and Repotting Iris

 What applies to spring bulbs also applies to irises: It is not recommended to plant them in pots year-round in climates with cold winters, as the rhizomes are exposed to freeze-and-thaw circles during the winter, which can affect root growth and even kill them. 

If you live in a climate with mild winters, you can plant suitable iris varieties in pots. Choose a 1-gallon pot with large drainage holes, one for each rhizome, and fill it with well-draining potting mix. The top of the rhizome should be exposed, just like when planting in garden soil. Potted irises need regular watering and more frequent fertilizer than irises in the landscape.

When the rhizomes fill the pot, it’s time to plant them in a larger pot or divide them into pots with fresh potting mix.

Pests and Problems 

 Irises are fairly pest-free, but iris borers are their one nemesis. This bug does most of its damage around blossom time, often between mid-April and mid-June. They chew their way into the leaves and then burrow down into the rhizome, leaving behind a trail of frass, a powdery brown residue. Once at their destination, the borers can eat several rhizomes and make their way through an entire bed. Their damage opens plants up to infection from bacterial rot, too. These insects are tricky to control as they are often hidden within the plant where pesticide sprays can't reach them. If you do find a damaged plant, dig it up and see if you can locate the culprits and dispose of them. The best method of control is often prevention, so make sure to clean up any debris around your irises in fall and early spring, which is where borers overwinter.

How to Propagate Iris

Bearded irises and other rhizomatous types will eventually need to be divided every two to five years, which is also the way to propagate them. If it has been quite some time since their last division and your irises seem to be flowering less and less, chances are it's time for them to be divided. The best time to divide and replant most irises is late summer to early fall. At this point in their life cycle, irises are somewhat dormant and are resting up for their fall growth cycle to begin.

To divide your irises, carefully lift the plants out of the ground (a pitchfork works great for this), and carefully tease apart the individual rhizomes. Don't worry if you break a few roots or rhizomes, as they are pretty tough plants. If there is large foliage attached to the rhizome, you can cut it back by half (this helps the plant lose less water while recovering from all the dividing upheaval).

Once they are all separated, you can replant by digging a small trench and setting each rhizome on a small mound of soil, then fanning out the roots around it. Then, backfill with soil around the
rhizome, making sure to tamp down any air pockets and bringing the soil level just to the top of the rhizome. Water your plants well. Give them a good drink once or twice a week for the first several weeks after planting until the new roots begin to grow.

Types of Iris

'Bennerup Blue' Siberian Iris

Iris sibirica 'Bennerup Blue' produces cobalt-blue flowers with small white blotches. It grows 2 feet tall. Zones 3-9

'Electric Rays' Iris

Iris ensata 'Electric Rays' is a Japanese iris with large double violet flowers streaked with white. This award-winning variety grows 3 feet tall. Zones 5-9

'Caesar's Brother' Siberian iris

This variety of Iris sibirica is a classic Siberian iris with stunning deep purple blooms. Zones 3-8

'Champagne Elegance' Iris

This Iris selection has glamorous fragrant flowers, several per stem, that have pale buff-apricot falls with amber beards and white standards faintly blushed pink. This tall bearded type may rebloom in late summer. It grows 3 feet tall. Zones 4-9

Black Iris

Black iris (Iris chrysographes) is known for its especially dark purple-red fragrant flowers. It blooms in early summer and grows 2 feet tall. Zones 7-9

Forrest's Iris

Iris forrestii, a striking yellow Siberian iris, grows about 16 inches tall and has mildly fragrant flowers. It blooms best when given some afternoon shade. Zones 4-9

Crested Iris

Iris cristata is a wild form native to areas of North America. It bears blue, white, or purple flowers in spring over tiny clumps of sword-shape foliage and grows to 1 foot tall. Zones 3-9

'Satozakura' Japanese Iris

This Iris ensata variety prefers to grow in water or damp places where soil is acidic. Its large, somewhat flat mauve to rosy-pink flowers are yellow at the throat and conspicuously veined. Clumps grow to 3 feet tall. Zones 6-9

'Immortality' Iris

This selection of Iris is a tall bearded type that offers pure-white flowers. It commonly reblooms in fall and grows 3 feet tall. Zones 4-9

'Harpswell Snowburst' Siberian Iris

This Iris sibirica cultivar offers blue-violet flowers edged in white. It grows 3 feet tall. Zones 3-9

Dwarf Iris

Iris danfordiae is an early-blooming bulb with single yellow flowers in late winter. It grows 6 inches tall. Zones 5-8

'Variegata' Iris

Iris pallida 'Variegata' is a lovely variegated bearded iris with rich purple blooms and a pleasant fragrance. Zones 4-7

Louisiana Iris

Iris fulva has 4-inch-wide coppery-red flowers that are yellow at the center but without beards. The swordlike leaves may reach 4 feet tall. Zones 4-9

'White Swirl' Siberian Iris

This variety of Iris sibirica makes tough clumps of narrow green sword-shaped leaves. In midsummer, each stem blooms with several 1- to 2-inch pure-white flowers touched with yellow at the base. Zones 3-9

'Yellow Flag' Iris

Iris pseudacorus 'Yellow Flag' grows vigorously in wet places. Its thick clumps of grayish spearlike leaves may reach 4 feet tall. In late spring and summer, 2-inch yellow flowers appear on strong stems. It may become invasive. Zones 5-8

Iris Companion Plants

Thrift

If you have a hot, dry spot with excellent drainage, you must give thrift a try. A small, tidy plant, it covers itself with adorable bobbing pink flowers. It's a charming groundcover, edging plant, or rock garden feature. When planted in large groups, it forms a mat of attractive grassy foliage and colorful marble-size balls of flowers. Also called sea pink, this tough plant tolerates wind, sea spray, and is drought-tolerant. They do need well-drained soil to prevent root rot.

Primrose

Take a walk down the primrose path and you'll never look back! Primroses are a classic cottage flower and are popular with collectors. They covet the hundreds of different primroses available, especially some of the tiny rare alpine types. Many are staples of cottage gardens and rock gardens, while others provide spring color to damp places, rain gardens, and bog gardens. Their basal rosettes of oval leaves are often puckered or are very smooth. The colorful flowers may be borne singly or rise in tiered clusters, or even spikes. Provide humus-high soil that retains moisture and some shade for best results.

Peony

Perhaps the best-loved perennials, herbaceous peonies belong in almost every garden. Their sumptuous flowers—single, semidouble, anemone centered or Japanese, and fully double—in glorious shades of pinks and reds as well as white and yellow announce that spring has truly arrived. The handsome fingered foliage is usually dark green and remains good-looking all season long. Provide deep rich soil with plenty of humus to avoid dryness, and don't plant the crowns more than 2 inches beneath the surface. But these are hardly fussy plants. Where well suited to the climate, they can thrive on zero care.

Lupine

Lupine draws the eye skyward with its gorgeously colored and interestingly structured flower spikes. Bicolor Russell hybrids are the most popular type. Their large pea-like flowers come in amazing colors and combinations, clustered in long spikes on sturdy stems. Lupine prefers light, well-drained soil that is slightly acidic, and it does not tolerate heat or humidity well. It performs best in areas with cool summers, especially the Pacific Northwest.