How to Grow and Care for Japanese Iris

Enhance Your Water Feature With This Versatile Iris

Japanese iris plant with bright purple, yellow and white flat petals in sunlight closeup

The Spruce / Evgeniya Vlasova

The Japanese iris is related both to irises commonly grown in the landscape, such as the bearded iris (Iris germanica), and to such wild iris plants as the Northern blue flag (Iris versicolor), a native of North America. Japanese irises come in several colors. and the flowers are flattish measuring about 3 to 6 inches across, rising above green blade-like foliage. The bulbs should be planted in the fall for a strong spring display. Because these irises like to grow around water during the spring and summer months, they are useful in certain damp and boggy problem areas of the landscape where other plants would fail miserably. The iris family, including the Japanese iris, is toxic to humans and animals

Common Name Japanese iris, Japanese water iris, Japanese flag, sword-leafed iris
Botanical Name Iris ensata
Family Iridaceae
Plant Type Herbaceous, perennial
Mature Size 2 to 4 ft. tall, 1.5 to 2 ft. wide
Sun Exposure Full, partial
Soil Type Loamy, clay
Soil pH Acidic
Bloom Time Summer
Flower Color Blue, lavender, pink, white
Hardiness Zones 4-9 (USDA)
Native Area Eastern Asia, Kazakhstan
Toxicity Toxic to humans and animals

Japanese Iris Care

Many landscapes suffer from depressions where water collects and the soil remains soggy for extended periods of time in the spring. Most plants do not grow well in such places, leaving homeowners at a loss as to what to plant there. Japanese iris can be a great boon in these cases, covering the ground while furnishing color at the same time.

Japanese iris is a bit fussy with its growing requirements. You need to give it just the right sun, soil, and spacing conditions. It is not as easy to grow as many other types of irises. Happily, Japanese iris is not a favorite food of deer. Partly because the deer tend to leave it alone, Japanese iris will sometimes naturalize in North America. Its chief enemies are compacted soil, improper irrigation, and overcrowding.

Japanese iris plants with thin stems and blade-like leaves with white flat flowers on top

The Spruce / Evgeniya Vlasova

Japanese iris plant with flat purple and yellow flowers and buds on thin stems with bee on top closeup

The Spruce / Evgeniya Vlasova

Japanese iris plant with dark purple flowers on thin stems and blade-like leaves

The Spruce / Evgeniya Vlasova

Japanese iris plant with purple, white and yellow streaked flat petals closeup

The Spruce / Evgeniya Vlasova

Light

This perennial performs best in full sun unless you are growing it in an area that experiences very hot summers. In the latter case, it profits from a bit of afternoon shade.

Soil

Japanese iris wants friable, loamy soil. When soil particles become too tightly packed together, the roots of the Japanese iris cannot breathe, and the plant suffers. This is why it is critically important to provide the plant with friable soil.

Water

Japanese iris needs a lot of water in the spring, less (but still a significant amount) in the summer, and way less in the fall and winter. Make sure that the Japanese iris is supplied with proper irrigation. The amount required varies based on the time of year.

In general, water Japanese iris daily in the spring and summer. This plant requires more water than many landscape plants during the growing season. This is especially true in the spring, which is the season when the plant is growing most actively. But, even in summer, keep the soil evenly moist; never let it dry out completely during the summer. Problem is, during the rest of the year (fall and winter), the plant can suffer from being in excess water. So you have to strike a careful balance.

Temperature and Humidity

This iris is cold-hardy and it tolerates a large range of temperatures, even deep freezes. Japanese irises thrive in humid conditions. You can mist them and they will be happy.

Fertilizer

This plant is a heavy feeder. Grow it in an area that has been amended previously with organic matter and work compost into the soil around it annually.

Types of Japanese Iris

Different types of Japanese irises have varying shapes, sizes, and shades of purple and white petals. Here are popular choices:

  • 'Japanese Harmony' features white petals and a center fluff of smaller purple petals.
  • 'Japanese Pinwheel' offers varying shades of flared purple petals with thin white rims.
  • 'Japanese Sandman' is a full-petaled light purple flower with a center fluff of yellow, white, and purple petals.
  • 'Freckled Geisha' has white papery-thin petals outlined with purple ruffles and a hint of yellow fluff in the center.
  • 'Dinner Plate' is a newer trademarked variety of Japanese irises with extra-large ruffled layers of petals of either white, purple-white, purple-pink, or purple-blue combinations.
'Japanese Harmony' iris
'Japanese Harmony' Iris

Maureen Bates Photography / Getty Images

Japanese Pinwheel Iris
'Japanese Pinwheel' Iris

Courtesy of Iris Encyclopedia

Japanese iris plant with purple, white and yellow streaked flat petals closeup
'Japanese Sandman' Iris

The Spruce / Evgeniya Vlasova

'Freckled Geisha' Iris
'Freckled Geisha' Iris

Courtesy of Iris Encyclopedia

Propagating Japanese Iris

The very best way to propagate Japanese iris is by division. These flowers spread underground by rhizomes and they will need to be divided either in the spring or fall by taking these steps:

  1. Dig up a root ball and bring it up from the ground.
  2. Gently break apart the root ball into two to three plants.
  3. With a sanitized garden-cutting tool, trim dead roots. Note that healthy live roots will look white.
  4. Keep the roots moist until you plant them back into the ground.

Potting and Repotting Japanese Iris

Because the plant is finicky about water (whether too little or too much), the easiest way to grow Japanese iris is as a potted plant for your water feature. When plants are grown in containers, they give you more flexibility because you can move them around based on their needs and your own needs. You can even place a potted Japanese iris in standing water during the spring and summer. Because the plant is intolerant of excessive water in the off-season, you can simply take the pot out of your water garden in the fall. Shallow and wide plastic pots with good drainage make ideal containers for Japanese iris plants that stand in water.

Overwintering

To keep the plants happy in harsh weather, plant your Japanese iris, pot and all, in the ground (somewhere else away from wet areas in your garden) to overwinter. And then transport the plant, pot and all, back to the water feature at the beginning of the next spring.

Common Pests

Japanese iris are vulnerable to pests such as snails and slugs. Aphids and iris borers are also attracted to this flower. Pesticides and insecticidal soap will help control these pests.

How to Get Japanese Iris to Bloom

If your Japanese iris' pretty purple, pink, or white blooms are weak and you can't detect the sweet fragrance they usually emit during their peak summer months, there could be a few issues. For better blooms, consider the following:

  • The plants may not be getting enough sun to bloom correctly, which means they need to be replanted in a sunnier spot.
  • The bulbs could have been buried too deeply in the soil which compromises the growth of rhizomes and flowering. Dig up the bulbs and replant no deeper than 3 inches for better results.
  • The irises could be overcrowded which also can decrease their ability to bloom, which means digging them up and dividing them so they are planted about 8 inches apart.
  • No blooms could also mean the rhizomes are mushy or rotted, which could be a result of compacted soil. Dig up, cut away affected rhizomes, and amend the soil to help restore blooms.

Common Problems With Japanese Iris

Though happy Japanese irises do not need much supervision there's generally one problem that can affect these plants, and that's fading colors. Usually, the Japanese iris boasts deep colors. But if it's fading, it could mean several things:

  • It may be shocked due to transplanting or extreme temperature changes.
  • Possibly a critter or pest has dug the soil around the plants exposing the rhizomes.
  • Another reason is age. If the iris has seen a couple of seasons, it could be ready to expire.
FAQ
  • Where do Japanese iris grow best?

    Because of its tolerance for wet soil in spring and summer, the Japanese iris is an ideal candidate for use around water features, ponds, and streams.

  • Do Japanese irises spread?

    Japanese irises spread via underground rhizomes. A colony of plants will grow vigorously and soon start to grow into each other's space, resulting in overcrowding. To solve the problem, transplant some of the plants out of their patch to another suitable spot in the yard. This will give sufficient "elbow room" to the remaining plants.

  • What should I do with Japanese irises after they bloom?

    You just need to deadhead the spent flowers.

Article Sources
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  1. Iris ensata. North Carolina Extension Gardener Plant Toolbox.

  2. Iris. Pet Poison Helpline.