How to Plant & Grow Japanese Water Iris (Iris ensata)

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Different Japanese water iris cultivars
The original Japanese water iris species is blue with yellow markings, but today many different cultivars exist. katorisi, CC BY-SA 3.0, via Wikimedia Commons

The Japanese water iris with its tall, sword-shaped leaves and large (3 – 6 inches or 8 – 15 cm), showy flowers is a delight in any garden. Also called the Japanese flag iris, sword-leaved iris, or simply the Japanese iris, it is part of the iris family, along with other popular garden plants such as gladioli, freesias, and other irises. The name Iris means “rainbow” in Greek, while ensata means “sword”, describing the shape of its leaves.

The Japanese water iris is originally from East Asia and Russia, where they have been cultivated extensively for many years. The original species is blue with yellow markings. Today there are many different cultivars available providing either standard or double bloom flowers in a wide variety of colors including blue, purple, white, red, and pink.

Apart from adding color and height to any pond garden, they can increase your water quality and remove pollutants from the water. The large plants can protect pond fish from predators by providing hiding places. They also provide a perch for birds who want a drink or bath from your pond.

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Japanese Water Iris Fact Sheet:
USDA 4 – 9, UK zones 1 – 5
Full sun to partial shade
Blue, purple, pink, red, or white
June and July
2 – 4 feet (60 – 120 cm)
2 inches (5 cm)
Acidic to neutral (pH 5.5 – 7)
Average to high
Clay, loam, sand

Japanese Water Iris Growth, Hardiness & Climate

Japanese water iris in a pond
Japanese water iris can be grown in bogs and ponds, but they must be dry in the winter. Rs1421, CC BY-SA 3.0, via Wikimedia Commons

Japanese water irises can be grown in any part of the garden providing they are kept moist during spring and summer. A surrounding mulch layer will help retain moisture in the soil. They can also be grown in bogs or ponds, but they need to be dry during winter. They prefer rich, slightly acidic soils, so give them lots of compost and fertilizer. They grow best in full sun, but in hot areas, they can take some shade in the afternoon.

How to Plant Japanese Water Iris

Japanese water iris blooms in a field
To stimulate new growth, you should trim your Japanese water iris’ leaves to a length of 4 – 6 inches. 663highland, CC BY-SA 3.0, via Wikimedia Commons

The Japanese water iris can be grown from seeds or rhizomes, or you can buy established plants from a nursery and plant them in the ground or into a new pot. You should always repot newly bought plants as soon as possible to ensure the roots are healthy and the pot is big enough. The best time to transplant an iris is after it has finished flowering. Trim the leaves to a length of 4 – 6 inches (10 – 15 cm) in an inverted v-shape to stimulate new growth and reduce transplant shock. Plant them 2 inches (5 cm) deep in the soil and keep them moist.

Seeds can be harvested as soon as the flowers die back and should be planted immediately. Plant the seeds 2.5 inches (6 cm) deep in trays and keep them moist. They need some cold exposure to stimulate germination, so you can keep the tray outside during the winter. They can take anything between 1 and 18 months to germinate, so don’t throw them away if they don’t germinate immediately. Seedlings can be transplanted in spring or fall.

Rhizomes can be planted in fall or spring. Soak the rhizomes overnight and plant them 2 inches (5 cm) directly into the garden or pot where you want them to grow. Add a layer of mulch to the soil to retain moisture and discourage weeds.

How to Care for Japanese Water Iris

Japanese water iris flowers by a river
Be sure not to submerge your Japanese water iris too deeply, as they can suffocate. Wikiodaiba, CC BY-SA 3.0, via Wikimedia Commons

Japanese water irises generally require little maintenance if provided with ideal growing conditions. General care includes cutting flower stems after flowering and removing foliage after they die back in the fall. For optimal flowering, plants should be divided and replanted every 3 – 4 years to prevent overcrowding.

Even though they are considered water plants, their natural habitat is marsh meadows or on the banks of water bodies where the soil is always moist but they are not submerged. The Japanese water iris can suffocate if submerged too deeply and are prone to root rot if they are kept wet during the winter months. Their soil should be loose, and not too tightly compressed, so that the roots can breathe.

They are not prone to many diseases, but you should keep an eye out for pests like slugs, snails, aphids, cutworms, caterpillars, leaf miners, iris borers, and rust. Larger pests, such as caterpillars and snails, can be removed by hand. Aphids can be wiped off the leaves with soapy water. For rust, remove affected leaves and dispose of them appropriately. Do not add the leaves to your compost heap and sterilize your secateurs afterward to prevent spreading the disease. For other pests, ask your local nursery for appropriate advice and treatment.

How to Winter Japanese Water Iris

Although they are perennial plants, meaning they live for multiple years, they lose their leaves and go dormant in winter. While they need lots of water during the growing season, they prefer to stay dry during fall and winter. The easiest way to provide these conditions is by growing them in a pot. The pot can be placed in a shallow pond during spring and summer and then removed during fall and winter. Bury the pot in the soil in a different part of the garden until spring to ensure the correct temperature while overwintering.

Is Japanese Water Iris Invasive or Toxic?

Similar to other irises, the Japanese water iris produces several toxic chemicals, especially in their rhizomes. Ingestion can lead to nausea, vomiting, diarrhea, salivation, abdominal pain, and lethargy. The plant sap can cause skin irritation, so you should preferably wear gloves when handling them and wash your hands afterward.

While they have spreading rhizomes and can form large clumps in the garden when conditions are favorable, they are not invasive.

Is Japanese Water Iris Edible? Do Animals Eat it?

Their toxicity is probably a mechanism to discourage herbivores from eating them, because deer and other animals tend to avoid them. They are poisonous to dogs and other pets. However, many insects seem to be resistant to their toxins, since aphids, caterpillars, and other pests will eat their leaves.

Where to Buy Japanese Water Iris & Seeds? (UK & US)

The Japanese water iris is a popular garden plant. Visit your closest nursery to see which cultivars they have or purchase them online. You can buy them as rhizomes, seeds, or potted plants. Common names can be confusing. “Japanese iris” can refer to either Iris ensata or Iris japonica, so make sure you buy the correct plant. Also, make sure you know which cultivar you are buying, so you get the flower and color you want. Different cultivars may have specific requirements, so ask your supplier for tips on how to grow them.

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Angeline L
About the author

Angeline L

I'm a passionate researcher and scuba diver with a keen interest in garden plants, marine life, and freshwater ecology. I think there’s nothing better than a day spent writing in nature. I have an academic and professional background in sustainable aquaculture, so I advocate for the responsible production of commercial fish, macroinvertebrates, and aquatic plants.

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