Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.
What are Japanese Iris?
Japanese iris (Iris ensata) may look exotic, but they are easy to grow. As their name implies, they are native to Japan where they have been grown and bred for 500 years. Japanese iris are beardless iris like their cousins, the Siberian iris, but their flower petals are larger and flatter, earning them the nickname "Butterfly Iris". The flowers range in color from purple to blue to pink to white and many bi-colors. They bloom at the end of the “iris season” after the Siberian iris, normally in mid- to late June. Their leaves are thinner and more grass-like than the bearded iris. They are also slightly shorter, about 40” tall. Japanese iris are hardy in USDA growing zones 4-9.
How to Grow Japanese Iris
Japanese iris require a minimum of 6 hours of full sun each day. In warmer climates, shade in the afternoon is preferred to keep them from drying out. They like moist, well-drained soil that is more on the acidic side. They are often planted on stream banks, their natural habitat, where the soil remains constantly moist. They are not pond plants, however. Their crowns must remain above the water line. Areas with a high water table are also good choices for planting your Japanese iris.
Japanese iris can be planted any time from spring until fall. Don’t allow your rhizomes to dry out before planting them. Soaking them overnight before planting is a good idea. Plant them so that the tops of their rhizomes are at least one inch below the soil surface. Keep your plants well mulched to make sure that the soil remains cool and moist and to keep down weeds.
Japanese iris are what is known as heavy feeders, rapidly depleting the soil of essential nutrients. It’s a good idea to fertilize them every year, either right before they bloom or just after they bloom. Use the same fertilizer that you would use on your acid loving plants such as azaleas and rhododendrons.
How to Divide Japanese Iris
Like most perennials, it’s a good idea to divide your plants every 3 to 4 years to maintain good health. You will notice that your iris clumps expand upwards each year because new roots grow above the old roots consequently forcing the bulbs upwards towards the top of the soil where it is dryer which is not conducive to good growth for these moisture loving plants. You should divide your clumps before they reach that point.
Late spring or early summer is the best time to divide and replant your iris. Divide them using a garden fork to carefully dig up the rhizomes about a month after they finish blooming. Use a sharp knife to cut them apart. Replant the outermost, young rhizomes which are healthy and discard the older center ones which will not grow and bloom as well. Also discard any dead or diseased rhizomes. Make sure that each rhizome has both leaves and roots. Replant them with the tops at least one inch below the soil surface. If you are not going to be replanting your rhizomes immediately, keep them moist by storing them in bucket of water until you are ready to plant them. Smaller divisions can take up to two years to reach a size where they will bloom so be patient.
How to Use Japanese Iris for Cut Flowers
Japanese iris make excellent cut flowers. Harvest your flowers early in the day. Choose buds that are just beginning to open rather than flowers that are fully open. The buds will open in a few hours in the vase. Place your flowers in a bucket of tepid water until you are ready to create your arrangement. When you are ready to arrange them, re-cut the stems about an inch above the original cut at an angle. To ensure the longest life for your flowers, keep your arrangement out of direct sunlight. It’s also a good idea to place it away from drafty doors and windows. Be sure to remove dead and dying flowers promptly.
Add an exotic look and prolong the iris season by planting Japanese iris. They are hardy, easy to grow plants for the moister areas of your yard.
Questions & Answers
Question: Can I add vinegar when watering Japanese Iris?
Answer: No, vinegar is a strong acid that is often used as a weedkiller. In this case, acidity refers to the pH of the soil, not acid like vinegar. Your best bet is to use a fertilizer specially formulated for acid-loving plants. I am an organic gardener, so I like to use pine needles which are acidic for my acidic soil loving plants. Pine needles are a natural, renewable source of acidity. I gather them from underneath evergreens and use them to mulch around my acidic soil loving plants.
Question: If I buy a Japanese Iris from a nursery and it is potted is it okay to just place it as potted in the ground?
Answer: Yes, you can plant it straight into the ground. Just make sure that the top of the soil that was in the pot is even with the top of the soil in your garden. If you plant too deep or too shallow, the rhizome will die.
© 2015 Caren White
Caren White (author) on January 19, 2015:
Flourish, I'm knee deep in catalogs and counting the days until spring. Thanks for reading and commenting.
FlourishAnyway from USA on January 18, 2015:
You have me looking forward to Spring!
Caren White (author) on January 18, 2015:
Sorry Will, but Japanese iris require moisture. They are best grown in wet soil.
WillStarr from Phoenix, Arizona on January 18, 2015:
Can they be grown in the Arizona desert? We live in Phoenix.
Caren White (author) on January 18, 2015:
Heidi, so sorry that you can't grow iris. They are one of my favorite flowers. Thanks for reading, voting and commenting.
Heidi Thorne from Chicago Area on January 18, 2015:
Irises are lovely, but I've never been able to grow them successfully. I think it's a soil quality issue. Thanks for the info! Voted up, useful and beautiful!