Growing Japanese iris at home, (Iris ensata) requires the correct combination of several components. And once you do this, you’ll have good success right up into a USDA zone 4 gardens because this plant is quite hardy.
This plant appreciates a full sun spot or, at the very least, an afternoon sunshine garden.
It demands acidic soil for long-term success. When I grow it in containers on the side of the pond, I generally grow it in straight peat moss. In the bog garden, I amend the soil to 50:50 peat and soil to get that acidity level dropped down. If you don’t use acidic soil, you’ll find the plant will fail to thrive.
It wants a great deal of moisture. There’s little point in trying to grow this plant in the dry, sunny garden as you would any other iris. This is a plant of the bog. I have had the best success with this plant when I grow it very wet in the spring and allow it to dry out a little in the fall (still damp but not water-logged)
So for best success, combine damp soil with an acidic one.
A shovel of compost in the spring is usually adequate food to keep them blooming during July.
The flowers are some of the largest of the iris family and have a faint scent (if any).
They do not repeat (not even if you cut them back).
Propagating Japanase Iris
I divide mine in the spring or fall (mostly spring).
You can also start them from seed as they do develop pods in the late summer. Sow in the early spring as you would for any other perennial. They are not difficult to germinate, although you might think you’re growing grass. Remember, they require acidic soils and damp conditions.
Overwintering Japanese Iris
I don’t do anything special to them in zone 4 – simply cut back the foliage.
I tend to divide my plants every 3-4 years, which gives them a bit of a renewed interest in flowering.
Japanese Iris Problems
As with all iris, you’ll sometimes see Thrips on the flowers. These are controlled using insecticidal soaps and hanging blue sticky traps next to the iris beds.
As long as you have the soil acidity correct and provide enough moisture, this plant will grow. If you don’t provide those conditions, you’re wasting your time and money.