Water iris or Japanese iris (Iris laevigata and Iris ensata) are perennial plants with rootstocks that grow in cool to wet soil, even with their bases in water. Very elegant, they flower from May–June to August, bringing a touch of colour and sophistication to borders, cool rockeries or pond and pool edges. To multiply these plants and obtain new specimens, the most effective method is division of the rootstocks. Here is our step-by-step guide to multiply them easily.
When to divide water irises?
Best time to divide water irises is late summer or early autumn, generally between August and October, after flowering. At this time the plant begins to enter dormancy, which reduces stress on the rootstocks during division and allows new plants to establish before winter, ensuring optimal flowering next spring. It is also possible to carry out division in early spring, just before growth resumes, but greater attention will then be needed for watering.
Equipment needed :
How to divide water irises?
Multiplying Japanese irises by sowing is possible, but usually not recommended, as it is a tedious and slow method that requires a lot of patience. That is why we recommend division of the rootstocks to multiply this handsome perennial that is at ease on pond edges or in waterlogged summer soil.
- Start by gently digging up the clump of irises. To do this, insert the spade around the plant about 10 to 15 cm from the base so as not to damage the rootstocks. Lift the clump gently taking care to preserve as many roots as possible.
- Once the clump is lifted, shake it lightly to remove excess soil. You can rinse the rootstocks with clear water to inspect them better. Examine them carefully and remove any soft, rotten or diseased parts to avoid spreading infection. Make sure each piece is healthy and vigorous.
- With a well-sharpened knife or a pruning shear, cut the rootstocks into sections 10 to 15 cm long. Each section should include at least one bud or a fan of leaves. Smaller sections can be potted to grow on before planting permanently in the garden.
- Replant the divided rootstocks immediately. Place them in moist soil, pushing them in so the rootstock is just covered with soil. Bury rootstocks shallowly, 7 to 12 centimetres, so they lie close to the surface.
- Leaves should be above the soil to prevent rotting. Space new plantings about 25 to 40 cm apart to allow good development.
- Water new young plants generously. Continue to water regularly, especially if division was done in spring or if climate is dry. Ensure soil remains moist, or at least cool.
Our tip: If water irises are well established, consider dividing again every 3 to 5 years. This will prevent clumps becoming too dense and rootstocks lacking space to develop. Regular division of Japanese irises also rejuvenates clumps and keeps them floriferous and vigorous.
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