Q. I am very fond of the bearded irises in my garden. Some of the clumps have become slightly overgrown and crowded. When should they be divided? Can I do it now?
A. To maintain bearded (German) irises in good health and optimal flowering, the clumps should be divided every few years. Depending on the plants themselves and their growing conditions, division can be undertaken every two to five years. For most, every three years is about right. Crowded clumps and diminished bloom are indications that it’s time for renewal.
The ideal time for lifting, dividing and replanting these irises is right after flowering, in July or August. This gives ample time for new roots to become well established before the arrival of cold, wet weather.
I would not take a chance on dividing and replanting now. These irises prefer soil that is on the dry side. An autumn planting would mean that healthy rooting would be compromised by cold, wet conditions.
When you do divide and replant, lift each clump carefully to avoid damage to the rhizomes. If the soil is dry, water the day before to make the lifting easier. Separate the clumps into divisions, each with at least one fan of leaves.
Cut the leaves back by around a half, making clean cuts with sharp scissors, and trim back any overlong roots. Trimming back the leaf fans reduces transpiration from the foliage and helps newly planted divisions to settle in. Check for damage, such as soft spots, on the rhizomes and discard any dubious ones.
Choose a planting site in full or nearly full sun, with a rich, loamy, pH neutral (not acid) soil that drains quickly of excess moisture. I form a ridge in each planting hole, so that I can settle the horizontal rhizome atop the ridge and arrange the roots down the sides before firming in the plant. The rhizome should end up just barely showing at the soil surface.
Avoid over-watering bearded irises.
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