When should we divide our irises
When should we divide our irises? I have several huge clumps that don’t bloom as well as they used to.
A: Irises (Tall-bearded, dwarf-bearded, I. pallida, and other traditional garden types) are amazingly tough and resilient. I have divided and moved them almost any time of the year. Ideally, though, late summer is the best time. Dividing in late July or August allows the semi-dormant plants to become well-established and to reserve plenty of carbohydrates. Iris divided in late summer are more likely to flower the next spring. Divided in fall or winter, they may not flower until the second spring. Also, leaving divisions with three or more growth tips (rhizomes) will increase the chances of first-year flowering.
When re-setting Iris, loosen the soil, dig in a cup-or-so of compost and a tablespoon of organic phosphate. Plant the rhizomes in an area that has full sun for at least six hours. Iris won’t flower in shade. Lastly, assure that the top of the rhizome is slightly ABOVE the soil level. Buried rhizomes ultimately rot.