When to Plant - The best time to plant bare root irises is during the late summer, typically July – September. Some gardeners in hot, southern climates prefer not to plant until October, avoiding the hottest part of the summer. In northern climates, irises should be planted at least 6 weeks before first frost to allow new roots to establish.
Potted irises purchased at local sales or garden centers can be planted early spring until 6 weeks before frost.
Location - Irises need at least 6 hours of sunlight per day to bloom and often thrive in full sun. Choose a well-drained location, as wet soils will facilitate rot in bearded varieties.
How to Plant - Bury all roots and the bottom half of the rhizome, leaving the top side of the rhizome exposed. Burying the entire rhizome will lead to rot. Mulch can be used around the plant, as long as the top of rhizome remains exposed.
Space rhizomes 12”-20” apart, depending on your preference. Rhizomes will slowly spread and will need to be divided in time. The closer you plant rhizomes, the sooner you will need to divide.
Watering - In the absence of rain, newly-planted rhizomes need to be watered for new roots to flourish. Allow the top couple inches of soil to dry between waterings. Once rhizomes are established, the American Iris Society suggests watering when the top three inches of soil dry out. The amount of watering will depend on your climate. Over watering will cause rot.
Fertilizing - Your soil type will determine if fertilizing is necessary. Contact your local extension agent to learn more about the soils in your area. If fertilizer is recommended, choose a fertilizer without much nitrogen, such as 5-10-10. Excess nitrogen will cause rot. Fertilizer can be applied in early spring, and approximately one month after bloom.
Dividing - Dividing iris clumps every 3-4 years is recommended. When irises become crowded, they will not bloom. Divide iris in late summer or at least 6 weeks before first frost. Dig the entire clump of irises, break or cut apart individual rhizomes to replant, and discard the old, center, “mother” rhizomes.