When to Plant Iris Flowers

Iris Growing Conditions

• Bearded iris grows in well-drained soil with low fertility levels. Soil that is too rich induces soft top growth that is susceptible to disease. Exposure to at least six hours of sun each day is important for growing most irises. The application of manure or excessive amounts of compost promote root rot, a common iris disease. Like the bearded iris, all the bulbing irises need well-drained soil.

  The most tolerant of soil conditions, and considered easiest to grow, is the Siberian iris, which is also tolerant of part shade. Unlike the bearded iris, the flag, Japanese and Louisiana Iris prefer moist rich soil, but still grow best in full sun.

When to Plant Bulbing Iris

• Bulbing types of iris plants are appropriate for planting in USDA plant hardiness zones 5 through 9. They are planted in the fall along with other spring blooming bulbs. Plant iris bulbs 3 to 4 inches deep and 6 inches apart. Divide bulbing iris every three years by digging up the bulbs in late summer or early fall after they are dormant and the foliage has completely faded. Gently pull the bulbs apart with your hands and plant as desired.

Planting Iris Rhizomes

• Plant the rhizomes of the iris in fall or spring. Bearded iris are planted 1 inch deep so the rhizomes are slightly exposed at the surface of the soil. Other types of iris rhizomes are planted 2 inches deep. Planting the rhizomes too deep causes them to rot, so follow planting instructions carefully after buying new plants. Plant rhizomes 24 inches apart to give them room to multiply.

Dividing Iris Rhizomes

• Iris rhizomes are divided and replanted every three to four years in late summer. Take note of the depth the rhizomes are growing in the soil before dividing. Cut the leaves back by two-thirds and carefully lift the rhizomes from the soil by pushing a sharp shovel or garden fork 8 to 10 inches below the rhizomes. When you pick up the rhizomes, you will see the old original rhizome and the younger offsets. Gently separate the newer rhizomes by slicing the plant material holding them together with a sharp knife. Discard the old rhizome as well as any mildewed, diseased or insect-damaged rhizomes.

Planting Divided Iris Rhizomes

• Plant the divisions at the same depth as they were growing in the previous location. The appropriate depth will vary slightly for each variety. Loosen the soil to a depth of 5 inches and plant the iris rhizome at the appropriate depth. Ensure the roots are spread out and the rhizome is planted deep enough to account for soil settling that exposes the shallowly planted rhizome more than expected after a rain.