Tough and durable Siberian iris lights up the landscape in late spring and early summer with its colorful blooms in shades of lavender, white, blue and yellow. When the blooms fade, the foliage remains attractive, turning gold before it dies down in autumn. To keep Siberian iris blooming its bodacious best, it should be divided whenever the plant outgrows its boundaries, or when the middle of the clump begins to die down. Divide Siberian iris in early spring, just after new growth emerges, or in late summer or early autumn, after the plant has finished blooming.
Prepare the ground for the newly divided Siberian iris. Remove rocks and large clods, then using a garden fork or a shovel, cultivate the soil to a depth of at least 8 inches. Work an inch of compost into the top of the soil.
Dig up the clump of Siberian iris with a garden fork. Dig at least 6 inches away from the clump, inserting the garden fork straight down into the soil. Continue around the perimeter of the clump, rocking the garden fork back and forth, until the roots are loose.
Lift the Siberian iris carefully out of the ground. Wash off the soil with a garden hose, then pull the clump apart at its natural divisions, leaving a large root on each clump. Discard the tough, woody middle section. Siberian iris is tough, so you may need to use a sharp knife to divide the clumps.
Cut the foliage back to about 6 inches with garden pruners. Trimming the foliage will allow the Siberian iris to focus its energy on developing new roots.
Dig a hole in the prepared spot, and plant the divided Siberian iris immediately. The iris should be planted at the same depth in the soil as it was planted before.
Water the Siberian iris well, and continue to keep the soil damp until you see new growth, which indicates the Siberian iris has rooted. Spread an inch of organic mulch around the Siberian iris to keep the roots warm during winter and to conserve moisture.