About every three years or so iris plants need to be divided, otherwise, they risk overcrowding and disease. The blooms will also suffer if they are not divided enough. The best time to divide bearded irises is from July through September, at least 6 weeks before the first frost.
Iris plants grow not from bulbs but from something called rhizomes. These form clumps with roots on the bottoms and leaves coming out the top. When these clumps get too big, they need to be divided and each new division will form a new plant.
So what do you do when it’s time to divide your bearded iris rhizomes? Using a garden fork, dig up your iris rhizomes and shake off the soil, then rinse them in water. You’ll want to look over each one for signs of iris borers (holes in the rhizomes) and soft rot. As you divide the rhizomes you’ll want to discard these sections and only plant the young, healthy rhizomes back into the ground. Using a sharp, sterilized knife, separate the rhizomes, making sure there are leaf fans on each section. Without the fan of leaves, the rhizomes are unlikely to grow.
Once you have separated the iris rhizomes and discarded all of the infested or diseased irises, you will want to begin digging a shallow hole big enough for three to five iris plants in an area that receives at least 6 hours of full sun a day. You will want less than an inch of soil above the top of the rhizomes, so the hole doesn’t need to be deep. Cut the fan leaves about 4–6 inches long before planting. Plant bearded iris groupings about 24 inches apart to allow room for growth. Cover the roots, but allow the planted rhizomes to remain visible at the soil surface.
Water newly planted iris rhizomes well, but do not continue to water unless it has been several weeks since the last rain. Keeping them moist will encourage rot.
Once your new irises are established, apply a low-nitrogen fertilizer twice a year—in early spring—and just after the bloom to keep them healthy and happy.