How long has it been since you last divided your bearded irises? If you don’t remember or worse, have never divided them since initially planting, then odds are it’s time. Other indicators that you may need to divide your irises include few or no blooms, and a large mass of rhizomes that are tightly grown together with little or no spacing. The general rule of thumb is to divide every three to four years according to the American Iris Society.
When To Divide
The best time to divide and plant bearded iris is at the end of Summer, or six weeks before your first estimated frost date based on USDA zone. If you are located in the South, where summer heat can extend well into October, you have a bit more time than those who see freezing temps much earlier. The point of planting your iris at this time is to allow your newly divided and planted iris a chance to establish its root system before winter sets in.
How To Divide
First, you want to lift a clump of iris rhizome from the ground. Some people use a blunt spade or fork, if the ground is especially hard like my clay soil I use a shovel. Being careful to not damage the rhizome place the shovel or spade close to the clump and lift the ground up. This will loosen the rhizomes enough to pull a clump loose.
After shaking off any debris and dirt, examine your clump of rhizomes. The original rhizome will have new rhizomes growing on either or both sides. You want to remove the original spent rhizome that will not bloom again. You may see the remnant of a bloom stalk or a nub from the spent rhizome.
Remove the new rhizomes from the spent rhizome by simply snapping it off at the joint. This feels much like snapping a carrot. If you prefer you can use a knife or garden clippers, but be sure to use sterilized tools to avoid contaminating the new rhizome. You can either compost the old rhizomes or throw them away if you dispose of your garden clippings in the trash.
Before replanting the divided rhizomes you want to trim the leaves or fan down to four to six inches. This cuts down on stress to the plant and eliminates the plant being blown away or falling over due to the tall leaves.
When you are ready to replant be sure your area receives at least 6 hours of sun a day. If shaded by new plant growth, you may need to cut back tree branches, shrubs, or relocate your irises to a sunnier spot. Dig a shallow trench and lightly cover the iris rhizome leaving the top of the rhizome exposed. If you fully bury the rhizome you risk plant health and decreased chance of blooming. Sometimes watering the newly planted iris in is enough to expose the top of the rhizome if it is planted the correct depth. You can mulch around them to control weeds if you want, but be sure to leave the rhizome exposed.
Now you have everything you need to know to divide your bearded irises. Happy Gardening!