The blooming of the bearded irises is something I eagerly await each spring. This year, I made a mental note that I should divide and replant the irises after flowering. The time is now! Here's how to divide irises—with step-by-step pictures.
Even though my irises were planted years ago and are terribly overgrown, the blossoms were still gorgeous this spring. But dividing bearded irises every three to five years allows the clump to rejuvenate and bloom better (not to mention a way to multiply your irises to fill in gap). Also, it's helpful to avoid the iris borer which is a very destructive pest typically attracted to older, over-crowded gardens.
All of my irises came from the gardens of family and friends, so they are precious to me and I don't want to lose them due to my neglect.
When Should You Divide Irises
It's important to divide and replant at the right time of year. In many regions, July and August are the best months to dig, divide and transplant bearded irises. You can probably get away with dividing through mid-September, too. Do NOT divide in the spring. You must do this job post flowering, during the summer.
This is hot, heavy work involving a lot of digging, so I waited until the weather cooled down a tad before starting. My goal was to rework the iris beds, add some compost, get rid of the invading sedums and tree roots, and divide and replant the crowded rhizomes.
The irises are getting crowded and encroached upon by pachysandra. The bed they are in is narrow and next to large rocks leaving the irises nowhere to go.
Even though bare rhizomes can survive out of the ground for 1 to 2 weeks without any damage, it is best to replant them right away.
How to Divide Bearded Irises
First, dig the rhizomes up and check them for disease or insect damage. Try to dig carefully around the plants so as not to do any unnecessary damage to the rhizomes.
We found some borer damage and mushy rhizomes that were discarded.
Snap or cut off the old part of the rhizome, since it will not flower again.
The roots are quite long and will help anchor the newly planted rhizome in place.
Amend the soil with compost and dig a shallow hole or trench. Make a mound of soil in the middle to place the rhizome on, spread the roots out over the mound and cover them with soil.
Leave the top of the rhizome exposed.
Next season's plant emerges from the fan end of the rhizome so when replanting, face it in the direction you want plant growth to travel. Space the pieces 12 to 24 inches apart for tall types, closer for dwarf ones.
I planted mine closer, for a more immediate display, knowing that I will have to divide them again sooner.
It is easier to plant if you cut the tops back to about 6 inches tall.
Water well to settle the soil around the roots and continue to water deeply once a week until new growth appears. Once established, bearded irises are drought-tolerant and won't need additional watering. Fertilize early in spring and again right after blossoming with compost or a low nitrogen fertilizer.
It was sweaty work, even on a cool day, but so rewarding to see it done. We had enough leftover rhizomes to plant another bed on the other side of the stone wall. Next spring we should have twice the blossoms!