If you're nervous about dividing plants, Bearded Irises are a great place to start. Shallow roots make them easy to dig. Big rhizomes make it easy to see where to split up the clump. And their inherent toughness means there's no way you'll kill them in the process.
Wait a few weeks after they bloom before dividing irises, as the plants won't then be in active growth. Try to replant rhizomes at least 4 to 6 weeks before freezing weather, so they can grow new anchoring roots. In most areas, this means dividing in July and August and planting by or before September.
Like mother, like daughter
The roots of bearded irises form tuberous structures called rhizomes, at or just below the surface of the soil. Rhizomes store food for making blooms and for making more rhizomes. Each rhizome blooms only once. Future iris flowers come from "daughter" rhizomes that bud and branch out from the "mother" rhizome. Knowing this, you can see why you need to divide irises periodically and how to go about it.
Start by digging up a whole clump of iris. It's OK to break off a rhizome or two from the edge of a clump to share with a friend, but the entire clump does need to be dug up and sorted out every 3 to 5 years. Since they're such shallow growers, a few angled cuts with a spade or shovel will let you pull up the clump pretty easily. Knock off the dirt, or rinse with the hose, until you can see what's going on with the rhizomes.
Rhizomes can be long or stubby, fat or thin, straight or twisty. Healthy rhizomes look firm and plump. Definitely discard any rhizomes that are soft, mushy, or damaged. Look for old dried flower stalks or for scars on top of the rhizome where a flower stalk came off. Knowing if a healthy-looking rhizome has bloomed yet or not will help you decide what to do with it.
You'll need to deconstruct the clump as you sort it out. Rhizomes are easy to separate. Just put one hand on either side of the "joint" between two rhizome sections, and snap it as if snapping a carrot in half.
The "toss it" pile
The oldest "mother" rhizomes are easy to spot. They look as worn out as they probably feel! They'll have larger rhizomes branching out from them, but they won't have much in the way of new leaves or little daughter nubs.
Don't be alarmed if you see a lot of little holes on the bottom of the older rhizomes. These aren't made by borers or other pests; they're just where old roots have fallen away.
These old rhizomes can be discarded. As long as your irises haven't had pests or diseases, they'll make a fine addition to your compost pile.
The "keep" pile
Look more closely, and you'll see newer firmer rhizomes branching out from the oldest gnarliest ones. The biggest of these may already be producing daughters of their own, or they may be saving all their strength for next spring's glorious bloom.
Large, firm, unbloomed rhizomes are what everybody hopes to get when they buy or trade for irises. If you're dividing a large clump, you'll have plenty of prime rhizomes to replant and to share. But even a small daughter rhizome has the potential to grow into a fine clump with patience and a little TLC.
The "maybe" pile
With a little practice, you'll be able to guess which rhizomes are likely to bloom next year. Once you've sorted out the "keepers" and the "yuk, toss it" rhizomes, you'll probably have a lot of smaller daughter rhizomes as well as some "these don't look so bad" mother rhizomes.
If a larger rhizome has a bloom stalk, you know it won't bloom again. But if even small daughter rhizomes are forming on it, it can be worth keeping. The food stored in the mother rhizome will help the smaller daughter rhizomes develop, and in another year or two they'll be blooming and producing daughters of their own.
Depending on how much room you have, sort out the best of these to replant. I figure if I plant a couple of "keepers" with a few "maybes," I'll have a clump of irises that will produce good blooming rhizomes for maybe another 5 years before needing to be divided again.
Cut & clean up
Tidy up the iris rhizomes in your "keep" pile by cutting back their leaves. They won't contribute much more to the rhizome at this point. Newly planted irises don't have good roots to anchor them in place, so shortened leaves make them less likely to topple right over. Trimming makes them easier to handle for washing, storing, or trading, also. You can be fancy and make angled cuts from each side, but the plant won't care if its leftover leaves look pretty.
Cleaning and sanitizing rhizomes (whether from your own garden or elsewhere) helps prevent the spread of pests or diseases. If you're not sure this has been down with irises you've received, just do it. It won't hurt the rhizomes to be cleaned once by their grower and again by you. Although irises are pretty easy growers, when it comes to pests like iris borers, an ounce of prevention is worth a pound of cure.
Swish and rub the rhizomes in a bucket of water, then give them a good spray from the hose. That should get off most of the visible dirt. Now, take care of the spores, bacteria, or insects that you can't see. Expert growers recommend soaking rhizomes for 10 minutes in a 6% bleach solution (about a cup of bleach per gallon of water). Rinse well, then lay them out to dry in a breezy spot.
Congratulate yourself! Your once-overcrowded irises are ready to spread a bounty of blooms in your garden and beyond. Replant several of the best rhizomes for a better-blooming clump in their original location. Increase their impact by tucking a few more here and there. Give away or trade the remaining "keepers" to grace other gardens.
The hardest part of dividing irises is getting started. Screw your courage to the sticking point, and pry up that first clump. Sorting them out doesn't require precision or expert technique. They're one of the most forgiving plants you can grow. So get out your shovel, and just get at them!
Many thanks to Ben Wilson for a companionable afternoon digging irises in his beautifully restored historic garden.
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Photos by Jill M Nicolaus.