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Jul 20, 2015 4:11 AM CST
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Name: Patrick
Toms River, New Jersey (Zone 7a)
Dog Lover Irises Lilies Region: New Jersey Orchids Region: Pennsylvania
I've seen some photo's here on the forum of some rather large clumps of iris. The visual effect is stunning, but I'm wondering how large should you let them get before you start to divide up the clump ?
Jul 20, 2015 6:30 AM CST
Name: Tom
Southern Wisconsin (Zone 5b)
Butterflies Vegetable Grower Keeper of Poultry Irises Keeps Horses Dog Lover
Daylilies Cat Lover Region: Wisconsin Celebrating Gardening: 2015
If the rhizomes start to grow over the top of each other, they will need to be thinned. Thinning some out every year allows you to maintain a clump for a quite a while without actually dividing it. When you divide, you can replant several of the same kind to give the same appearance of a large clump. Each cultivar seems to have it's own way of growing, and some just seem to grow nice and don't get too crowded for a long time, others seem to grow all over themselves and need thinning more often. So the answer to your question is "It depends on the individual iris" and your preference to how large an area you want covered with one kind of iris. Smiling
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Jul 20, 2015 6:43 AM CST
Name: Sharon
McGregor IA (Zone 4b)
Good advice, Tom. I planted several rhizomes of Ocelot in a circle, with the growing side out, and every year since I have added soil around the clump and kind of levered up the outside of the clump so the rhizome was back on a level instead of going down the slope. Occasionally I cut out a section with a knife if I see one area is getting too crowded and plant it elsewhere. I think this is a winning method for me.
Jul 20, 2015 8:09 AM CST
Name: Lee-Roy
Bilzen, Belgium (Zone 8a)
Region: Belgium Composter Region: Europe Ferns Hostas Irises
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Depends on the kind of Iris you're referring to. Rhizomatous iris like germanica need to be divided much earlier than let's say sibirica because the inside gets congested/old and because of that don't produce blooms anymore.
Germanica needs to be divided every 3-5 years while sibirica can last up to 10 I've heard.

General rule is if the flowering decreases: divide
Jul 20, 2015 8:11 AM CST
Name: Dee Stewart
Willamette Valley OR
Snowpeak Iris
Charter ATP Member I was one of the first 300 contributors to the plant database! Hosted a Not-A-Raffle-Raffle Birds Garden Ideas: Level 1 Region: Pacific Northwest
Region: Oregon Irises Hummingbirder Garden Art Dog Lover Cat Lover
most commercial growers divide yearly for better crop, my home ones divide every 3 years.

Denise Stewart
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Jul 20, 2015 8:13 AM CST
Name: Mary Ann
Western Kentucky (Zone 7a)
Bee Lover Irises Hummingbirder Hostas Keeps Horses Farmer
Daylilies I was one of the first 300 contributors to the plant database! Container Gardener Cat Lover Region: Kentucky Birds
Yes -- you're referring to Tall Beardeds and Dwarfs, right Dee? I divide at about the three-year mark too. I will try to postpone to four-year if I can.
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