David Robson Extension Educator, Springfield Extension Center, University of Illinois
Yard & Garden
The Month to Divide Iris
It’s too hot and dry to plant anything so dig up and divide irises
When you think of spring flowers, you’ll probably think about tulips, daffodils, peonies and irises. All make great displays in the perennial garden, coming back year after year, though tulips tend not to always follow this rule of thumb.
If you throw in the qualifier of unusual colors and shapes, then iris rise to the top, leaving the others far behind.
If you want blue blues, irises are for you. Dark purple? The same plant. Vivid yellows and oranges? Yep, still irises. Even greens, though they’re somewhat murky. Pure whites and darkest purples approaching blacks are also in the iris range. The only color that doesn’t seem to exist is red.
To add to the glory, the iris has a plant shape that’s unlike most plants. Instead of a rounded form, it’s more of an architecturally flattened fan. Bearded iris take it to another level by having a bluish cast to the leaves, though by the end of summer, that color tends to be more green than blue.
Siberian iris, on the other hand, have a grass-like texture, but if you look at them carefully, you’ll see the flattened fan.
The one thing most irises have in common is a horizontal stem called a rhizome. It’s fairly evident on the bearded form with the fans of leaves separated by a two- or three-inch woody stem lying on the ground. Siberian iris is more compressed, though if you look close enough, you’ll see them.
Irises tend to bloom once per fan, and then produce new fans. In the ideal world, the one fan would produce two, which would then produce two, so the next thing you know, you’d have a zillion fans producing a zillion flowers. However, as the fans start to grow, they grow back on themselves. And if they can’t produce new fans, they just stop blooming.
The old fans still come back year after year, giving the plant some nice garden interest, though most people would prefer the flowers and the foliage.
This all comes back to August.
August is the month to divide iris, even though we think of this month as 31 days of do-nothing. It’s too hot to plant. It’s too dry to start a new lawn. It’s the month to enjoy air conditioning and the State Fair.
Every five years, you need to dig up your clumps of iris and separate them. It’s not a hard process, just time consuming. If you do it in the morning, it won’t be as hot.
Start by lifting out the clump. Take care to note the outer fans, because those are the ones you want to save. Those in the center of the clump are best discarded to the compost pile. This applies to bearded, Siberian and Japanese iris.
Use a sharp butcher’s knife or spade to divide the clumps. The goal for bearded iris is to have a 3- to 5-inch piece of rhizome, some roots and a fan of leaves. Having two fans is the upper limit. If you have three, you haven’t divided enough.
For Siberian and Japanese iris, a clump of fans about 2 to 3 inches in diameter is enough.
Look the fans over carefully for rhizome rot, which can come from too much water or the iris borer. Anything that is soft and mushy should go on the compost pile.
Work the soil, adding organic matter. Pull out grass weeds at the same time. Remember, irises prefer full sun.
Plant the rhizomes about an inch below the soil and lightly firm the soil. Rhizomes ideally should be at ground level, but when the soil settles and the plant becomes rooted in place, the rhizomes will probably be pretty close to the surface.
It’s at this time you can get happy with the scissors and cut each fan back by two-thirds, otherwise the fan will fall over. It still may, but at least it won’t look as bad.
David Robson is an Extension Educator, Horticulture, at the Springfield Extension Center, University of Illinois Extension, P.O. Box 8199, Springfield, IL 62791. Telephone: 217-782-6515.
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