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For beautiful spring fern peonies and iris, now is the time to split them

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Know It and Grow It
Lead Summary
By
George Bonnema, Luverne Horticulturalist

August is the month to thin or transplant German bearded iris and peonies.
Dividing fern leaf peonies is a bit more complicated than regular peonies in that they have a “neck” between the eye of next year’s growth and the tuber. Just cutting into the clump with a spade will often ruin a lot of the potential new divisions because of the way they intertwine.
The safest way to divide fern peonies is to dig the entire clump and use a strong stream of water to wash the soil from the tuber mass.  Then it is easier to see how to divide single or groups of the tubers with eyes attached.  If a tuber breaks off without the eye, it most often will not produce a new plant.
When replanting, be sure the eye is within one inch of the soil surface. If planted too deep, this type of peony will grow but will not bloom.
It may take a year or two for the peony to establish enough to resume blooming. Be sure to plant in an area that will receive a minimum of six hours of direct sun per day.
Regular peonies have the eye for next year’s growth directly above the tuber so that clump can just be cut into sections to replant. Same planting depth and sun light requirements.
Iris, either Siberian or German bearded, are also divided or transplanted now in order to give them time to get established before winter. Both types of iris clumps tend to get too thick or too big, and dividing results in more flowers.
The German bearded type grows from a rhizome that lies on the surface of the soil, and it is important that they be planted that way when you work with them. Rhizomes planted too deep will rot.
We cut the fan of leaves back to about 6 inches so they don’t get floppy in the wind, which would make it difficult for them to root in.
Looking at the rhizome, you will notice that the new growth projects forward each year, so plant it pointing in the direction where it can do that without becoming a problem later.  That is one reason that these iris do not work well in a landscape that is mulched with plastic or fabric and rock.
Siberian iris grows in a clump form, and that clump can just be cut into sections for replanting with a spade or knife.
If you have clumps of daylilies that have not been divided for several years – or maybe ever – this is also the best time to divide them. They have time to get established prior to winter and will present a much stronger bloom cycle as a result.
I prefer not to use fertilizer at the time of transplanting because there is not an established root system to utilize that fertilizer. Also, we do not want to encourage new growth this late in the season. It is more important that the roots get established, and then give them a treat of fertilizer in the spring to encourage the new growth.

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