Peonies and iris are long-lived favorite perennials for Minnesota. Peonies can easily outlive their gardeners and may not need to be divided or moved. Iris, especially bearded iris, benefit from regular division and flower better when they are regularly divided. August is a good time to divide these perennials.
Iris tend to go dormant and slow their growth in August, so it's good to divide them when the plant is not actively growing.
Any yellow foliage can be inspected for the dreaded iris borer, which by August has moved down the foliage and into the rhizome. Discard all rhizomes with borer damage, or if minimal, you may be able to save them, just be sure you have killed the borers.
- Cut the foliage back to 6-8 inches and keep one or two “flags” or foliage clusters per rhizome.
- Bearded iris prefer well drained soil, they rarely grow well in wet soils and can decay in standing water. Full sun produces the best blooms on iris.
Peonies do not need division to continue to grow and flower, however, I am often asked about when to divide them because someone is moving, or they want to give a plant to a friend. Fall is the opposite season for flowers on peony and iris, so that is the perfect time to divide these two perennials.
If you have dug a peony, you know it’s got a big fleshy root system that provides lots of stored food to keep the plant growing for many years. But trying to get all of the root can be a challenge. Older peonies have large roots.
Peony flower buds have a cold or chilling requirement before they will actually flower, so they grow well in cold climates. Peony flower buds are below ground and if too protected and deep, they may not accumulate their chilling requirement and fail to flower.
Peony eyes or buds at the root-shoot junction should be planted 1.5 inches deep. Too shallow or too deep planting may inhibit flowering.
The other good thing about fall peony division is you can remove any diseased foliage, such as the black spots or botrytis blight that all peonies seem to get by September. Sanitation is a good prevention for botrytis, so cleaning up the peony foliage in fall is a good idea.
For more information see How and when to divide perennials.