I am about to try fall division of a peony bush. Wish me luck.
The recommendation for the time to divide perennial flowers is following their bloom cycle. Summer and fall bloomers should be divided in the spring before new growth starts. Spring bloomers should be divided in the fall several weeks after their bloom cycle has finished, but at least six weeks before the ground is expected to freeze.
U of MN Extension publications remind us that the best times of year to plant any tree or shrub in Minnesota is in the spring or fall. That is when temperatures are warm enough to allow the plant to establish itself, but are not so hot as to stress the plant.
Dividing many perennials on a regular basis is good for a healthy garden. Overcrowded plants compete for nutrients and water. Restricted airflow can lead to diseases. Dividing the plants into smaller sections stimulates new growth, and the smaller the plant, the more quickly it will take to its new location. Division is also an easy and inexpensive way to increase the number of plants in your garden.
To get started, stop summer fertilizing. Pick the new spot and pay attention to sunlight, wind, airflow, competition, soil and drainage conditions, as well as visual choices. Then give the above ground foliage a good trim. When you dig, aim to get all the roots and cause minimal damage. Dig the new hole large enough so that you can spread the roots. The goal is to focus the plants energy on re-establishing its root system, not the upper foliage. Make sure that you maintain the proper soil depth. Then tamp gently to close any air pockets that can dry the roots.
With all transplants, good watering (not too much, not too little) is essential during this time as dry soil is the number one cause of transplant failure. Continue to water until the soil freezes. Add a two-inch layer of mulch around the base of plants to help keep everything stable. Be careful to keep the plant stem mulch-free to allow air flow by keeping the mulch two inches away from the stem. Ground up fall leaves, compost, or dry straw are all good choices. And finally, do not fertilize. Wait until new growth begins in spring before applying any fertilizer.
In Minnesota, peonies should be planted or moved by early September. Set tubers with the eyes facing up and the roots two inches below the surface to guarantee survival, but not too deep. Old foliage of many perennials should be left in the garden to hold snow and insulate the roots from winter winds. However, peony stalks often carry a fungus blight that can survive the winter so they should be carefully cut down almost to the tuber.
You can check out this U of MN document, extension.umn.edu/planting-and-growing-guides/dividing-perennials, for guidelines on what perennials divide well in fall. The list includes those with fleshy taproots like peonies and oriental poppies. Among many others are astilbe, black eyed susan, catmint, coral bells, echinacea, goldenrod, hens and chicks, hostas, several irises, Jack-in-the-pulpit, ligularia, some lilies, mint, phlox, Russian sage, shasta daisy, speedwell, violets and wild ginger.
For additional peony planting information check The Minnesota State Horticultural Society’s northerngardener.org/how-to-care-for-peonies.