When it comes to caring for garden peonies, one of the most frequently asked questions is how to prevent the tall perennial plants with their large, heavy flowerheads from flopping over. However, cutting back peonies is just as important for proper peony care.
Here's why, when, and how to cut back and deadhead your peonies to get the most out of these garden favorites.
Is Cutting Back Peonies Necessary?
In addition to the aesthetic considerations, the main reason for cutting back peonies is plant health. Peony diseases can be controlled by removing diseased foliage during the growing season, and by cutting back the entire plants after the end of the growing season in the fall.
Cutting back peonies is different from deadheading, which is the removal of the spent flowers after the bloom. If the flowers stay on the plants, they will produce seeds instead of storing all the plant energy in the roots, which ensures plant health and next year’s bloom. It does not matter how much you trim the flower stalks. However, for a neater appearance, it is best to cut them below the foliage so they don’t stick out.
Deadheading also extends the bloom time of Itoh peonies, which bloom later than garden peonies, but are deadheaded the same way.
Removing Flower Buds
Newly planted peonies benefit from removing the flower buds entirely before they open in the first spring. Not every gardener eager to see the peonies bloom is willing to make that sacrifice. But preventing the peonies from blooming in the first year after planting helps the plants get established, and the reward is a fuller bloom in the subsequent years.
Removing Peony Foliage
Cutting back peonies during the growing season is optional; it should be done as needed to prevent the spread of disease. Cutting back peonies in the fall, on the other hand, is always required.
Pruning for Disease Control
Two common fungal diseases of peonies are peony leaf blotch and powdery mildew. The spores of both fungi survive in dead leaves and infected plant debris through the winter. Good sanitation, which consists of cutting back infected plant parts during the growing season and disposing of the plant material in the trash, helps to control the spread of disease.
Peony leaf blotch is caused by the fungus Cladosporium paeoniae. Because of the typical glossy purple or brown spots or blotches on the upper surface of the leaves, it is befittingly also called measles or red spot. As the season progresses, older, bigger leaves may also become distorted, and the stems show reddish-brown streaks.
The other fungal disease affecting peonies is powdery mildew. Unless it is severe, the chalky residue on the leaves from powdery mildew is more of an eyesore than a disease that affects the plant. Still, to keep it under control, cut off infected leaves and collect any infected leaves that have dropped to the ground to prevent reinfection.
When pruning out diseased leaves, have a bucket filled with one part chlorine bleach and nine parts water on stand by to disinfect your pruners. Dip them in the mix and wipe them dry with a clean rag, so you don’t spread the fungal spores from plant to plant.
Lack of good air circulation in combination with moisture buildup can also worsen the spread of fungal disease in peonies. If the foliage of your peonies is healthy but very dense, it’s a good idea to thin the foliage a little to let more air and sunlight in.
In the fall, cut the peonies back but wait until the foliage has died or fully yellowed after a hard frost. If you cut the peonies back too early, you reduce the time during which the foliage is absorbing sunlight to build the plant’s energy reserves. This will result in reduced bloom the next year.
How to Cut Back Peonies
1. Cut off the stems at or near ground level after a hard frost.
2. Thoroughly remove all plant debris and dead foliage regardless of whether the peonies have been infected with a disease or not. The dead foliage might contain fungal spores that are not visible to the human eye. If the plants were diseased, safely depose of them in the trash; do not compost it under any circumstances.
"Growing garden peonies". Horticulture and Home Pest News, Iowa State University.