People worry about cutting down their peonies. “I don’t want to hurt it. Is it too soon?”
Meanwhile the peony is silently begging, “Please, cut me! Let me get ahead or stay ahead of this creeping crud!”
The peony’s plea has to do with one of the most common, most debilitating peony problems: botrytis. It is a fungus specific to peonies that infects damaged tissue in spring, spreads during summer to disfigure leaves and stems with purple brown splotches, works its way into the stem and thence down into the crown. The infection can worsen to the point of killing all flower buds even as they form in spring.
If your peony has brown- to purple-red splotches and streaks like the plant in this photo, cut it down, right away, even if it’s midsummer.
In early fall no matter what color a peony’s foliage is, cut it down to keep it from developing this botrytis trouble.
Cut a peony all the way to the ground. Do not leave stubble. The plant resumes growth each spring from below-ground buds. Any stem bases you leave will simply be dead clutter that may be harboring botrytis spores.
If stems are hollow or discolored inside it’s too late to fully control infection through sanitation this fall.
New shoots emerging in spring may be picking up botrytis spores as they grow. Botrytis is not a strong aggressor and usually can’t make headway into healthy tissue. So protect the spring growth from late frost. Cover the plant against frost and consider applying a prophylactic fungicide or antitranspirant (such as Wilt-pruf) to new growth as it emerges and expands.
It's late September in USDA zone 5-6 and we have already cut all traditional herbaceous peonies and intersectional hybrids.
We know – the intersectionals (including Itoh types) are woody and such a cut removes budded stems. Cut it! It will grow new from lower buds and remain healthy.