Peonies (Paeonia spp.) are among the most beloved of garden perennials. The large, fragrant flowers wane by early summer, leaving a clump of attractive foliage and stems. Removal of these leaves is warranted only when diseased or when they are dying or turning yellow, at the end of the growing season. Removing healthy green foliage weakens the plant and eventually causes plant death.
After flowering, the old flowers and forming seed clusters may be cut off to tidy the overall look and aesthetics of the peony plant. Cut the stem back only to the first or second leaf under the flower. Keep healthy, lush, green peony foliage in order to strengthen the plant’s roots and encourage a more robust flowering display next spring. After September, peony foliage may be trimmed back further for aesthetics, but only reduce the yellowing leaves, not green tissues. Allow the killing fall frost to end the life-cycle of peony foliage before full removal.
Wet and humid conditions during spring and summer can cause peony foliage to become afflicted with fungal diseases. Leaf blotch and stem spot, the formation of reddish purple blotches on foliage and stems, calls for removal of the entire stem, cutting it at ground level. Dispose of diseased foliage far from the peony plant, and do not place it in the compost pile, where the disease may proliferate or linger.
Other fungal blights can cause foliage and stems to completely turn black. If caught early, these inflicted leaves and stems can also be cut away and destroyed. However, if poor air circulation and wet soils remain, the blight will spread to other stems and plants. Relocating peonies from soils that are fungus-infested and too wet is the best solution.
In the fall, when peonies are best dug, divided and transplanted, peony foliage may be reduced by no more than 50 percent after being planted in its new location. Retaining all foliage is beneficial, but the removal of yellowing foliage and particularly long stems can decrease the physical pressures on newly planted peony roots and lessen the amount of water the roots must absorb to sustain the foliage.