My itoh peony is full of powdery mildew. It is beyond the prevention stage. I am worried it may spread to the phlox nearby. Can I cut it to the ground in June after blooming, or do I have to wait until fall? Will it effect next year’s growth if I cut it to the ground in June?
Thanks for contacting Toronto Master Gardeners. Generally the powdery mildew on one host will not affect another genus so the risk of your phlox contracting the disease from the peony is low. The problem with cutting down the peonies to the ground now is that they need the leaves to replenish food stores for creating blossoms for next year. So cutting them back now will affect growth next year. It is best to cut the peony back in the late fall to four inches. Dispose all of the plant debris in the garbage. To prevent the powdery mildew from returning, spray the plant with a fungicide in the early spring.
June 9, 2021