I have raised peonies for years in Maryland and Pennsylvania, and I have always trimmed back mine after they are done blooming to 15 inches tall. I let the rest die down to feed the root ball. I never trim down to the ground in the Fall. The remainder rots away by Spring, and I always have a healthy supply of blooms. One year I did have some powdery mildew, and cutting them back like this and spraying with a light soapy water straightened them right out.
I have raised peonies for years in Maryland and Pennsylvania, and I have always trimmed back mine after they are done blooming to 15 inches tall. I let the rest die down to feed the root ball. I never trim down to the ground in the Fall. The remainder rots away by Spring, and I always have a healthy supply of blooms. One year I did have some powdery mildew, and cutting them back like this and spraying with a light soapy water straightened them right out.