This usually happens to my peonies in August. It is probably too early for this to be happening to your peony if you are located in my zone. Though if you are in a warmer zone, it is probably normal Hopefully you have good drainage where your peony is planted and the yellowing is not due to rot. Best thing that you can do now is cut off the peony foliage, clear the weeds so that the plant has better air circulation, and mulch to keep the roots cool.
