You didn't say what you were trying to get rid of by spraying your peonies so giving you a couple of ideas
Hi Judy: If you are spraying for ants, don't! Peonies need the ants to open the buds. They eat the glue that keeps the buds tight. If you spray for other bugs, you might kill the ants and your Peonies won't open, or a lot of them won't. If you are talking about Japanese Beatles, I spent many summers as a child, picking them off :I Good luck
If your talking about cut flowers, I hang them upside down to dry then Spray with a couple coats of hair spray.