Opening a discussion for recommendations for fertilizing peonies. Usually, the same procedures and products will work just fine with many other perennials.
My first choice is Miracle-Gro bloom booster. 15-30-15 with many micro-nutrients. I like to add a tablespoon of magnesium to about 3 tablespoons of the MG and mix in roughly 5 gallons of water for use during the hot summer. Otherwise in the Spring, just mix it to the same ratio and sprinkle it on the ground, making certain to keep it at least a foot from the crown or at the foliage drip line. I buy the medical magnesium, Epsom salts, at Walmart rather than the gardening bags at Lowes. The medical type is 100% where the garden type is diluted. It is always better to fertilize lightly several times rather than one heavy application.