Fertilizer for Peonies
Peonies, despite the delicate beauty of their blooms, are tough plants with serious survival instincts. Given their long lifespan, stories abound of plants whose owners have abandoned them, but the plants go on blooming every spring. I even know a grower who had no idea she had a peony farm on her new property until it grew up through the tangle of weeds! (I try really hard not to be jealous, but really?)
So whether peonies actually need fertilizer is up for debate. They'll do better and be more able to sustain those resource-intensive blooms if you help them out a bit, so it's probably a good practice. Also, if you're cutting flowers from your plant, you'll need to replace nutrients to make up for what you've removed.
A soil test is always the first line of information that you'll need for your garden. My initial soil test, for instance, showed that I have very high potassium in my soil, so I'm careful not to use fertilizers with much potassium at all. A soil test is very helpful to understand the quirks of your garden.
Commercial fertilizers are labeled with a three-digit code, such as 10-10-10. The first number is the ratio and concentration of nitrogen the fertilizer contains, the second the phosphorous, the third the potassium. These are vital nutrients every plant needs to flourish.
If peonies receive too much nitrogen relative to the other nutrients in a fertilizer, they'll spend all their time and energy on beautiful green leaves instead of flowers. A low-nitrogen fertilizer is best for them. Epsoma's Bulb-tone (3-5-3) or Flower-tone (3-4-5) meet this requirement, are widely available, and are made of organic materials.
Apply your fertilizer at the rate recommended on the bag, in the spring when the peony stems are 2-3" tall. Make sure that the fertilizer doesn't touch the tender stems, or it will burn them. Scratch the fertilizer in to the soil, and you're done. Your plant will thank you. I think.