There are two main times of the year to fertilize peonies, before your peonies bloom in early spring and after they have bloomed in the later summer / early fall. This is a good time to add a slow release organic fertilizer or composted cow manure around the drip line of your peony plants. Slow release, organic nutrients will be available throughout the season for your peonies. This includes the early spring when they are expending lots of energy to push up new shoots of foliage, and even into the fall when they are storing energy and nutrients to produce next year's foliage and blooms.
Peony Plant to be Fertilized
The first step in fertilizing your peony is to remove the layer of mulch covering your peonies. This layer of mulch is just another barrier between your peony and the nutrients you are trying to give it. If it is not possible or perhaps too time consuming, you can put the fertilizer on top of the mulch. However the peony will have easier access to fertilizer that is directly applied to its soil. Also less of the fertilizer is likely to get washed or blown away.
Peony Plant with Mulch Removed
Since peony plants only produce one set of foliage per year, you will want to make sure to keep it green and healthy. I don't recommend any quick release water soluble chemical fertilizers. These fertilizers will actually 'burn' the leaves of your peony plant and make them turn yellow. Avoid those types of fertilizers and look for something slow release, natural, or organic instead. Since peonies are not continuously producing new above ground growth throughout the season, they do not need a high nitrogen, quick release fertilizer.
Organic Slow Release Fertilizers are Best for Peonies
Some fertilizers that I like to use are Espoma Organic Plant-tone or EcoScraps Natural and Organic Rose & Flower Plant Food. Both of these fertilizers are slow release and organic. They can both be found at local super stores and hardware stores, or you can purchase them online. I also like to use Black Kow composted manure as a fertilizer for my peonies. If you have your own compost pile, then homemade compost is also a great fertilizer. When using compost, you will want to make sure that you use a very thin layer and don't bury your peony with the compost. Most of the time, I will use the bagged organic fertilizers for herbaceous peonies, and the compost for tree peonies since herbaceous peonies's are sensitive to planting depth.
You can use a garden scoop or measuring cup, whatever you're comfortable with to apply the fertilizer. I like to use a measuring up so I know exactly how much I am putting on each plant. Sprinkle the fertilizer lightly over the soil. Make sure to apply the fertilizer around the drip line of the peony's outer most leaves in a round circle like shape. Be careful not to dump the fertilizer directly on the peony's crown. It likely won't do as much good in this location since most of the storage roots are under the crown, and the feeder roots are generally around the outer edge of the foliage drip line. Optionally you can work the fertilizer into the soil a bit with your fingers or a small garden cultivator, taking care not to disturb the peony's roots. If you are short on time or you are worried about disturbing your peony, you can skip working the fertilizer into the soil.
Replace the Mulch Around Peony
After you have applied your fertilizer, the last thing to do is to replace the mulch around your peony. This will keep the fertilizer from washing away, and give nature a chance to incorporate the fertilizer into the soil during your next few rainstorms. If it has been particularly dry lately, you can also water in the fertilizer yourself with a garden hose or watering can. Now you're all done. This should give your peony a great start to the next growing season!
