Peonies do great with Osmocote and I always fertilize in the spring once I see the stems and leaves above ground. Just be careful not to add too much mulch on top of peony beds. The plants do not bloom when planted too deep and a thick mulch can bury the crowns.  My other peony tip is to cut back the old  foliage in the fall once it starts to turn yellow or brown. This helps prevent leaf spot and other diseases that can overwinter in the foliage. Keep growing, Marianne Binetti