Early spring is the best time to fertilize hydrangeas but keep in mind your big leaf hydrangeas or Macrophyllus varieties will be pink when the soil is not acid -  we call this a more basic or neutral ph. Potting soils are more basic while the native soil of the PNW is most often acid so the flowers turn blue or purple. Adding a fertilizer to change the acidity of the soil can take several years but is worth the effort because changing flower color is just another way to play and have fun in your garden - and that is always a good idea. Keep growing, Marianne Binetti