Hydrangeas are not heavy 'eaters' like roses are. They do not a big growth spurt so they do not need as much fertilizer. If your soil has adequate amounts of minerals, you would not need to fertilize them at all! It probably feels weird but you could skip fertilizing on some years and have no effect. The one assumption that I make is that your soil has adequate amounts of minerals (a soil test can provide this information).

You can fertilize them in Spring using either compost, composted manure, cottonseed meal and a general-purpose slow-release chemical fertilizer. For a newly planted shrub, you can use the 'less is more' saying and give them 1/2 cup to 1 cup per application. A single application will last all year. If using a chemical fertilizer, apply according the label instructions. I begin fertilizing about two weeks after my average date of last frost or when the shrubs have started to leaf out.

Remove the mulch; water a little; add the fertilizer; water again and cover with mulch. If you have dogs like I do, turn "on" one of those Star Trek spaceship shields around the fertilizers for a few weeks so the pooches stay away from the good smelling organic fertilizers! While fertilizing, add more mulch if you need to... in order to maintain 3-4' up to the drip line.

Hydrangeas in pots will need more frequent feeding. See the link below for more information.

Here is a link that might be useful: More information