Wait until June. Add compost, composted manue or cottonseed meal. Half a cup will do for the whole year in the northern half of the country. You can also add some coffee grounds, liquid seaweed or liquid fish; these are not strong fertilizers. But stop all forms of fertilizers by the end of July or so (in order for the plant to go dormant before early frosts and winter arrive).

Here is a link that might be useful: Fertilizing hydrangeas

Hi ~ The first 2 weeks of Apr were absolute gifts from Ma Nature dry & quite sunny altho very gusty winds, that made for challenging clean-up days ~ even had 2 of those days when temps climbed up to near 90s! Erring on the side of the predicted following weeks of no sub-freezing temps I went ahead & uncovered in layers to take advantage of Spring's wake-up calls of warmth & Sun! Altho no sub-freeze temps the next 2 weeks the much needed rain came nearly daily w/ colder & gusty winds. All the newly uncovered marginal bloomers just proceeded to go on stretching & all the fat leaf buds were simply happy to be out of their confines ~ now near end of Apr are filled with young leaves in different degrees - some fuller than others. The ES family group have large foliage from the bases & their fat buds just cautiously stretching. Only 3 of my 10 Divas are taking time to venture into early Spring calls but they're alive!

It's still cool & quite windy but Mighty Sun is finally warming the Earth once more to make our pieces of soil look a bit like heaven this season once more!

Our thoughts & prayers go out to our friends in the South!!!