Hello, pfckce. Hydrangeas are not picky feeders and they do not respond to extra fertilizer amounts like roses do so feel free to apply any of these: organic compost, cottonseed meal, composted manure or a chemical fertilizer that is general purpose slow release like Osmocote. One application in June is recommended in the northern half of the country where the growing season is shorter. In the southern half, apply fertilizers twice, approximately in April and July. A half a cup will do for a newly planted shrub.

Coffee grounds can be added -in addition to the above mentioned fertilizers- throughout the growing season but discontinue all fertilizers (including coffee grounds) after July-ish. That prevents the shrubs from "staying awake" as winter arrives due to too much nitrogen in the soil.

Note: assuming your soil has no mineral deficiencies, no harm would occur if you were to forget to fertilize on some years.

Luis