I would use one, not both, since hydrangeas do not get stimulated as much as roses do when you add nitrogen rich fertilizers. The granules decompose over time and slowly release the fertilizer. Water soluble rose food would tend to stay in growth mode at times when they should be thinking of going dormant. The mix can be as much as 20% nitrogen. General purpose slow-release products like Osmocote and cottonseed meal can also be used.

If you feel like feeding the plant something, use weak fertilizers like liquid seaweed, liquid fish or coffee grounds. These products contain small amounts of other mnerals that plants should have but that get leeched out by all the watering ones gives to potted plants. I would also limit the amount of nitrogen on/after July so the plant goes dormant.

To put things into perspective, if this shrub was in the ground, you would feed it 1/2 cup or 1 cup of cottonseed meal or manure in June and that would be "it". For the whole year. You could add those weak fertilizers that I mentioned too but stop in July.

Luis:
How about alfalfa tea? Is that weak enough? So far, I've only fertilized with compost in early spring and my hydras are budding out nicely. Last year I fertilized with triple phosphate and nothing spectacular happened so I decided to cut back to just the minimum since I've read that you shouldn't overdo it with them.
Jm

newbiegardener, you don't need to be overly concerned about your containerized hydrangea not getting enough nutrients - if it's lacking something, it will 'tell' you by exhibiting symptoms you can then address, it won't curl up and die overnight. You'll have time.

My experience is that liquid products are much safer in containers, less chance of burning roots or overdoing it.

My perennials and shrubs in the ground are rarely (some never) fertilized including hydrangeas, they are top dressed with compost each spring and that's been sufficient for my own soil type....no symptoms of nutrient deficiencies to address. I do fertilize plants in containers, but I usually mix to about half the strength given on the label directions. Fertilizer companies can be a little like laundry soap companies, they are in the business of selling product - the more we use, the more they sell even though less is often adequate.

Thank you all. Your input is very helpful.