Don't worry the Osmocote is great for hydrangeas and it is what I use on my own hydrangea collection. I do not think you also need to use another fertilizer as hydrangeas are not heavy feeders like roses and vegetables. You may want to experiment and use the Rosetone on just one hydrangea and only the Osmocote on the others. See if you can tell the difference. The amount of Osmocote sprinkled around the base of flowering shrubs like hydrangeas is important (usually one capful for a 4 foot area) but do not worry that your husband damaged the hydrangeas by using too much or too little. Slow release plant foods like Osmocote are not going to burn or damage plants the way that liquid or high nitrogen, fast acting  plant foods can. I predict some beautiful hydrangeas in your garden this summer. Keep growing, Marianne Binetti