Hydrangeas are best fertilized twice a summer. Apply a slow release 10-10-10 fertilizer in the early summer, like Osmocote, to the amended soil around the base of the plant. Apply a liquid or powder fertilizer in May and July for best results, such as Miracle Gro. Follow the directions on quantity applied and err on the side of too little amount as opposed to too much. Never fertilize a plant that is wilting or looking sick. The plant will only be under more stress with the fertilizer application. Do not fertilize after August!