Fertilizing plants at an inappropriate time can be counterproductive. Hydrangeas (Hydrangea) enter a growth phase in spring in which they benefit from additional nutrients.
A fertilizer can also help species that are pruned in the spring to resprout or produce flowers. For older plants, the question arises as to which fertilizer to use. There are both mineral and organic options. The color of the flowers is also important. If you prefer a blue color for a farmer's hydrangea that blooms predominantly pink, you must use alum.