Hydrangeas should be fed during the growth season. This is typically in early spring, and in some cultivars, late summer. Choose a slow-release fertilizer that will supply the plant with little amounts of nutrients over time since hydrangeas do not require a lot of fertilizer. Most fast-release fertilizers for hydrangeas are most effective when used during early spring and mid-summer. More importantly, follow the feeding times indicated in the fertilizer because the times may differ for different fertilizers.