Experts and BHG readers answer.
fertilizer for hydrangeas
"Yes, spring is a good time to fertilize your hydrangea. Look for something like 5-10-10, where the phosphorous and potassium numbers (the latter two) are higher than the nitrogen (the first number). This will be better for flowering and plant health, and won't cause excessive growth as nitrogen might.
Luke Miller, editor
Garden Ideas & Outdoor Living magazine"