There aren't many plants that you can change the color of flowers by fertilizing. Plus, there aren't too many blue flowers with 10-inch heads. This forced feeding needs to take place a couple times in early spring and spring. You can also change flowers to a deeper pink by feeding with garden lime in spring. The most widely grown garden varieties are the big-leaf hydrangeas or Hydrangea macrophylla.
Hortensias are the most common, and they are the types with the huge globe-shaped flower heads.
Many home gardeners have come to love the lighter, more natural look of the flat lace-cap types. Two cultivars of lace-caps include Bluebird and Blue Wave. As mentioned, hydrangeas vary in their reaction to chemicals in the soil. As the pH decreases, the aluminum in the soil becomes more available and hydrangeas take up more. It's always kind of fun to feed hydrangeas to see what shades of blue or pink you can come up with.