To turn hydrangeas blue, ensure you have a color changing variety, acidify the soil and add aluminum sulfate. Some varieties of white hydrangeas do not change color. In varieties that respond, lowering the pH provides a darker blue blossom.Continue Reading
Hydrangeas that begin with blue blooms are most likely to respond to changes in soil condition. For the darkest blue color add products to lower the soil pH to the 5.2 end of the scale. You can purchase pH testing kits at your local hardware store.
The ideal soil pH for blue hydrangeas is between 5.2 and 5.5. Add compost, coffee grounds or pine needles to the ground surrounding the hydrangea. Work these materials into the soil around the plant. They provide nutrients and adjust the pH to ensure the plant produces the desired color of bloom.
Aluminum in the soil helps bring out the blue color. Aluminum sulfate mixed with water is the easiest way to increase the soil's aluminum content but is hard on plants, as too much aluminum sulfate burns the roots. Only apply aluminum sulfate to established plants that are at least 2 years old, and water plants before applying.