Aluminum

• Aluminum is an essential ingredient in the soil for producing blue flowers. The pH balance of the soil determines whether aluminum is available to the plant. The lower the pH, the more aluminum will be available for uptake into the plant. Higher pH soils release less aluminum. Aluminum is naturally available in soil, especially in piedmont and mountain soils that are reddish in color. Mathilda Gurges and Nikko Blue cultivars produce pink flowers if there is no aluminum in the soil, but blue flowers if there is aluminum.
  
  

Soil pH

• Nikko Blue or Endless Summer hydrangeas turn blue in containers if you grow them in an azalea or shrub mix below a pH of 6.5 and add 1.5 oz. of aluminum sulfate for every 3 gallons of soil. Apply the aluminum sulfate just as the flower buds form in mid-spring. Without the added aluminum, the plants will produce lavender or pink flowers. For in-the-ground hydrangeas, apply 1 oz. of aluminum sulfate per gallon of water and water the plant with it. You may have to double the amount for larger plants or bushes.
  
  

Surprise Color Changes

• Sometimes hydrangeas planted with roots and soil that have no aluminum will change to blue mid-season as the roots extend themselves into the surrounding soil, especially if the soil has a lower pH and aluminum in the soils.
  
  

Preventing Blue Flowers

• You can do two things to prevent your flowers from turning blue so they remain pink or white. The first is to surround the plant with aluminum-free soil or containerize the flower. Without aluminum the hydrangea will not turn blue. Then treat the soil with lime to lower the pH below 6.5.
  
  

Color Intensity

• Hydrangea bloom colors will be more intense if you improve the health of the plant. Regular watering and an application of hydrangea-specific fertilizer twice a year will not only keep the plant healthier, but will also make the colors of the blooms more vivid. Talk to your nurseryman or extension service agent to select the right fertilizer for your area and soil.