Bigleaf Hydrangea

• Despite its very American nickname, bigleaf hydrangea comes from Japan. The species is rated as hardy in USDA cold-hardiness zones 6 through 10, but some of its many cultivars thrive in zone 5 as well. Bigleaf cultivars bloom in early spring. They produce large inflorescences (flower heads) that group large, flashy blossoms with smaller flowers. It is often the inconspicuous blossoms that produce seeds. Bigleaf flowers range from pink to blue depending on the acidity of the soil.
  
  

Hydrangea Color

• If you transplant a hydrangea from one area to another, the blossoms may well change color. Hydrangea flowers develop in the popular sky-blue pigment only if aluminum is present in the soil and the shrub can access it. Although most soil contains sufficient aluminum, the plant cannot access the aluminum in soil with a high pH. Hydrangeas require acidic soil with a pH 5.5 or lower to access adequate aluminum to produce blue flowers. Neutral or alkaline soils -- those with a pH 6.5 or higher -- produce pink flowers. When soil pH falls between 5.5 and 6.5, expect purple or lavender flowers.
  
  

Changing Hydrangea Color

• One of the many delights of hydrangeas is the possibility of changing flower hue by altering the pH of the soil. Those gardeners who wish their blue flowers were pink need to raise the pH by adding lime to the soil. More often, gardeners seek the bright blue shade for which the hydrangea is noted. Those wanting to change from pink hydrangea flowers to blue must lower the pH in the soil to make the aluminum available to the plant. Adding aluminum sulfate adds acidity to the soil (and lowers its pH).
  
  

Solution

• To change your hydrangea flowers to blue, add 1 tbsp. aluminum sulfate per gallon of water. Apply this solution to plants at least two years old, and continue the application during the growing season. Dose the plants with regular water before you apply with the solution; it can burn dry roots. This procedure works best in soil that already contains some aluminum and is not highly alkaline. It is almost always successful with potted plants.