Changing Hydrangeas Color
Generally, most strains of French hydrangeas (Hydrangea macrophylla), will develop color based on the acidity in the soil. Strong acidic soil with a pH below 6 in coordination with high aluminum content will turn the flowers blue. Acidity can be increased by adding Aluminum Sulfate or Iron Sulfate (available at most nurseries). You can increase the aluminum content by adding a high potassium low phosphorus fertilizer. Alkaline soil with a pH above 7 will turn the flowers pink or even red. You can do this by adding lime to soil and mixing a fertilizer with a high phosphorus content. Depending on how the flowers react to the pH balance, you may need to apply it several times as the results can take a month or so. Be patient they will turn!
A tip on white hydrangeas – they seem perfect for turning color. Unfortunately, regardless of the pH in the soil, they will remain white!