How to Change Hydrangea Color
How to change hydrangea flower color by adjusting soil pH. Acidic soil produces blue, alkaline produces pink. Only works on bigleaf hydrangeas, not panicle or oakleaf.
Only bigleaf hydrangeas (Hydrangea macrophylla, the mophead and lacecap types) change color based on soil pH. White varieties of bigleaf stay white regardless of pH. Panicle hydrangeas (Limelight) are not affected. Oakleaf hydrangeas are not affected. If you have a panicle or oakleaf type, pH adjustments will not change the flower color.
Use a soil pH test kit or send a sample to your extension lab. Current pH determines what you need to change and by how much. Below 5.5: flowers are already blue. Above 6.5: flowers are already pink. Between 5.5 and 6.5: flowers are purple or mixed. The adjustment is about moving the pH one direction or the other.
Apply aluminum sulfate at 1 tablespoon per gallon of water, applied as a soil drench around the base of the plant. Apply 3 times in spring (March, April, May) as buds form. Aluminum sulfate both lowers pH and makes aluminum available to the plant (aluminum is what actually creates blue pigment). You can also use sulfur to lower pH but it works slower.
Apply dolomitic lime at 1 cup per plant, scratched into the soil surface in early spring. Lime raises pH and ties up aluminum so the plant can't access it. Without aluminum, the pigment shifts to pink. This takes longer than going blue: expect 1 to 2 seasons for full pink conversion. Adding superphosphate (0-20-0) also helps block aluminum uptake.
Color change takes one full growing season minimum. Flowers already formed won't change mid-bloom. The pH adjustment affects next year's buds as they form. Apply amendments in spring, expect color change the following spring. Some flowers may show mixed colors (purple, half-pink half-blue) during the transition. This is normal.
