How can I turn pink hydrangeas blue and vice-versa?
French hydrangeas are unique in the fact that the soil pH affects the flower color. Acid soil turns flowers blue, alkaline soil turns them pink, and near-neutral soil turns them purple or half blue and half pink. The causative agent for blue color is the aluminum ion. It’s available to plants in acid soil, but not in alkaline soil. To acidify soil, add chopped oak leaves, sphagnum peat moss, pine straw, coffee grounds, iron sulfate, and garden sulfur. To make soil alkaline, add lime. Don’t expect overnight results. The change takes at least a year. To show you what I mean about soil and color, look at the hydrangea above. It’s a H. serrata called ‘Blue Billow.’ Not very blue, is it? The soil has turned the sterile flowers pink. You’ll notice many of the hydrangeas you buy at garden centers bloom pink the first year. That’s because the soil mix in the pots is slightly alkaline. Once you plant them in acid soil, however, they’ll turn blue. Exceptions — selections like ‘Pia,’ ‘Ami Pasqui
