“Lime makes ’em pink, sulfur makes ’em blue,” or is it the other way around? Indeed, adding lime to the soil around the common French hydrangea will make the plant change flower color from blue to pink. The lime counteracts the acidity of the soil and makes aluminum in the soil less available to the plant. Since soils naturally become acid as the years pass, most hydrangeas display blue flowers. Except for a few genetically pink-flowered varieties, lime must be applied regularly to maintain the pink blooms on a hydrangea.
To change blue to pink, sprinkle 1 cup of lime on the soil around a plant every year. Over a period of three years, the blue flower color will gradually change to pink.
To change pink to blue, mix 2 Tbs. of aluminum sulfate per gallon of water. Water small (3′ x 3′) shrubs with one gallon each year. Larger shrubs can handle 2 (or perhaps 3) gallons of the mixture each year.