Answer: The acidity or alkalinity of a soil influences the shades of color in our hydrangeas. When the blooms of some varieties develop muddy, indeterminate coloring it signifies a pH in the neutral range and no clear color directive to the flowers.
Before taking steps to produce a certain color in hydrangeas you have to first consider what the flower color originally was, since different soil treatments will have different effects on the natural color of the variety. Red-flowering hydrangeas, for example, can be kept red or made more red by applying wood ashes, lime, or bone meal to the soil. All these materials have an alkaline effect on soils.
Blue-flowering hydrangeas will turn pink in a neutral or alkaline soil. A friend of mine once asked me why one of her blue hydrangeas in a group of three had produced pink bloom. It turned out that the pink one was at the front of the grouping and had received a dumping of wood ashes late in the winter.
Acidifying the soil with peat and powdered sulphur, or applying aluminum sulphate, keeps the blue in hydrangea bloom, and the same treatments will turn vivid red hydrangeas purple. Any treatment to adjust the flower color in hydrangeas needs to be started well ahead of bloom to be effective. It is best to apply soil amendments before the plants leaf out in the spring.
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