Why can hydrangea flower color change?
Bigleaf hydrangea, Hydrangea macrophylla, is a deciduous shrub. In some pink-blue cultivars, color relates to available aluminum and soil chemistry. Not all hydrangeas respond alike, and white cultivars do not follow the same color-change rule.
Soil and light conditions
Acidity influences aluminum availability, but cultivar genetics also matter. Adding chemicals indiscriminately to change color can damage roots. Use organic, moisture-retentive but draining soil. Morning sun and afternoon protection can help in hot climates. Large leaves show water stress readily; temporary midday wilting is distinct from persistent root trouble.
Pruning and flower buds
Some cultivars flower on previous-season wood, while others also flower on new growth. Heavy pruning at the wrong time removes buds, and winter injury can have a similar effect. More fertilizer is not always the answer. Hydrangeas are not food plants and should not be eaten. Identify species and flowering habit before planning pruning or winter protection.