Answer: Hydrangea flower color is determined mostly by cultivar and secondly by soil pH ? acid soil for blue, alkaline for pink. To keep your blue blooms constant, apply an acid concoction each spring and fall: 1/4-ounce of aluminum sulfate mixed with one gallon of water. For gardeners who want to keep pink blooms pink, apply a layer of lime around the base of the shrub in spring and fall. While soil amendments do encourage color changes in certain hydrangea blooms, it?s important to remember that other factors (like sunlight) play a role as well.
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