Answer: Many of the bigleafed hydrangeas (Hydrangea macrophylla) will respond to soil acidity in their flower coloration, although not all of them will do this. The more acid the soil, the bluer the flowers should be. (Some varieties are bred to remain more or less pink in most soils or to show several colors at one time.)
The acidity can be changed somewhat by regularly using a granular fertilizer for acid loving plants such as Hollytone, and by using an acidic mulch such as pine needles. Also, never add lime near the plant as this makes the soil less acid and would encourage the pink. Often this kind of care regime is enough to make the difference.
You can also increase the acidity (meaning lower the pH) by adding an acidifier gradually so as not to shock the plant. A goal of 5 to 5.5 would be adequate to accentuate the blue, while 6 to 6.5 will bring out the pink. To do this, you would want to run some basic soil tests to determine where you are now and then adjust it over time. Your county extension or knowledgeable nursery personnel should be able to help you with the tests and interpreting the results.
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