Answer: Although many hydrangeas can be forced to change or accentuate color depending on the soil pH, Nikko Blue was selected because it should be more consistently blue over a range of soils. A goal of 5 to 5.5 (acidic soil) should be adequate to accentuate the blue.
You can increase the acidity (meaning lower the pH) by adding an acidifier gradually so as not to shock the plant. The acidity can be maintained somewhat by regularly using a granular fertilizer for acid-loving plants such as Hollytone (read and follow the label instructions) 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 paleness. Often this kind of care regime is enough to make the difference.
To begin adjusting pH, 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.
Often hydrangea blooms open and begin very pale and then the color develops over time. The whitish or silvery coloring could be due to drought stress, where the flowers are simply of poor quality and not lasting well -- I am seeing this in my locale this summer where the blooms seem bleached and are not lasting very well at all. Sometimes, too, a plant will take several years to settle into its new location and bloom its best.
However, if the flowers are truly white then it is possible your plants were mislabeled and are not actually Nikko Blue but something else such as H. arborescens instead.
I hope this helps you trouble shoot.
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