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 such as aluminum sulfate 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. Add aluminum sulfate early next spring, just before the flowers begin to develop. By the time the flowers open, the treatment should help deepen the color of your Nikko Blue.
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