Answer: The main influence that affects flower hydrangea color is soil pH, which is a measure of the soil?s acidity. The pH scale is from 0 to 14. Seven is the midpoint and is neutral. PH levels above 7 indicate alkalinity, while those below 7 represent increasing degrees of acidity. The average garden soil is slightly acidic in the range from 5.5 to 6.5. The pH range of 4.5 to 7.5 is not uncommon in garden soils. Blue hydrangeas require a highly acid soil, while pink hydrangeas are produced in a neutral or slightly acid soil.
A starting point to manipulating hydrangea flower color is to know exactly what you soil pH is. Your local cooperative extension service or garden center may be able to instruct you on this procedure. The results of this test will let you know exactly how acid or alkaline your soil is. If you?re growing hydrangeas in two different soils, you may want to check each soil type for varying pH levels.
If you have a pink hydrangea and would like to make it blue or purple, you need to lower the pH to make the soil more acid. This can be done by adding aluminum sulfate to the soil around the base of your plant. Aluminum sulfate is available at most garden centers. Follow the label directions carefully and don?t overdo it. In addition, you do not want to add nitrogen or phosphorus to your soil, as these elements help promote pink colors. Since many general-purpose fertilizers contain these elements in large amounts, it is better to add single-element fertilizers such as muriate and potash.
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