Answer: Sulfur or aluminum sulfate will lower the soil pH. Magnesium sulfate is found in Epsom salts and contains magnesium, which can also affect soil pH, but not to the same extent as aluminum sulfate. Aluminum sulfate generally works much quicker than sulfur, though sulfur is generally cheaper. Changing the soil's pH will help change or maintain the color of the flowers but will not affect the flowering ability of your hydrangeas. Usually non flowering is caused by winter dieback or pruning at the wrong time of the year. Some of the newer hydrangeas will bloom on new growth of the year but the older varieties bloom on then growth that develops on old wood. If you have big leaf hydrangea, the type that blooms on new growth that develops on old wood, and if the plant dies down to soil level each winter, there will be no old wood on which new flowering stems can be produced. Therefore your hydrangea will not flower (except in occasional warm winter weather where the plants do not die down to soil level). Hope this sheds some light on the problem.
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