Answer: Aluminum Sulphate (sometimes marketed as just plain "sulfur") is usually available in garden centers. If yours doesn't carry it, it's probably because it's not an item that sells well in your local Home Depot. Look for it under the organic garden amendments/supplies. As for changing the color of your hydrangea, flower color is dependent on cultivar and aluminum availability. Aluminum is necessary to produce the blue pigment for which bigleaf hydrangea is noted. Most garden soils have adequate aluminum, but the aluminum will not be available to the plant if the soil pH is high (alkaline). For most bigleaf hydrangea cultivars, blue flowers will be produced in acidic soil (pH 5.5 and lower), whereas neutral to alkaline soils (pH 6.5 and higher) will usually produce pink flowers. Between pH 5.5 and pH 6.5, the flowers will be purple or a mixture of blue and pink flowers will be found on the same plant. So, to change the color of the blooms, you need to change your soil's pH. It helps to know what the pH is now so you will know how to go about changing it. For instance, if your soil is acidic, 4 ounces of lime around the base of your hydrangea plant can change the pH of your soil by one point.
You can add lime in four-ounce increments until you reach the color of pink you desire. You need to try and raise your pH to 6.5. Depending on your soil texture and how acid your soil was to begin with, this can be a long process, taking up to 2 growing seasons if you have an extremely acid soil high in organic matter. Adding nitrogen and phosphorus also aids in preventing the absorption of aluminum and gives a good, clear pink hydrangea color.
Conversely, 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 (or just plain garden sulfur) 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.
Remember that hydrangea color changes will not occur overnight, and are sometimes not predictable. The plants genetics and soil both play a part in manipulating its flower color. It can be very exciting watching the results of your soil amendment and the degrees of color change.
Best wishes with your garden.
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