To achieve the blue bract color on Hydrangea macrophylla a pH of 5.5 or less is required along with the availability of aluminum. The addition of aluminum sulfate to the soil is required. Variables in this process, though, are the amount of lime in potting mixes, the quality of irrigation water, and the source and rate of applied fertilizer.

This research project determined that the addition of 1.5 oz. of aluminum sulfate per gallon pot appears to be adequate to achieve the desired results unless the pH is very high. Great care must be taken in the application of higher levels of aluminum sulfate as that can result in very stressed plants. Consequences of excessive aluminum sulfate application would be plant stunting, some leaf drop and smaller flowers.

(from "Decreasing pH on Glory Blue Hydrangea" by Dr. James T. Midcap, University of Georgia, 2000 Research Reports published by the Center for Applied Nursery Research).

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