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Blue or pink colors are predicated on the amount of aluminum in the soil solution which can be absorbed by the roots. Although pH is often listed as the agent of color change, it is actually an instigator of (a precursor to) the process. If soils are acid, aluminum is available; if more alkaline, then aluminum is tied up in insoluble forms and not readily available for uptake. So the true story is that high acidity, i.e., low pH, solubilizes (or makes available) aluminum; the reverse occurs at low acidity (high alkalinity), i.e., high pH. Excess phosphorus in the soil will also tie up the aluminum in insoluble precipitates, even in acid soils. Hydrangea macrophylla grown in pine bark medium, pH 5 to 6, are typically pink. Why? The acidity is high, but almost no aluminum is present in the substrate (bark). Soil is composed of minerals, typically aluminum, silicon, iron, etc., and therein resides the difference. So how do growers produce blue hydrangeas in pine bark? Aluminum sulfate is added to the surface of the container at a prescribed rate, usually 0.75 to 1.5 ounces evenly distributed on the surface of the 3-gallon container medium. Greenhouse growers also apply it as a drench at the rate of 2.4 ounces per gallon solution with 8 ounces applied as a drench per 6-inch container. Greenhouse treatments start at budbreak and continue every 2 weeks for three additional applications. Growers have variable timetables for application but in our work as soon as flower buds are visible, a single application at the 1.5 ounce rate per 3-gallon is made. Water thoroughly after application to ensure solubilization of the aluminum and movement into the root zone. Too much is worse than too little: I have dwarfed and killed plants with excessive applications. Hydrangea macrophylla displays a high tolerance to aluminum. Research showed that aluminum complexes with citric acid in the cell sap and may be detoxified in this manner.
Occasionally, elemental sulfur (flowers of sulfur) is recommended for acidifying the soil and thus mobilizing (solubilizing) aluminum. This is a borderline crazy approach and slow to effect the desired change. If the soil pH is high, live with pink, rose, and red hydrangeas — they are beautiful — or create raised beds, laden with acid organic matter, and apply aluminum sulfate that over time will lower pH and supply aluminum for ready blueing. Hydrangea macrophylla or H. serrata, in any shade of pink to red, is satisfying. Consider nature's gift to the garden, accept and enjoy. On the other hand, if soils are acid as lemons, lime may be added to raise the pH if pink, rose, and red flowers are desired.
