The secret to blue hydrangeas

Editor's note: This is an excerpt from the second in a three-part series on hydrangea production. Part one (August 2017) covered mineral nutrition. Part two focuses on flowering and bluing of H. macrophylla, and part three addresses effective use of plant growth regulators.

Hydrangeas that have blue flowers readily in the landscape or field are often thought of as easy to grow in container production. This is true when substrate, irrigation, and aluminum are managed closely. Be prepared to experiment with these factors to maintain a pH in containers between 4.5-5.5; provide available aluminum without causing toxicity; amend with calcium and magnesium; and irrigate with enough water to prevent desiccation injury, but not leach nutrients necessary to produce plentiful, verdant canopies laden with vibrant, blue flowers.

Growing mophead hydrangeas with blue flowers is easy in southeastern U.S. landscapes because aluminum and other macronutrients are plentiful and soil pH is acidic (<7). Trying to replicate those conditions in containers, however, is not easy, because bark substrate pH is low, aluminum (Al) needs to be abundant, macronutrients, including calcium and magnesium, need not be limiting, and micronutrients need to be present to mitigate any unforeseen deficiencies, such as iron. Therefore, aluminum sulfate (AlSO4), ground or coarse dolomitic limestone, and micronutrients are incorporated in the substrate as amendments. Controlled release fertilizers are used to provide nitrogen, phosphorus, potassium and in some cases micronutrients. Your dolomite amendment should result in maintaining a substrate pH of 4.5 to 5.5 throughout production. In general, the sepal color intensity is determined by a cultivar’s inherent ability to produce blue pigment, the amount and availability of aluminum in the substrate managed by aluminum addition and pH, and a cultivar’s ability to accumulate aluminum from the substrate. When the substrate pH is acidic (4.5 to 5.5) and aluminum is present, sepal color can be expected to be blue because aluminum is generally highly available at a low pH. At pH = 6.0, aluminum is less available in the substrate regardless of the amount; therefore, sepal color is most likely pink. Between pH 5.5 to 6.0, sepal color might be pink, blue, or lavender, or a mixture of pink and blue flowers may be present on the same plant. This is highly dependent upon cultivar. The flower color is not permanent.

Click here to read the full article in the September issue of Nursery Management, including substrate and production plans to help you grow a plant that is blue at the time of sale.

Photo: hydrangea "bluing" testing at Hampton Roads Agricultural Research and Extension Center in Virginia Beach, Va.