Hydrangeas are beloved for their bold blossoms and vivid color. But for some gardeners, the plants never seem to bloom as intensely as they did at the garden center.
What should be bright, blue blossoms are dull and muted, or worse, the flowers are all pink. If your hydrangeas have yet to get the blues, don’t despair – this common problem is related to soil nutrition and can easily be corrected.
Controlling Color 101
Interestingly, flower color in bigleaf hydrangea, Hydrangea macrophylla, is strongly influenced by soil pH and its resulting effect on the availability of aluminum in the soil. A blue hydrangea needs aluminum in order to produce the rich, blue color. The more aluminum a bigleaf hydrangea can take from the soil, the more intensely blue the flower color will become. In contrast, less aluminum available to the plant results in pinker flowers. Genetics of the cultivar also influence the shade of blue or pink found in a hydrangea flower.
So what does this have to do with soil pH? Whether the hydrangea is growing in a container or garden soil, pH and the presence of other nutrients will determine whether aluminum is available to the plant. Aluminum is more readily available for uptake by plants in soils with low pH (acidic) and less available in higher pH soils (alkaline). As such, in acidic soils hydrangea bloom color is blue, while alkaline soils create pink blossoms.
How to Change from Pink to Blue
Gardeners can alter soil pH and therefore flower color, with the addition of powdered or pelletized sulfur to the soil. Before amending soil, conduct a soil test to determine the present soil conditions, and therefore, how much sulfur is required to achieve the desired change in pH. For blue flowers, aim for a soil pH between 5.0 and 5.5. Soil tests will also determine if aluminum is present in the soil at adequate levels. Conduct soil tests every two years to monitor soil pH and aluminum.
Ideally, gardeners amend soil pH before planting, but if you are working around existing plants, be sure to work the sulfur into the soil. Altering soil pH takes several months, so do not expect to see results immediately. Amending soil in autumn will impact hydrangea flower color the following summer. Once soil has been amended, annual fertilization with aluminum sulfate will help maintain soil pH and encourage deep blue flowering. You may also find specialty fertilizers for hydrangeas that will serve this purpose.
A Quick Fix
Many of the hydrangeas on display in garden centers have benefited from an application of aluminum sulfate during bud development. For individual plants, you can ensure blue blossoms for the current season by applying a liquid soil drench around the plant as flower buds are forming in mid spring. Researchers recommend mixing one ounce of aluminum sulfate per gallon of water and applying this to each hydrangea. For very large plants, the application may be doubled. Practice restraint and continue to conduct soil tests, as too much aluminum in the soil can be problematic for many plants. Aluminum sulfate applications should not replace efforts to amend soil pH, which will yield much longer-lasting results.
