Hydrangea Flower Color

Dick Bir
NC State University

There are a couple of references to hydrangea flower color on my website at www.ces.ncsu.edu/fletcher/staff/rbir but the questions keep coming so I will try to summarize here.

Some hydrangea shrubs have the capacity for flower colors from blue through white to deep pink or red with differing cultivars under differing conditions having the potential to produce shades everywhere within the spectrum. These are known collectively as the bigleaf hydrangeas. Nomenclature varies but you are likely to see them listed as H. macrophylla or serrata.

Hydrangea arborescens, paniculata and quercifolia are also common shrub hydrangeas grown in NC. However, they all have white flowers, some tinged in pink, which differ in how the flower color fades to pink or red but they do not turn blue unless someone has dyed the flowers.

For a bigleaf hydrangea cultivar to turn pink or blue, it has to possess certain pigments and copigments. If they are not present, the flower color is white. If they contain the pigment and there is aluminum present then they are some shade of blue. If they have the pigment and there is no aluminum present, they are a shade of pink. The intensity and shade of the color depends upon how much pigment exists in that cultivar and how much aluminum as well as copigment is available. This is why the cultivar 'Mathilda Gutges' is always going to have more intensely colored flowers than 'Nikko Blue' when they are growing under the same conditions. If there is no aluminum present, 'Nikko Blue' will be a lovely pastel pink but if there is aluminum present, it is pastel blue.

For naturally growing plants, whether in containers or garden soil, pH and the presence of other nutrients often determines whether aluminum is available to the plant. Aluminum is more available in lower pH soils and less available in higher pH soils. There is usually abundant aluminum in NC piedmont and mountain soils, particularly those with a reddish color. In soilless container growing media such as those commonly used by the nursery industry, there is no aluminum or just a trace.

If you are in the container nursery business and want your 'Nikko Blue' or 'Endless Summer' hydrangeas to turn blue, you should grow them in your normal azalea or shrub mix with a pH below 6.5 and supply some aluminum. Research by Dr. Midcap at the University of George suggests that 1.5 ounces of aluminum sulfate per 3 gallon container applied as flower buds are forming in mid spring will result in blue flowers. If no aluminum is applied, they may be lavender or pink flowered.

What happens to these same cultivars when they are planted in the landscape depends on whether there is aluminum in the soil . . . and there usually is in most of NC. After the roots grow out into the native soil, the plants flower blue. It may be the first season they are in the ground or the second but they eventually turn blue. Usually there is no reasonable way to turn them back to pink because of the abundance of aluminum in our naturally acidic soils. I have seen unreasonable amounts of lime applied to plants in our red clay loam soils and all that happened was that iron chlorosis was induced . . . the plants never became the pink they were originally. If folks want their pink hydrangeas to stay pink in Western NC, they probably should plant them in soil less media in raised beds and lime to a pH of about 6.8 then be ready with supplemental iron for the chlorosis that may ensue.

If your client has plants they want to flower blue but they are flowering pink and they are already in the garden, apply aluminum. However, it is easy to get too much of a good thing. IF the pH is below about 6.5, the standard suggestion is an ounce of aluminum sulfate in a gallon of water and water the plant with it. If it is a big plant, use two ounces of aluminum sulfate in two gallons of water (this is obviously not very precise). THEN wait to see what the results are the next year. If the flowers are still too pink, repeat the process . . . including waiting another year.

If your client has high pH soil . . . the plant, for example, is at their beach house, . . . then they may need to replant after removing existing soil around roots into soil where abundant organic matter has been used as an amendment or plant in a raised bed then use an organic mulch. This MAY work but just as our red soils in the west can overwhelm with too much aluminum, the seashell and sand soils can overwhelm with an abundance of calcium, etc. from the seashells. Beautiful blue flowers can be obtained with aluminum sulfate in pure acidic, well-drained sand but the situation gets complicated when seashells enter the picture.

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North Carolina State University