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Blue or Pink - Which Color is Your Hydrangea

FOR IMMEDIATE RELEASE
June 18, 2013

The hydrangeas are beginning to bloom and show their colors. Beautiful pinks, blues, whites, greens – there are a number of different species of hydrangea to choose from. You may have even seen a beautiful Nikko Blue Hydrangea but when it's planted somewhere else, it blooms pink instead of blue. So what happened? It's the same plant – but just planted in a different location?

The colors of the blooms for those hydrangeas that are supposed to bloom pink or blue are directly connected with the pH of the soil they are planted in. The hydrangeas directly affected by the pH are Bigleaf Hydrangeas – Hydrangea macrophylla. Aluminum is what causes the flowers to turn blue and typically there is enough aluminum in the soil, but the pH can lock up the aluminum particles in the soil making them unavailable to the plant. It should be said that there are a few varieties of Bigleaf Hydrangeas that have mostly white flowers and are tinged with color or shift to another color as they mature – and these can still be affected by soil pH. In fact there are some pink blooming Bigleaf Hydrangea like 'Pink Elf' and 'Glowing Embers' that can never turn blue and may only turn a purplish color.

To put it simply – acidic soils will cause Bigleaf Hydrangeas to bloom blue and alkaline soils will cause the plant to produce pink flowers. You can find out the pH of your soil by sending in a soil sample to a soil testing lab. When looking at your soil test – a pH of 7 is neutral, below 7 is acidic, and above 7 is alkaline. That is the common breakdown of soil pH, but when we starting looking into soil pH for hydrangeas, the ranges where flower color is impacted shift a bit. It's not a simple above 7 or below 7. For hydrangeas – soil pH below 5.5 will cause flowers to be blue, pH above 6.5 will be pink and typically if the pH is between 5.5 and 6.5 we can see a melding of the two colors bringing it closer to a pinkish-purple colored blooms.

If you wish change the color of your Bigleaf Hydrangea, you can do so by applying aluminum sulfate or sulfur to lower soil pH or limestone to raise pH . Sulfur is a safer bet when trying to lower pH as sulfur reduces the chance of aluminum toxicity that can occur from using aluminum sulfate. Assume that changing the color of your hydrangea is a gradual process – it's not necessarily going to be immediate, it may take a few years. Patience is key when it comes to influencing hydrangea flower color. Whatever you are using to influence the color of the flowers, make sure to apply according to the label and only use the recommended amounts. Some basic recommended guidelines are 1/2 cup of wettable sulfur (lower pH) or 1 cup of lime (raise pH) per 10 square feet.

With Bigleaf Hydranageas it is also a good idea to know if they bloom on old or new wood. Sometimes late frosts can damage buds that formed on previous year's wood. With Bigleaf Hydrangeas that bloom on new and old wood can provide a longer bloom time – varieties such as 'Endless Summer' and 'All Summer Beauty' bloom on old and new wood.

Hydrangeas are high impact plants for full sun to partial shade locations and by providing evenly moist, well-drained soil can provide you with summer long color.

Source: Kari Houle, Extension Educator, Horticulture, khoule@uiuc.edu