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Why is a Blue Hydrangea now pink?

Soil pH plays a major role in flower color

By: Sandy Feather ©2013
Penn State Extension


Q. I have a ‘Nikko Blue’ hydrangea that was blue when I bought it, but has been pink since I planted it near my porch. I know the flower color has something to do with soil nutrients – can you tell me what to do to get blue flowers?

A. Bigleaf hydrangeas (Hydrangea macrophylla) have pink flowers in alkaline soil and blue flowers in acid soil. ‘Nikko Blue’ is included in this group. Soil pH measures soil acidity/alkalinity on a scale from 0 – 14, with 7 being neutral. Higher numbers indicate alkaline soil while lower numbers indicate acid soil. Bigleaf hydrangeas are blue if the pH is between 5.0 and 5.5. The flowers turn pink when the pH gets around 6.0.

blue hydrangea
'Nikko Blue' Hydrangea

The flower color is dependent on the concentration of aluminum ions (Al+++) in the soil. Aluminum is more available to the plant when the soil is acidic, less available when the soil is alkaline. In this case, you should use aluminum sulfate to lower the pH of your soil and supply additional aluminum.

 

Have your soil tested to determine the pH so that you know exactly how much aluminum sulfate to use. It is possible to use too much. Soil test kits are available from your local Penn State Cooperative Extension office. In Allegheny County, soil test kits are $12 for the first kit and $9 for additional kits ordered at the same time. They come with complete information for taking representative samples and understanding your soil test results. You can send a check for the cost of the number of kits you want to Penn State Extension, Soil Test Kits, 400 North Lexington Street, Pittsburgh, PA 15208. Make checks payable to Penn State Extension.

Pink Hydrangea
Pink Hydrangea

Once you purchase the kit, take the sample and fill out the paperwork, and then send it to Penn State’s Agricultural Analytical Laboratory. The fee covers the cost of the kit and the actual testing. Your only other cost is the postage to send it to the university. The kit is a self-contained mailer with the lab’s address pre-printed on it.

Although we tend to have acid soils in western Pennsylvania, there are situations where the pH can move into the alkaline range. For example, garden beds located near cement structures (patios, porches, walls, sidewalks and driveways or the foundation of your house) often have a slightly alkaline pH. This is because lime leaches out of concrete. We use limestone to raise soil pH when the soil is too acid for crops such as vegetables, flowers or turfgrass. If your porch is made of concrete, or there are other concrete structures near the hydrangea, it explains why it is pink now, even if it was blue when you bought it.


MORE

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